Field of Science

Gastrotrichs and their Tacky Little Tubes

When I was a student, I was taught that known animal diversity could be divided between somewhere in the region of a couple of dozen 'phyla'. These were the fundamental units of animal classification, the basic archetypes of animal morphology. Many of these were the major assemblages with which we all were familiar: chordates, arthropods, molluscs and the like. But many were the so-called 'lesser phyla', those taxonomic orphans that, whether small in size or small in number or both, tended to escape observation and study by the majority of people. One such 'minor phylum' was the collection of small worm-like animals known as the Gastrotricha.

Polymerurus nodicaudus, a paucitubulate gastrotrich, from Balsamo et al. (2015). Scale bar equals 100 µm.


Gastrotrichs are, in general, minute (Todaro et al. 2019). The largest reach about three-and-a-half millimetres in length, the smallest are about sixty microns, and there are probably many more at the lower range than the higher. They are dorsoventrally flattened with numerous cilia, and their cuticle may often be differentiated into a covering of scales or spines. Gastrotrichs are aquatic and are often referred to as part of the meiofauna, the assemblage of animals specialised for living within and crawling through the spaces between sand grains. That is indeed the preferred habitat for many species and gastrotrichs may be among the most abundant inhabitants of this milieu, edged out only by the nematodes and copepods. However, other species live above the sediment surface, crawling over the surface of aquatic vegetation or even floating among the plankton. Over 850 species are known to date, of which are a bit over 500 are marine (with all marine species being meiofaunal) and the remainder are found in fresh water. They feed on micro-organisms such as bacteria and algae, swallowing them by means of a muscular pharynx.

Gastrotrichs differ from other animals in a number of significant features. Among these is the differentiation of the outer cuticle into two distinct layers. The outermost of these layers, the epicuticle, covers the entire outer surface of the body, including coating the cilia. Gastrotrichs also possess characteristic tubular outgrowths ending in adhesive glands. Their relationships to other animals remain uncertain. Most authors now agree that they represent an early-diverging branch of the Lophotrochozoa, the animal superclade including such creatures as molluscs and annelids. It is possible that they are more closely related to flatworms than anything else but even then the relationship would hardly be close.

Pseudostomella etrusca, a macrodasyidan gastrotrich, from Todaro et al. (2011). Scale bar = 50 µm.


Historically, gastrotrichs have been divided between two orders, the Macrodasyida and Chaetonotida. This division was supported by structural features of the pharynx and the body wall but is also reflected in the distribution of the adhesive tubes. The Macrodasyida, which are usually vermiform, possess adhesive tubules at both the anterior and posterior ends of the body, as well as laterally. Macrodasyidans are always interstitial in habits and usually marine. The Chaetonotida, on the other hand, lack anterior tubules. Chaetonotidans were further divided between two major taxa. One of these was the isolated genus Neodasys which is vermiform and interstitial like a macrodasyidan, and possesses both lateral and posterior tubules. The remaining Chaetonotida were recognised as the suborder Paucitubulatina. As indicated by their name (meaning 'few tubules'), members of this suborder are characterised by the reduction in number of adhesive tubules, usually to a single pair at the end of the body (a few species have two pairs of tubules, others lack distinct tubules and have the adhesive glands opening directly on the main body). They are short, generally shaped more or less like a bowling pin, and are the most ecologically diverse major gastrotrich group, including both marine and freshwater forms.

A phylogenetic analysis of gastrotrichs by Kieneke et al. (2008), however, questioned the established classification of the group. Rather than affirming a basal division between Chaetonotida and Macrodasyida, their results placed Neodasys as the sister group of all other gastrotrichs. Such a division may be reflected in the nature of their adhesive tubules: Neodasys has tubules containing a single gland but Macrodasyida and Paucitubulatina have two glands per tubule (unfortunately, because of the lack of close outgroups, it's hard to know which tubule type was ancestral). Within the Macrodasyida + Paucitubulatina clade, the macrodasyidans were then paraphyletic to the paucitubulates. Interestingly, the sister group to the Paucitubulatina was a clade of the only two known freshwater macrodasyidans, Marinellina and Redudasys. The implication was that gastrotrichs may have made the move to fresh water on just one occasion (followed by a number of returns to the sea among paucitubulates). This is not an isolated case: a number of phylogenetic studies of micro-organisms have found deep divides between marine and freshwater lineages. It seems it's hard to adjust to a life less salty.

REFERENCES

Kieneke, A., O. Riemann & W. H. Ahlrichs. 2008. Novel implications for the basal internal relationships of Gastrotricha revealed by an analysis of morphological characters. Zoologica Scripta 37 (4): 429–460.

Todaro, M. A., J. A. Sibaja-Cordero, O. A. Segura-Bermúdez, G. Coto-Delgado, N. Goebel-Otárola, J. D. Barquero, M. Cullell-Delgado & M. Dal Zotto. 2019. An introduction to the study of Gastrotricha, with a taxonomic key to families and genera of the group. Diversity 11: 117.

To Dung and Beyond

When most people think of a fly, odds are that they imagine one of the group of flies known as calyptrates. This is the clade that includes, among others, such animals as house flies, blow flies and flesh flies. Calyptrates are often reasonably large as flies go and they often have life styles (such as larvae feeding on decaying matter) that bring them close to humans and their homes. One of the most recognisable features of this clade, and the inspiration for its name, is enlargement of the lower calypter, a lobe at the base of the wing. This lower calypter can be moved semi-independently of the rest of the wing which is how calyptrate flies are able to fly acrobatically and avoid being swatted. Nevertheless, there is one significant subgroup of 'calyptrate' flies that has foregone the advantages of an enlarged calypter, commonly recognised as the family Scathophagidae.

Yellow dung fly Scathophaga stercoraria, copyright Derek Parker.


The Scathophagidae are a modestly diverse family of flies with about 250 known species, the great majority of which are found in the Holarctic region (Vockeroth 1987). Only a handful of species are found in more southerly regions, mostly at higher altitudes. They are medium-sized flies, ranging between three and eleven millimetres in length, fairly similar to a house fly in overall appearance but generally more slender and bristly. They are commonly referred to as 'dung flies', in reference to the larval diet of one of the most widespread and best known species, Scathophaga stercoraria (whose scientific name broadly and appropriately translates as 'shit-eater, thing of shit'). However, despite the unremarkable number of species, scathophagids are actually more diverse in their larval habits. As far as we know, adult scathophagids are all predators on other insects.

Scathophagids are divided between two subfamilies, distinguished by features of the male terminalia. In Scathophaginae, the sixth abdominal tergite (dorsal plate) of the male is hairy and usually separate from the following fused syntergosternite 7 + 8 (with dorsal and ventral plates of the segments fused to form a ring). In Delininae, the sixth tergite lacks hairs and is always fused to the following syntergosternite. The subfamilies also differ in life history. Larvae of Delininae are leaf-miners on monocots, hatching from eggs laid on the leaf surface. The Scathophaginae are more diverse. As already indicated, some are saprobes. As well as the dung-feeding S. stercoraria, the genus Scathophaga also includes species which specialise on rotting seaweed on the sea-shore (a milieu which, offhand, supports a range of fly species belonging to numerous families). Other species are, like the Delininae, miners in plant tissue though they are found in a wider range of hosts (both monocots and dicots) and their eggs are inserted by the female directly into the plant tissue. A handful are aquatic or semi-aquatic predators, feeding on small invertebrates along lake shores or in sewage, or on the eggs of caddisflies in fast-running streams.

Cordilura pubera, a plant-feeding scathophagid, copyright Aleksandrs Balodis.


Considering the more derived character of the male terminalia in the Delininae, and the more disparate life habits of the Scathophaginae, some authors have suggested that the latter may be paraphyletic to the former. It has also been presumed that the Scathophagidae as a whole is ancestrally saprobic, considering that saprobic habits are also the norm in related fly families such as the Muscidae (house flies). However, a molecular phylogenetic analysis of the Scathophagidae by Kutty et al. (2007) supported monophyly of both subfamilies. Their results indicated that the original scathophagids were plant-feeders with saprobic lineages arising within the family on two separate occasions. Predatory larvae also evolved twice, once as a further development from saprobes and once direct from plant-feeding ancestors. The diet of this family started out fresh but, somewhere along the line, some species decided they'd rather eat muck.

REFERENCES

Kutty, S. N., M. V. Bernasconi, F. Šifner & R. Meier. 2007. Sensitivity analysis, molecular systematics and natural history evolution of Scathophagidae (Diptera: Cyclorrhapha: Calyptratae). Cladistics 23: 64–83.

Vockeroth, J. R. 1987. Scatophagidae. In: McAlpine, J. F. (ed.) Manual of Nearctic Diptera vol. 2 pp. 1085–1097. Biosystematics Research Centre: Ottawa.

Tadpole Shrimps: Living, Not Fossils

The concept of a 'living fossil' has a fraught history. It tends to appear a lot in popular publications where it conveys a sense of drama and mystery to the organisms so described. In the actual scientific literature, however, it tends to be heavily criticised, due to being poorly defined and of uncertain significance. It commonly gets used to refer to the modern representatives of relictual lineages, often glossing over ways the modern taxa differ notably from their forebears. A reference to 'living fossils' may say more about the writer making it that it does about the intended subject. Consider, for instance, what some have described as the ultimate living fossils: the tadpole shrimps of the Notostraca.

Triops longicaudatus, dorsal and ventral views, copyright Micha L. Rieser.


The tadpole shrimps are a cosmopolitan group of freshwater crustaceans that grow up to ten centimetres in length. They feed on detritus on the bottom of marginal habitats such as temporary pools, brackish lagoons or marshes. Their vernacular name refers to their characteristic body shape, with a flattened oval carapace covering the front of the body (beneath which are concealed the limbs) followed by an elongate, legless abdomen. A pair of compound eyes is visible dorsally near the front of the midline. The body ends in a pair of long caudal rami. Notostracans are a subgroup of the branchiopods, the class of crustaceans that also includes brine shrimps and water fleas. Like other branchiopods, tadpole shrimps have legs with numerous leaf-like branches, adapted for swimming rather than walking. However, whereas many branchiopods have a long pair of second antennae that is used for swimming, tadpole shrimps have both pairs of antennae quite short, presumably in connection with their more benthic lifestyle. The thorax bears eleven pairs of legs, the first of which is elongate and serves to replace some of the sensory function of the rudimentary antennae. The last pair of legs sits on the same segment as the reproductive organs, and in females is modified to form a basket for carrying egs. Like brine shrimps, tadpole shrimps can form a resistent cyst to survive the drying out of their habitat. Also like brine shrimps, this has lead to them being cultured commercially, either as a food supply for fish or as a curiosity in their own right.

Modern tadpole shrimp species are divided between two genera, Lepidurus and Triops, distinguished by the shape of the end of the abdomen. Only about a dozen species are distinguished all up though molecular studies have suggested that there should be more. Notostracans have a sporadic but extensive fossil record, going back as far as the Upper Devonian (Lagebro et al. 2015). And this is where the 'living fossil' concept comes in. The overall form of the tadpole shrimps has been established for a very long time. Indeed, fossils from the Triassic and Permian periods have been described as subspecies of the living Triops cancriformis, which would make it the oldest known species on Earth. Even older fossils from the Carboniferous have been assigned to the genus Triops.

Lepidurus arcticus, copyright Per Harald Olsen.


The problem with these grandiose claims is that their basis is fairly weak. Tadpole shrimps are not heavily sclerotised so their finer features tend to be preserved only rarely. Notostracan fossils often will not preserve much more than the overall proportions of the carapace. And if one wishes to describe Triops cancriformis as a living fossil simply because it has an oval carapace and narrow abdomen, one might as well describe lizards as living fossils because they still have four legs and a tail. A study of the ontogeny of the Triassic 'Triops cancriformis minor', originally described as indistinguishable from the modern species except for its overall smaller size, by Wagner et al. (2017) found notable differences from its modern relative. Both forms have a carapace that becomes longer and narrower over time but, whereas that of T. cancriformis is an oval shape from birth, 'T.' minor begins life rounder and becomes oval with time.

Reconstruction of Almatium gusevi, from Olesen (2009).


One fossil group of crustaceans closely related to the Notostraca is the Kazacharthra, known from the Triassic and Mesozoic of Asia; the two groups have been united in a clade dubbed the Calmanostraca. Kazacharthrans share a number of features with modern tadpole shrimps such as the broad, flattened carapace (albeit one that is proportionately broader than that of Notostraca) and reduced antennae. However, whereas the legs of tadpole shrimps differ from front to back, those of the kazacharthran Almatium gusevi are all similar in structure. In particular, kazacharthrans lack the antenniform first legs of modern notostracans. As it happens, the first legs are also not modified in one Recent species: the little-known Lepidurus batesoni, so far collected once from a location in Kazakhstan (suggesting that the genus Lepidurus is not monophyletic). A phylogenetic analysis of calmanostracans by Lagebro et al. (2015) placed both Almatium gusevi and 'Triops' minor outside the notostracan crown group. It also left open the possibility that 'notostracans', with their much earlier fossil record, are paraphyletic to the Mesozoic kazacharthrans. Lepidurus batesoni was placed closer to other crown notostracans than Almatium or minor, owing to its relatively narrow carapace compared to those taxa and the presence of a rounded anal plate.

So all up, there are problems with describing tadpole shrimps as 'living fossils'. The label focuses on superficial habitus while ignoring the possibility of noteworthy changes in less commonly preserved features. In particular, the antenniform first legs of most modern tadpole shrimps, never yet identified in any fossil species, may be a quite recent innovation. In his 2012 thesis on branchiopod phylogeny, Thomas Hegna ended up concluding that "it seems that Triops cancriformis has no fossil record at all—a dramatic twist of fate for the 'oldest living species'". Hard to qualify as a living fossil when you're not even a fossil!

REFERENCES

Lagebro, L., P. Gueriau, T. A. Hegna, N. Rabet, A. D. Butler & G. E. Budd. 2015. The oldest notostracan (Upper Devonian Strud locality, Belgium). Palaeontology 58 (3): 497–509.

Olesen, J. 2009. Phylogeny of Branchiopoda (Crustacea)—character evolution and contribution of uniquely preserved fossils. Arthropod Systematics and Phylogeny 67 (1): 3–39.

Wagner, P., J. T. Haug, J. Sell & C. Haug. 2017. Ontogenetic sequence comparison of extant and fossil tadpole shrimps: no support for the "living fossil" concept. Paläontologische Zeitschrift 91: 463–472.

The Origins of Song

The world is currently home to roughly ten thousand known species of bird. These come in a significant range of varieties and sizes: ostriches, hummingbirds, penguins, sandgrouse. But one particular clade of birds accounts for roughly half of all living species: the true songbirds of the Euoscines.

Brown treecreeper Climacteris picumnus, a representative of an early-diverging Australian clade of songbirds, copyright Patrick Kavanagh.


Though the name 'Euoscines' doesn't appear to have received a whole lot of usage in the literature, the clade it refers to actually has a long history of recognition. The Euoscines are one of the major subgroups of the well-recognised order Passeriformes, the perching birds. Members of the Euoscines include such familiar animals as finches, crows, wrens, swallows, skylarks, sparrows, and a whole host of others. On a morphological basis, Euoscines are mostly united by the distinctive structure of their syrinx, or voice-box, which is controlled by five pairs of intrinsic muscles (Ericson et al. 2002; by way of contrast, the lyrebirds and scrubbirds that form the clade most closely related to the Euoscines have only three pairs). This complex syringeal structure is doubtless a factor in the elaborate songs that characterise many representatives of the clade and from which the group gets its vernacular name. Molecular data has further strengthened the case for the Euoscines.

Whereas the phylogenetic integrity of the Euoscines is no considered by most researchers to be beyond reproach, its exact origins are a little more mysterious. Outside the Euoscines, the members of the Passeriformes fall into three well supported clades. As noted above, the immediate sister group of the Euoscines is a small Australian clade, the Menurae (the Menurae and Euoscines together form the singing birds, the Oscines). Another very small clade, the New Zealand wrens of the Acanthisittidae, is thought to represent the sister group of all other Passeriformes. The largest clade of Passeriformes outside the Euoscines is the Suboscines, whose members include such examplars as the broadbills and pittas of the Old World tropics, and the antbirds, tapaculos and tyrant flycatchers of the New World. The Suboscines form the sister clade to the Oscines.

Australian logrunner Orthonyx temminckii, another early-diverging exemplar, copyright JJ Harrison.


We also have a fairly clear idea of basal relationships within the Euoscines, primarily from molecular data. I won't dwell on details here (I am aware that while litanies of names can hold a lot of interest for myself, others may find them more tedious) but a detail that has garnered attention is that a preponderance of the basal euoscine lineages are enitrely or predominantly Australasian. This, together with the Australasian distribution of two of the other three major passerine clades, has lead to the proposal that Australasia represents the ancestral homeland for the Euoscines as a whole. But when did the Euoscines first make their appearance?

This is where things begin to get fuzzier. The fossil record of Passeriformes, as for many other birds, is very patchy and often difficult to interpret. Possible passerine bones have been identified from the early Eocene of Australia but they are fragmentary and their identity has been questioned. The earliest well-preserved passerines come from the early Oligocene of Europe (Bochenski et al. 2021). These fossils preserve features indicating that at least the oscine and suboscine lineages had diverged by this time. Attempts to apply molecular dating to the passerine phylogeny, however, have lead to proposals that the major lineages of passerines diverged much earlier, during the Cretaceous era in fact. The divergence of the passerines has then been linked to the break-up of Gondwana, beginning with the isolation of the New Zealand wrens as New Zealand separated from Antarctica about 80 million years ago.

Spotted pardalote Pardalotus punctatus, representing the meliphagoid lineage of Australasian songbirds, copyright Patrick Kavanagh.


Personally, I find this completely incredible. Firstly, it implies a gap of at least 25 million years or so at the beginning of the passerine fossil record (if we accept the Australian fossils as passerines). I've already noted that passerines do not have a great fossil record overall, particularly in the Southern Hemisphere where they are supposed to have originated, but other small birds do have a decent fossil record in the Northern Hemisphere during this time period. The absence of passerines from Europe and North America in the Palaeocene and Eocene does seem likely to be genuine. Secondly, it implies the survival through the devastation of the end-Cretaceous extinction event of not just at least three lineages of passerines but also those bird lineages that diverged before the passerines. At a bare minimum, that requires at least ten clades of birds surviving the Cretaceous and more than likely requires significantly more, most of those lineages also having no recognisable Cretaceous fossil record. Meanwhile, all other non-bird dinosaurs that we do know were around, many of them ecologically very similar, were completely wiped out. Thirdly (and this is perhaps the one that really gets me), it requires that these passerine lineages divided by continental drift then failed to disperse enough over the next eighty million years to obscure the imprint of said drift. Need I remind you that birds can fly? Hand-waving explanations such as the members of many of these early-diverging lineages being poor fliers, or the northern and southern continents being further apart at the time, just don't cut it in my opinion. Why should we assume that if modern Acanthisittidae or Menurae are poor fliers, their extinct relatives also had to be? Eighty million years seems like more than enough time for variation in flight strength to evolve. And a re Suboscines even any more prone to being poor fliers than Euoscines? As for the greater distance between continents, passerines have made their way to isolated oceanic islands (such as those in the mid-Atlantic) that were never close to any landmass. Phylogenetic evidence suggests that some modern passerine groups are indeed the descendants of long-distance dispersals, such as the South American vireos being apparently descended from Asian ancestors, or Hawaiian honeycreepers originating from near the Arctic. And of the previously mentioned European Oligocene passerines, some such as Wieslochia weissi were possibly not part of the Suboscines + Oscines clade (Manegold 2009), indicating that passerines of this grade could indeed make the ocean crossing. So no, the idea of Cretaceous songbirds is just not something I buy right now.

REFERENCES

Bochenski, Z. M., T. Tomek, M. Bujoczek & G. Salwa. In press 2021. A new passeriform (Aves: Passeriformes) from the early Oligocene of Poland sheds light on the beginnings of Suboscines. Journal of Ornithology.

Ericson, P. G. P., L. Christidis, M. Irestedt & J. A. Norman. 2002. Systematic affinities of the lyrebirds (Passeriformes: Menura), with a novel classification of the major groups of passerine birds. Molecular Phylogenetics and Evolution 25: 53–62.

Manegold, A. 2009. The early fossil record of perching birds (Passeriformes). Palaeontologia Africana 44: 103–107.

To Drop Jaw or Not?

The vast majority of living ray-finned fishes (that is, all of them except for bichirs, sturgeons and paddlefishes) fall under the auspices of the clade Neopterygii. I have commented on this clade in earlier posts and in those posts I have noted that modern neopterygians can theselves be divided between three basal lineages. By far the largest of these is the teleosts with only a handful of species representing the other two: the seven or so species of gar in the Lepisosteidae, and the phylogenetically isolated bowfin Amia calva. However, the exact relationships between these three lineages have been the subject of debate.

Close-up on bowfin Amia calva head, from Big Fishes of the World. Note the membranous attachment of the back of the upper jaw.


Historically, the bowfin and the gars were recognised as a group Holostei in apposition to the Teleostei. When first established, this division was motivated primarily by the nature of their scales: the heavy, solid scales of the holosteans contrasted with the thinner, lighter scales of the teleosts. Hence the name 'Holostei' meaning 'entirely bone': the holosteans have both a completely bony skeleton on the inside (as opposed to the partially cartilaginous skeletons of more basal fishes) and a complete covering of bony scales on the outside. However, the heavy scales of the Holostei are a primitive feature, indicating that the two lineages diverged before the evolution of the lighter teleost scales but not indicating a direct relationship with each other.

With the increasing emphasis on evolutionary relationships as the primary informer of classifications, a different system was proposed. This saw the gars as the most divergent lineage of the Neopterygii with the bowfin being united with the teleosts as a clade Halecostomi. This time, the primary evidence for this division was in how their jaws worked. The ancestral condition for vertebrate jaws has them working much as our own still do. The upper jaw, the maxilla, is largely fixed in place against the base of the neurocranium (the brain-holding bit) while the movement of opening and closing the mouth is achieved by the lower jaw, the mandible, pivoting around its hinge towards the back of the skull. In the bowfin and teleosts, however, the maxilla is hinged with the skull at its anterior end and with the mandible at the back. When the mouth opens, the maxila pivots downwards from this anterior hinge, dropping the mandible as a whole downwards. The bowfin and teleosts also possess a bone in the cheek, the interopercular bone, that is not found in other fishes; a muscle attached to this bone rotates the gill operculum as the mouth opens (Lauder 1980). Functionally, the expansion of the mouth cavity in this manner of opening the jaws creates a suction that pulls prey or other food into the fish's mouth.

Though it was by no means universally accepted, it is probably fair to say that the halecostomes vs gars picture of neopterygian evolution became the majority view. But then came the advent of molecular phylogenetic analysis, all ready and willing to cast the proverbial spanner. Rather than confirming halecostome monophyly, molecular analyses pointed the other way, back towards a clade of the bowfin and gars. Following this, a detailed study of gar systematics published by Grande (2010) also supported a gar plus bowfin monophylum on morphological grounds and resurrected the concept of Holostei (albeit redefined on phylogenetic grounds).

Skull of a longnose gar Lepisosteus osseus, from Grande (2010). In the lower diagram, the maxilla is labelled 'mx' and the lacrimomaxillaries are labelled 'lmx'.


Gar jaws, it should be noted at this point, are a bit weird. Rather than being primarily composed of a single maxilla on each side, the upper jaws are made up of a series of tooth-bearing bones, each bone carrying just a few teeth, that have been dubbed the lacrimomaxillaries. When the jaws open, as well as the lower jaw opening in the standard manner, the flexible upper jaw also bends upwards. Rather than using suction to draw in their food like other neopterygians, gars capture prey by sneaking up to it then using a quick sideways jerk of the head to bring the open jaws around the prey (Lauder 1980). Gars were excluded from the Halecostomi on the basis of their lack of a long, mobile maxilla but, as explained by Grande (2010), a mobile maxilla is indeed present in gars but reduced to a remnant splint at the back of the jaw (in mature alligator gars Atractosteus spatula, the maxilla does not ossify). In very young juvenile gars, the mobile maxilla remains a significant part of the upper jaw with the lacrimomaxillaries being added in front of it as the jaw lengthens. As for the interopercular, this is genuinely absent in modern gars but it is present in close fossil relatives of gars such as semionotids. Rather than retaining a primitive jaw structure that was superseded in the bowfin and teleosts, it appears that gars evolved their own derived jaw structure from 'halecostome' ancestors.

Given that suction-assisted feeding is generally regarded as a major advance in fish evolution, how did gars end up abandoning it? That I can only speculate about. Is it related to the evolution of their elongate rostra? Long beaks are certainly a thing for a number of teleosts, but I don't know if any have a beak as long and robust as a gar's. Could it be that the greater precision of gars' snapping mode of feeding is an advantage in the low-oxygen, muck-filled waters in which gars thrive? Or could it be a side effect somehow of gars' more heavily armoured condition than other early-diverging neopterygians?

It's only fair to note that monophyly of Holostei is still not universally accepted; there are sill researchers who are inclined to think the bowfin closer to teleosts. But even if the 'Halecostomi' hypothesis was to rise once more to the surface, it would not be for the same reasons it did before.

REFERENCES

Grande, L. 2010. An empirical synthetic pattern study of gars (Lepisosteiformes) and closely related species, based mostly on skeletal anatomy. The resurrection of Holostei. Copeia 2010 (2A): iii–x, 1–871.

Lauder, G. V., Jr. 1980. Evolution of the feeding mechanism in primitive actinopterygian fishes: a functional anatomical analysis of Polypterus, Lepisosteus, and Amia. Journal of Morphology 163: 283–317.

Tears of a Baby

For many people, the most familiar members of the plant family Urticaceae are the stinging nettles. However, the nettles make up only one part of this cosmopolitan family and there are many representatives that do not sting. One such plant is Soleirolia soleirolii, commonly referred to by the vernacular name of baby's tears.

Baby's tears Soleirolia soleirolii growing around dwarf horsetail Equisetum scirpoides, copyright Carnat Joel.


The only species of its genus, Soleirolia soleirolii is a small creeping herb with more or less succulent stems and subcircular leaves half a centimetre or less in diameter (Harden 1990). It grows in damp habitats and may even grow submerged in water. Baby's tears form a dense flat mat with stems rooted at the nodes. The tiny white flowers reach only a millimetre in size. Wikipedia lists a number of vernacular names for this plants, such as baby's tears, angel's tears, Corsican creeper, or mind-your-own-business (I have no idea what this last name refers to).

In its native range, Soleirolia soleirolii is mostly restricted to islands of the western Mediterranean, including Corsica, Sardinia and Majorca, with a localised mainland population near Rome in Italy (Schüßler et al. 2019). A population was also recently discovered near the coast of Algeria (Hamel & Boulemtafes 2017). On the basis of molecular phylogenetic dating, Schüßler et al. (2019) suggested that its current range may be relictual, having gone extinct over most of mainland Europe as the climate changed. However, as those who glanced at the references for this post may have already guessed, Soleirolia has now become established in many parts of the world outside its native range. It has often been grown as a ground cover or houseplant. If it finds itself somewhere it likes, it may become invasive; though easily uprooted, its proclivity for vegetative reproduction means that it can easily return if not thoroughly cleared. And so we have a paradox, where what is regarded as a valuable relict in one location may be considered a vexatious weed in another.

REFERENCES

Hamel, T., & A. Boulemtafes. 2017. Découverte d'une endémique tyrrhénienne Soleirolia soleirolii (Urticaceae) en Algérie (Afrique du Nord). Flora Mediterranea 27: 185–193.

Harden, G. J. (ed.) 1990. Flora of New South Wales vol. 1. New South Wales University Press.

Schüßler, C., C. Bräuchler, J. A. Reyes-Betancort, M. A. Koch & M. Thiv. 2019. Island biogeography of the Macaronesian Gesnouinia and Mediterranean Soleirolia (Parietarieae, Urticaceae) with implications for the evolution of insular woodiness. Taxon 68 (3): 537–556.

Eurotiomycetes: Small but Significant Fungi

Mention the word 'fungi' and most people's thoughts will probably go to images of mushrooms or toadstools. A few may conjure up pictures of lichens. Nevertheless, the great majority of fungal species are microscopic and likely to pass unremarked by most observers. That does not, however, mean that they are of no consequence. Today's post involves one major group that, for all their visual insignificance, include some of the most significant fungal species for modern human society: the Eurotiomycetes.

Developmental stages of Aspergillus glaucus, with cleistothecia as figs 21–23, from Raper & Fennel (1965).


The class Eurotiomycetes has been recognised in recent years as including a diverse assemblage of fungi, associated with a wide range of morphologies and habitats, that are united as a clade by molecular analyses. Réblová et al. (2017) recognised five subclasses within the Eurotiomycetes of which the two largest (or at least the most studied) are the Eurotiomycetidae and the Chaetothyriomycetidae. The Eurotiomycetidae are, for the greater part, saprobes. They were largely recognised as a distinctive group even before the advent of molecular phylogenetic analysis owing to the production by sexually reproducing forms of a distinctive type of fruiting body, the cleistothecium. In cleistothecia, the fruiting body is completely enclosed with no openings to faciliatate the release of spores, which only escape when the fruiting body itself breaks down. Cleistothecia are most commonly produced by fungi that grow in enclosed locations such as underground (the Eurotiomycetidae are not the only group of fungi to produce cleistothecia though they are one of the most diverse). Within the cleistothecium, spores develop within globular asci with a single wall that breaks down shortly after maturity (Geiser et al. 2015).

Penicillium expansum on rotting pear, copyright H. J. Larsen.


For many people, though, the most familiar members of the Eurotiomycetidae are likely to be asexually reproducing forms. This is the clade containing the moulds of the genera Aspergillus and Penicillium. Even before a species of the latter achieved fame as the shource of the first known antibiotic, penicillin, members of these genera had a great impact on human lives. Species of Penicillium are the moulds used in the production of cheeses such as Roquefort and camembert. Species of Aspergillus are used to ferment soy beans and rice in the production of comestibles such as soy sauce and sake. On the flip side, a number of species of Eurotiomycetidae act as pathogens of mammals including humans, causing conditions such as respiratory illnesses or tinea, with the former being of particular concern in immunocompromised individuals. Eurotiomycetid moulds may also cause problems for food storage and the like, particularly as many species are capable of growing under remarkably hot and/or dry conditions. Some Aspergillus moulds produce dangerous toxins, capable of causing acute poisioning or cancer development.

Verrucaria maura, copyright Richard Droker.


The Chaetothyriomycetidae are less clearly defined morphologically than the Eurotiomycetidae but fruiting bodies are mostly produced as perithecia: flask-shaped structures with an apical pore through which spores are released. The asci within the perithecium usually possess a double wall. Like many eurotiomycetids, chaetothyriomycetids have a tendency to be associated with habitats where water availability is a concern such as in very dry and/or saline environments. A number of chaetothyriomycetid species form lichens. One genus, Verrucaria, is often found as a thin black lichen growing on rocks along the seashore. Some species grow within the cavities of myrmecophytes, plants that form mutualistic associations with ants (the plant provides food and/or accomodation for the ants and the ants help keep the plant clear of grazers or sap-suckers). The fungi are cultivated by the ants that use them for food.

The other three subclasses of the Eurotiomycetes are less well known and recognised as containing a single order each. The Sclerococcales were first recognised as such by Réblová et al. (2017) via molecular analysis. Fruiting bodies, where known, are apothecia (open bowls) bearing single-walled asci. Representatives are known from marine and terrestrial habitats, growing on wood or lichens, and some have been isolated from within the digestive tracts of bark beetles. The Coryneliaceae, living as parasites on podocarps, have been considered as morphologically intermediate between chaetothyriomycetids and eurotiomycetids. Molecular analysis positions them as sister to the latter (Wood et al. 2016). Finally, the Mycocaliciales live as parasites or commensals of other fungi, particularly lichens.

There are other representatives of the Eurotiomycetes that I haven't even had the time to gloss over, such as endophytes and ectomycorrhizal truffles. You may not know they're there but that doesn't mean they don't mean anything to you.

REFERENCES

Geiser, D. M., K. F. LoBuglio & C. Gueidan. 2015. Pezizomycotina: Eurotiomycetes. In: D. J. McLaughlin, & J. W. Spatafora (eds) The Mycota 2nd ed. vol. 7. Systematics and Evolution part B pp. 121–141. Springer-Verlag: Berlin.

Réblová, M., W. A. Untereiner, V. Štěpánek & W. Gams. 2017. Disentangling Phialophora section Catenulatae: disposition of taxa with pigmented conidiophores and recognition of a new subclass, Sclerococcomycetidae (Eurotiomycetes). Mycological Progress 16: 27–46.

Wood, A. R., U. Damm, E. J. van der Linde, J. Z. Groenewald, R. Cheewangkoon & P. W. Crous. 2016. Finding the missing link: resolving the Coryneliomycetidae within Eurotiomycetes. Persoonia 37: 37–56.

The Glyceriforms: Stabby Worms and Grabby Worms

Historically, the annelid worms have been considered a difficult group to classify. Whereas most of the recognised families have been fairly well established, higher taxa uniting these families have tended to be a bit on the vague side. Nevertheless, there are some supra-familial groups that can be considered well established, one such group being the Glyceriformia.

Specimen of Goniadidae (head to the right), from NOAA Fisheries.


The glyceriforms are two families of marine worms, the Glyceridae and Goniadidae. More than a hundred species are known in this clade (over forty glycerids and over sixty goniadids), found in habitats ranging from the intertidal to the abyssal. They range in size from about a centimetre in length to well over half a metre. The front end of the body tapers to a narrow, elongate conical point in front of the mouth, bearing two terminal pairs of small, slender appendages that may correspond to the antennae and palps of other worms. Eyes may be present or absent. The pharynx forms a remarkably elongate, eversible proboscis. In Glyceridae, the proboscis ends in a ring of four hook-shaped jaws, all similar to each other. In Goniadidae, the arrangement of jaws is more complex with the usual arrangement being small micrognaths on one side of the ring and larger macrognaths on the other. Glycerids usually have a transparent skin and an overall red or white colour reflecting the coloration of the internal fluids (red-coloured individuals are sometimes known as 'bloodworms', as are many other similarly coloured worm-like invertebrates). Goniadids have a more opaque cuticle and often have an iridescent sheen (Rouse & Pleijel 2001).

Glycera dibranchiata with everted proboscis, from the Yale Peabody Museum.


Glyceriforms most commonly live as burrowers in muddy or sandy substrates though some live on the surface of rocks. Most are carnivores of active invertebrates such as crustaceans or other worms; some may be detritivores. They may be vagile or they may construct permanent galleries of burrows with multiple entrance and exit openings in which they wait to lunge at anything foolish enough to pass nearby. In glycerids, the stabby jaws are associated with venom glands leading to ducts opening through pores on the jaw's underside. In some species, this venom is strong enough to cause a painful reaction in humans (though I haven't come across any references to long-term consequences). Goniadids lack venom glands and seem to rely on the physical use of their jaws to capture prey. As with many other marine worms, reproduction happens via pelagic epitokes. As a suitable time approaches (Prentiss, 2020, records goniadid epitokes emerging only during a full moon), the glyceriform worm undergoes a metamorphosis involving the break-down of the digestive system and enlargement of the parapodia. The transformed epitokes swim towards the surface where they release gametes through ruptures of the body wall, ending their life in a suicidal orgasm.

Close-up on proboscis of Glycera alba, copyright Hans Hillewaert.


Because of their hardened jaws, which are mostly constructed of protein but partially mineralised, glyceriforms have quite a good fossil record compared to many other worms (Böggemann 2006). Fossilised glyceriform jaws have been found as far back as the Triassic and are little different from those of modern glyceriforms. Body fossils are, unsurprisingly, much rarer but a worm from the Carboniferous Mazon Creek fauna, Pieckonia helenae, has been identified as a stem-group goniadid. The glyceriform body plan seems to have been a very successful one, remaining essentially unchanged over hundreds of millions of years.

REFERENCES

Böggemann, M. 2006. Worms that might be 300 million years old. Marine Biology Research 2: 130–135.

Prentiss, N. K. 2020. Nocturnally swarming Caribbean polychaetes of St. John, U.S. Virgin Islands, USA. Zoosymposia 19: 91–102.

Rouse, G. W., & F. Pleijel. 2001. Polychaetes. Oxford University Press.

Piercing Fruit and Piercing Souls

The moths of the superfamily Noctuoidea are one of the most diverse subsections of the Lepidoptera, with probably somewhere between fifty and seventy thousand species known to date (Zahiri et al. 2012; as with other massively diverse clades, the lack of proper checklists and revisions makes the question of species number surprisingly difficult to answer). For many people, the classic image of a 'moth' will evoke a noctuoid: broad-winged, often nocturnal, often predominantly brown or grey in colour. Obviously, a group this size is going to have a complex taxonomy, and one of the significant subgroups of the noctuoids is the tribe Ophiusini.

Variable drab moth Ophiusa mejanesi, copyright Bernard Dupont.


Historically, the classification of noctuoids has been something of a mess. One researcher commented in 1975 that "It is exceptional to find any two authors who use the same combination of subfamily names within the Noctuidae" and Zahiri et al. admitted in 2012 that the validity of this statement still stood. Until recently, the majority of noctuoids were dumped in a broad family Noctuidae but recent studies (particularly influenced by molecular data) have lead to a significant rearrangement. As a result, the Ophiusini went from being usually placed in the family Noctuidae, subfamily Catocalinae, to the family Erebidae, subfamily Erebinae. A number of genera previously included in the Ophiusini were also transferred elsewhere; most notably, these included all New World representatives so the Ophiusini are now regarded as an exclusively Old World group.

Thyas juno, copyright Alexey Yakovlev.


The Ophiusini are mostly robust-bodied moths with wings of a fairly uniform background colour marked with simple, linear lines on the forewings. The males lack well-developed coremata (eversible structures used for dispersing pheromones) on the genital valves. The caterpillars are elongate semi-loopers with the front two pairs of abdominal prolegs much reduced compared to the rear two pairs. Larvae have been recorded from a wide range of host plant families but the most commonly exploited hosts are members of the Combretaceae and Myrtaceae (Holloway 2005). The pupa lacks the waxy bloom found in many other erebines.

Caterpillar of guava moth Ophiusa disjungens, copyright Robert Whyte.


Many members of the Ophiusini also have a modified apex to the adult proboscis bearing strong, enlarged spines and reversed, erectile hooks (Zahiri et al. 2012). This formidable apparatus is used to pierce the skins of fruits, allowing the moth to feed on their juice. As well as damage caused by browsing caterpillars, ophiusins may therefore also be of concern to horticulture due to damage from this fruit-piercing behaviour. As well as the damage caused by the moth itself, the resulting holes may allow the fruit to be attacked by disease or other insects not capable of breaching the rind themselves. The modified proboscis may also function in what is somewhat daintily referred to as lachrymal feeding: the process of applying the proboscis to the eyes of mammals (more rarely birds) and feeding on secreted fluids. Yes, these are moths that can potentially destroy an orchardist's crop... and then proceed to drink his tears.

REFERENCES

Holloway, J. D. 2005. The moths of Borneo (part 15 & 16): family Noctuidae, subfamily Catocalinae. Malayan Nature Journal 58: 1–529.

Zahiri, R., J. D. Holloway, I. J. Kitching, J. D. Lafontaine, M. Mutanen & N. Wahlberg. 2012. Molecular phylogenetics of Erebidae (Lepidoptera, Noctuoidea). Systematic Entomology 37: 102–124.

Booklice: The Cutest of Pests

Humans have a tendency to think of 'nature' and the 'environment' as something distinct from our own society. Environments unmodified by humans are seen as 'natural' whereas structures created by human activity, such as buildings, are not 'natural' and thought to be somehow outside the 'environment'. As such, people often react strongly to the idea of things associated with the 'environment', such as non-human wildlife, encroaching on their homes. But of course, human houses are as much an environment of their own as any other of the world's habitats, and many animals find them to be places where they can thrive. Among the animals that most regularly share our houses with us are booklice of the genus Liposcelis.

Liposcelis bostrychophila, copyright Andreas Eichler.


Representatives of Liposcelis can be found almost anywhere in the world except in the coldest of regions. About 130 species have been described in the genus to date (Yoshizawa & Lienhard 2010) with doubtless more yet to be discovered (by comparison, Broadhead's review of the genus in 1950 recognised only 22 species, with a six-fold increase since then). The family Liposcelididae, to which Liposcelis belongs, differ from other free-living members of the Psocodea (or 'Psocoptera') in their flattened body form, as well as being smaller than most other examples (Liposcelis grow little more than a millimetre in length). In the flattened habitus, they resemble the parasitic true lice of the Phthiraptera, and recent studies have agreed that the liposcelidids represent the closest relatives of true lice (Yoshizawa & Lienhard 2010). Liposcelis species are readily distinguished from other liposcelidids by the shape of the hind legs: an obtuse tubercle on the outer margin of the hind femur gives it a distinctly broad appearance* (indeed, the genus name Liposcelis translates into English as 'fat thigh'). Liposcelis are also distinctive in being invariably wingless; other liposcelidid species typically come in both winged and wingless forms. Though the genus as a whole is easily recognised, distinguishing individual species is often a far more challenging prospect requiring microscopic examination of fine features of the chaetotaxy (arrangement of bristles on the body) and cuticular sculpture. Authors have divided Liposcelis species between a number of diagnostic sections and subgroups based on these and other features but the monophyly or otherwise of these subdivisions is largely unstudied.

*This feature is also shared with a cave-dwelling species from Ascension Island currently placed in its own genus, Troglotroctes ashmoleorum, but it seems more than likely that this species is itself a derived offshoot of Liposcelis.

Liposcelis species can feed on a wide range of organic matter but, like other 'Psocoptera', their primary source of food is probably yeasts and fungal spores (their vernacular name has been attributed to their feeding on yeasts growing on the glue binding books, though I would note that they are also probably more likely to be seen crawling on the light background of a book's page than in other, less closely examined corners of the house). Turner (1994) provided a detailed review of the natural history of one of the most widespread domestic pest species in the genus, L. bostrychophila, and reports that he was able to maintain cultures on "'Weetabix'™, 'Shreddies'™, baby rice, soya granules, sage and onion stuffing mix, skimmed milk powder, 'Oat Krunchies'™, red lentils, and yellow split peas". Other stored foods from which complaints had been received of booklice included "sugar, bread, salt, bay leaves, gelatine powder, poppadoms, custard powder, dried yeast, instant potato, nuts, dried fruit, baby food, sauce mix, dried mushrooms, pasta, coconut, cocoa, milk powder, spices, glace cherries, garlic, baking powder, icecream mix, dried soup, cracked wheat, carob powder, maize meal, wheat germ, jellied sweets and bread crumbs". They have also been found on cured meat and may damage curated insect specimens. As well as obtaining moisture from their food, Liposcelis are also able to extract water directly from the atmosphere owing to the hygroscopic properties of their saliva. A booklouse will hold a drop of saliva inside its mouth, then swallow it when the ball has absorved enough water from the air.

Liposcelis sp. (possibly L. meridionalis?) from southern France, copyright Jessica Joachim.


Female Liposcelis bostrychophila generally reach maturity and begin producing eggs about two weeks after hatching and may produce two or three eggs a day. As each egg is about one-third the size of the adult, this means that a female at peak fecundity is producing her own body mass in eggs in a single day. Most Liposcelis species reproduce sexually but some are parthenogenetic. Domestic L. bostrychophila, for instance, seem to be entirely parthenogenetic with males of the species only known from isolated collections in Hawaii, Arizona and Senegal (Georgiev et al. 2020). Studies on an unnamed species of Liposcelis from Arizona found that sex determination seemed to be facultative, determined by the mother, with no evidence for differentiated sex chromosomes (Hodson et al. 2017). Females seemed to produce more males early in life and more females later. The same studies also established the occurrence of paternal genome elimination in this species, where chromosomes inherited from the father were inactivated in the offspring and not passed on to their own progeny (which raises the question that, if males are effectively a genetic dead end, why would a female produce male offspring at all?) Paternal genome elimination has also been found in the human louse Pediculus humanus, and may be characteristic of the broader clade encompassing these species, but other species remain unstudied. Liposcelis genomes are also remarkable in the occurrence of fragmentation of the mitochondrial genome. Whereas some Liposcelis species have only a single mitochondrial chromosome, as is standard for most other animals, some species have the mitochondrial genome divided between two, three, five or seven chromosomes (Feng et al. 2019). The functional significance, if any, of this feature remains unknown.

Though booklice may be found in houses and stores on the regular, they are mostly only minor pests, only causing distress when reaching large numbers (an exceptional case quoted by Turner, 1994, involved a house in New Jersey at the beginning of the 1900s that became so infested "'that a pinpoint could not have been put down without touching one or more of these bugs"). They are not believed to transmit pathogens, except perhaps incidentally by carrying microbes from one store to another. For the most part, these little beasties are just another part of the wildlife that shares our homes with us, whether we are aware of them or not.

REFERENCES

Feng, S., H. Li, F. Song, Y. Wang, V. Stejskal, W. Cai & Z. Li. 2019. A novel mitochondrial genome fragmentation pattern in Liposcelis brunnea, the type species of the genus Liposcelis (Psocodea: Liposcelididae). International Journal of Biological Macromolecules 132: 1296–1303.

Georgiev, D., A. Ostrovsky & C. Lienhard. 2020. A new species of Liposcelis (Insecta: Psocoptera: Liposcelididae) from Belarus. Ecologica Montenegrina 29: 41–46.

Hodson, C. N., P. T. Hamilton, D. Dilworth, C. J. Nelson, C. I. Curtis & S. J. Perlman. 2017. Paternal genome elimination in Liposcelis booklice (Insecta: Psocodea). Genetics 206: 1091–1100.

Turner, B. D. 1994. Liposcelis bostrychophila (Psocoptera: Liposcelididae), a stored food pest in the UK. International Journal of Pest Management 40 (2): 179–190.

Yoshizawa, K., & C. Lienhard. 2010. In search of the sister group of the true lice: a systematic review of booklice and their relatives, with an updated checklist of Liposcelididae (Insecta: Psocodea). Arthropod Systematics and Phylogeny 68 (2): 181–195.

Sea Spiders

With arthropods being such a massively diverse sector of the global biota (and even that feels like an understatement; describing arthropods as 'very diverse' seems a bit like describing the Andromeda Galaxy as 'very far away'), it is only to be expected that it contains some very weird corners. And definitely among the weirder of those corners are the Pycnogonida, commonly known as the 'sea spiders'.

Anoplodactylus evansi, copyright Mick Harris & Claudia Arango.


Pycnogonids are a group of marine arthropods found around the world (not actual spiders, of course, though honest-to-goodness marine spiders are a thing that does exist). Their relationships to other arthropods have long been in dispute but the majority view is that they are distant relatives of the terrestrial arachnids. Pycnogonids are not uncommon in both coastal and deep-sea habitats but tend to go unnoticed: they feed on rock-encrusting colonial animals such as hydrozoans and are often coloured to disguise themselves against their prey. If one ever does see a sea spider, the first thing to stand out about them is how they are made of legs. The central body is often remarkably small compared to its limbs, to the extent that the dubbing of pycnogonids as 'no-bodies' by an early 20th Century author has become something of a cliché. Certain major organs, such as the gonads and parts of the digestive system, have even been diverted into the legs to make up for the lack of space in the body. Most pycnogonids possess four pairs of walking legs though there are species with more. At the front of the body on the underside of the head is a large proboscis that is used for sucking the juices out of prey, flanked by pairs of pincer-bearing chelifores and/or palps used for tearing it open. Near the first pair of walking legs there is often a pair of slender leg-like appendages known as the ovigers, used for carrying bundles of eggs until they hatch. The greater part of the body behind the head is taken up by the leg-bearing thorax; the legless abdomen is reduced to the merest nub like the docked tail of a dog.

Close-up on preserved male Anoplodactylus lentus, from Florida Museum of Natural History.


One of the largest recognised genera of pycnogonids is Anoplodactylus, with over 130 species worldwide and many continuing to be described (Lucena et al. 2015). This genus can be distinguished by the possession of chelifores with functional chelae (pincers) but palps are absent or reduced to buds. Both the chelifores and the proboscis are relatively short (Child 1998). Ovigers are five- or six-segmented and present in males only (male care of eggs is the standard pattern among pycnogonids). Species vary from 0.6 to 6 millimetres in body length. The majority of species of Anoplodactylus are found in shallow waters in temperate and tropical regions with a smaller number of species found in polar and deep waters. Alvarez & Ojeda (2018) record finding a single specimen of the species A. batangensis among vegetation on the surface of an anchialine pool in the Yucatan Peninsula of Mexico. Though the surface of these pools is more or less fresh water, deeper sections are saline owing to subterranean connections to the sea. The collection of a pycnogonid near the surface of this pool suggests an ability to adjust to very low salinity though one questions whether it would be able to survive indefinitely.

Larvae of Anoplodactylus are very small compared to those of other pycnogonids and have what has been termed an 'encysting' development (Burris 2011). As bizarre as the appearance of adult pycnogonids is, their larvae are arguably even weirder, being essentially nothing more than a head bearing chelifores, proboscis, and two pairs of undifferentiated appendages. The remaining segments of the body are added over the course of development. In Anoplodactylus, the larvae develop as parasites, forming a cyst in the gastrocoel (the stomach cavity) of cnidarians (having presumably been placed there somehow by their fathers, though I haven't found if we know how). They become free-living upon reaching the first juvenile stage, emerging from their host to pursue their predatory lives.

REFERENCES

Alvarez, F., & M. Ojeda. 2018. First record of a sea spider (Pycnogonida) from an anchialine habitat. Latin American Journal of Aquatic Research 46 (1): 219–224.

Burris, Z. P. 2011. Larval morphologies and potential developmental modes of eight sea spider species (Arthropoda: Pycnogonida) from the southern Oregon coast. Journal of the Marine Biological Association of the United Kingdom 91 (4): 845–855.

Child, C. A. 1998. The Marine Fauna of New Zealand: Pycnogonida (Sea Spiders). National Institute of Water and Atmospheric Research (NIWA).

Lucena, R. A., J. F. de Araújo & M. L. Christoffersen. 2015. A new species of Anoplodactylus (Pycnogonida: Phoxichilidiidae) from Brazil, with a case of gynandromorphism in Anoplodactylus eroticus Stock, 1968. Zootaxa 4000 (4): 428–444.

Of Hawks and Marble

The acanthomorph fishes (a major clade of fishes mostly characterised by the presence of spines at the front of the dorsal fin) have long been recognised as a particularly thorny problem for higher-level systematics. Morphological relationships between many of the large number of families recognised in this clade have been almost impossible to unravel, and it is only in recent years that molecular analyses have been able to start making sense of the rapid divergences. Nevertheless, there are some subgroups of the acanthomorphs that have been recognised for a long time and which recent analyses have continued to support. One such group is the cirrhitoids.

Spottedtail morwong Goniistius zonatus, copyright Joi Ito.


Variously referred to in recent sources as the Cirrhitoidea, the Cirrhitoidei, or the Cirrhitiformes, the cirrhitoids include about eighty known species usually divided between five families. These are the hawkfishes of the Cirrhitidae, the trumpeters and morwongs of the Latridae, the Cheilodactylus fingerfins, the Chironemus kelpfishes and the Aplodactylus marblefishes (the morwongs were historically placed with the fingerfins in the Cheilodactylidae but have recently been transferred based on molecular data—Ludt et al. 2019). The largest cirrhitoid is the dusky morwong Dactylophora nigricans of western and southern Australia, growing to 1.2 metres in length, but most species are only a fraction of this size. Some of the largest species are of note to fisheries. Cirrhitoids are generally inhabitants of reefs, mostly feeding on benthic invertebrates such as crustaceans. They have long been recognised as a coherent group owing to their distinctive fin structure. The lower rays of the pectoral fins are not branched, and in a number of species they are thickened and protrude past the fin membrane (observant readers of this post may have already noticed a theme in many of the genus names given to cirrhitoids, relating to this feature). The pelvic fins are set well behind the pectoral fins. Other notable features of the clade include a relatively high number of vertebrae, a relatively low number of rays in the caudal fin, and the presence in juveniles of a fatty sac running along the fish's underside (Greenwood 1995).

Coral hawkfish Cirrhitichthys oxycephalus, copyright Aquaimages.


Both morphological and molecular studies have agreed that the hawkfishes of the Cirrhitidae represent the sister clade to the remaining cirrhitoids. Hawkfishes are brightly coloured inhabitants of the tropics, usually well under a foot in length. They are distinguished by bundles of trailing filaments emerging from the ends of the spines on the dorsal fin. Perhaps the most familiar member of the group is the longnose hawkfish Oxycirrhites typus, a regular in marine aquaria. However, this is also perhaps the most atypical member of the family as other species do not have the elongate snout. Hawkfishes commonly perch atop corals on the uppermost part of the reef, protected by the coral's sting and able to maintain a clear view of their surrounds. Wikipedia suggests that this behaviour is the inspiration for the name of 'hawkfish', but I'm not sure I buy this. I mean, it sounds plausible, but it also sounds like the sort of thing you would have to be diving below the reef to see. Vernacular names for fish tend to more often refer to things you might observe while hauling them onto a boat.

Marblefish Aplodactylus arctidens, copyright Peter Southwood.


The remaining cirrhitoids are all found in cooler waters, mostly in the Southern Hemisphere. Two species of Latridae, the redlip morwong Goniistius zebra and the spottedtail morwing G. zonatus, are found in the northern Pacific off the coast of eastern Asia (the kind of distribution shown by the genus Goniistius, where species are found in northern and southern temperate waters but not in the intervening tropics, is known as 'anti-tropical' and it's an interesting question how such a distribution would come to be). They are mostly found among rocky reefs, with the kelpfishes Chironemus and marblefishes Aplodactylus being particularly associated with patches of seaweed. The marblefishes feed on algae (particularly reds) as well as on some invertebrates and are characterised by a transverse mouth that is little or not protractible (Regan 1911). As noted above, the family Latridae has been inflated recently by the inclusion of most of the species previously included in the Cheilodactylidae. Cheilodactylus itself is now restricted to two species found around southern Africa. They differ from the remaining species in the latrids by the absence of a gas bladder as well as by elements of the skeleton. Many of the latrids are favourites of anglers, being well regarded as eating fish. By contrast, the herbivorous marblefishes are maligned as very poor fare and avoided. There's something to be said for eating your greens.

REFERENCES

Greenwood, P. H. 1995. A revised familial classification for certain cirrhitoid genera (Teleostei, Percoidei Cirrhitoidea), with comments on the group's monophyly and taxonomic ranking. Bulletin of the Natural History Museum of London (Zoology) 61 (1): 1–10.

Ludt, W. B., C. P. Burridge & P. Chakrabarty. 2019. A taxonomic revision of Cheilodactylidae and Latridae (Centrarchiformes: Cirrhitoidei) using morphological and genomic characters. Zootaxa 4585 (1): 121–141.

Nelson, J. S., T. C. Grande & M. V. H. Wilson. 2016. Fishes of the World 5th ed. Wiley.

Regan, C. T. 1911. On the cirrhitiform percoids. Journal of Natural History, series 8, 7: 259–262.

The New Centaury

In an earlier post, I described the South American flowering herbs known as the Coutoubeinae. In this post, I'm going to take a step back and look at a clade of which the coutoubeines form a part, the Chironieae.

Seaside centaury Centaurium littorale, copyright Anne Burgess.


The Chironieae are one of the major tribes of the flowering plant family Gentianaceae, including about 160 known species. Representatives are found in most parts of the world, though as part of the native flora in Australasia they do not extend past the north of Australia (some exotic species have been introduced further south). The Chironieae seem to primarily be supported as a clade on the basis of molecular data (Struwe et al. 2002). All members are herbs, from annuals to short-lived perennials. Most have an erect growing habit; members of the Caribbean genus Bisgoeppertia are annual climbers and some species of the Mexican genus Geniostemon are creeping perennials. There may or may not be a basal rosette of leaves, and a number of genera have winged stems. Flowers are solitary or borne in cymose or racemose inflorescences. These flowers are most commonly salver-shaped (that is, shaped like a flat dish) or tubular, and usually have four or five petals (some species may have up to twelve). The calyx is usually comprised of fused sepals and is unwinged and tubular. The fruit is usually a septicidal capsule (splitting along the septa between carpels), more rarely a berry.

Yellow centaury Cicendia filiformis, copyright Hajotthu.


Members of the Chironieae are divided between three subtribes that are mostly distinct both morphologically and biogeographically. As described in the previous post, the Neotropical Coutoubeinae are characterised by releasing their pollen in tetrads whereas the other subtribes shed individual pollen grains. The Canscorinae are mostly found in the Old World tropics and have white or cream-coloured flowers (less commonly yellow, pink or purple) with the calyx tube longer than the calyx lobes. The Chironiinae mostly includes found in northern temperate regions, as well as the southern African genera Chironia and probably the South American Zygostigma. Their flowers come in a range of colours—pink, yellow, purple or blue, but less commonly white or cream-coloured—and may have calyx lobes longer than the tube. Many chironiine flowers also have anthers that become spirally twisted after releasing pollen whereas those of Canscorinae are always straight. Molecular data usually support the monophyly of the three subtribes and the majority view seems to be that the temperate Chironiinae represent the sister group of a tropical clade of Canscorinae and Coutoubeinae.

Cultivated Eustoma, copyright Rameshng.


Perhaps the best known members of the Chironieae are the centauries of the genus Centaurium. Historically, about fifty species across the Holarctic have been included in this genus. However, phylogenetic studies have demonstrated that this broad sense of the genus is polyphyletic and thus it has been cut down to a group of about twenty species found in Europe and western Asia. The name 'centaury' refers to the use of common centaury Centaurium erythraea as a medicinal herb, after the legendary centaur healer Chiron. Other Old World species are now placed in the genus Schenkia whereas North American species form the genera Gyrandra and Zeltnera. The yellow centauries of Cicendia are small, filiform annuals native to Europe and the Americas that have been introduced to Australia. The rose gentians Sabatia of North America bear pinkish-purple flowers, often in lax cymes. There are also the prairie gentians of the genus Eustoma. Native to southern North America, these plants bear large, showy flowers that have become popular in cultivation. Commercially, they are labelled as lisianthus. This is not to confused with Lisianthius, a distinct genus of Gentianaceae, or Lisyanthus, a name that has been used in the past for members of yet another gentianaceous genus. Both of these belong to completely different tribes in the family, and may be subjects for another day.

REFERENCE

Struwe, L., J. W. Kadereit, J. Klackenberg, S. Nilsson, M. Thiv, K. B. von Hagen & V. A. Albert. 2002. Systematics, character evolution, and biogeography of Gentianaceae, including a new tribal and subtribal classification. In: Struwe, L., & V. A. Albert (eds) Gentianaceae: Systematics and Natural History pp. 21–309. Cambridge University Press: Cambridge.