Field of Science

A Second Look at Scallops

In a post that appeared at this site over eight years ago, I described some of the distinctive features of the Pectinoidea, the group of bivalves commonly known as scallops. It's time to look in a bit more detail at some of the points mentioned in that post.

Fossil of Pernopecten, the earliest scallop genus, from ammonit.ru.


Pectinoidea, in the sense recognised by Waller (2006), first appear in the fossil record way back in the late Devonian. They were probably derived from earlier members of the Aviculopectinoidea, an extinct group of bivalves that closely resemble scallops in their overall appearance and were included in the Pectinoidea by many earlier authors (such as in the 1969 Treatise on Invertebrate Paleontology volume on bivalves). However, the shell ligament of aviculopectinoids was reinforced by aragonite fibres (a primitive feature for bivalves) rather than having the specialised rubbery core found in pectinoids. As such, aviculopectinoids would have lacked the swimming abilities of true scallops. The Palaeozoic pectinoids belong to a single genus, Pernopecten, that possesses a number of features such as details of the shell crystalline structure that indicate a position outside the pectinoid crown group. In the early Triassic, Pernopecten begat the family Entolioididae that includes the ancestors of living pectinoids.

As mentioned in the previous post, four pectinoid families survive to the present day: the Pectinidae, Propeamussiidae, Entoliidae and Spondylidae. The first three families diverged in the early Triassic. Spondylids (usually classified in a single genus, Spondylus) were not to appear until the mid-Jurassic and Waller (2006) argued for their derivation from within the Pectinidae. The Pectinidae are otherwise distinguished from other pectinoids by a structure called the ctenolium. This is a row of teeth that develops on the shell in the gap between the disc and one of the auricles (the triangular 'wings' at the top of the shell). During the earlier part of the scallop's life, when it lives attached to the ocean bottom by a byssus (what in mussels we call the 'beard'), the ctenolium functions to hold the byssus threads in place and help stop the shell from twisting. In those pectinid species that lack a byssus in the latter part of their life, the ctenolium may end up getting overgrown by the expanding shell and disappearing, but all pectinids (ignoring the aforementioned Spondylus question) have a ctenolium for at least part of their life.

The propeamussiid Cyclopecten secundus, copyright Museum of New Zealand Te Papa Tongarewa.


The Pectinidae is the largest scallop family in the present day, followed by the Propeamussiidae. The Entoliidae were diverse during the Mesozoic but declined dramatically after the end of the Cretaceous (I'm not clear whether or not their decline was a direct part of the end-Cretaceous mass extinction). Indeed, entoliids are completely unknown from the fossil record between the Palaeocene and the late Pleistocene; like the tuatara, it might be that the post-Mesozoic survival of entoliids could have gone completely unrecognised were it not for the single surviving relictual genus.

In the earlier post, I implied that propeamussiids lack the eyes and guard tentacles of other pectinids; it turns out that this was a mistake on my part. Many propeamussiids found in the deep sea do indeed lack these features but they are present in shallow-water propeamussiids. It appears that these features are ancestrally common to all crown-group pectinoids but have been lost as an adaptation to life below the photic zone. The anatomy and lifestyle of many propeamussiids remains poorly known but those species that have been investigated have simplified gills compared to pectinids. The filaments of the gills are free rather than being connected by ciliary junctions. The lips of the mantle are also simplified, lacking the complex lobes found in pectinids. These features may be related to the carnivorous diet of many propeamussiids that feed on zooplankton rather than smaller phytoplankton and organic particles.

REFERENCE

Waller, T. R. 2006. Phylogeny of families in the Pectinoidea (Mollusca: Bivalvia): importance of the fossil record. Zoological Journal of the Linnean Society 148 (3): 313–342.

Trichosternus

Trichosternus vigorsi, copyright Udo Schmidt.


Many of the carabid ground beetles tend to attract a lot of attention from amateur entomologists due to their size and striking appearance, but it must be admitted that they are often not the easiest of animals to work with from a taxonomic perspective. The larger species tend to fall into the category of 'big, black, massive sharp mandibles' and it can require a lot of practice to reliably identify which genus a specimen belongs to, let alone species.

Trichosternus is a genus of ground beetles found in far eastern Australia, from the base of Cape York in Queensland to a bit north of Sydney in New South Wales, in the band of land between the coast and the Great Dividing Range. There is also a single isolated species T. relictus in the southwest corner of Western Australia, and apparently another in New Caledonia (Darlington 1961). However, considering the difficulty that many authors have had in the past in providing an exact definition for Trichosternus relative to other closely related genera, it would be interesting to see if future studies corroborate the inclusion of these outlying species. By way of contrast, a reasonable number of New Zealand species assigned at one time or another to Trichosternus have all long since been moved elsewhere.

Trichosternus species are all flightless and in most the elytra are fused and cannot open (the exception is T. relictus). Most species have a distinctive male genital morphology, with the genital opening deflected to the right and the right paramere (the parameres are two sclerotised 'arms' on either side of the genitalia) modified into a specialised falcate shape, the exact functional significance of which seems to remain unknown. Again, the outlier in this regard is T. relictus in which said paramere retains a primitive styloid shape. Similar falcate parameres are also known from members of related genera such as Megadromus and Nurus; the latter is particularly similar to Trichosternus with the only real difference between the two being that Nurus is more robust with longer mandibles. Trichosternus relictus also has a distinctive female genital morphology, in which the internal passage between the median oviduct (where emerging eggs are fertilised by sperm stored in the spermatheca) and the vagina is remarkably extended and concertina-like. Again, the function of this structure is unknown though Moore (1965) suggested that it might be related to viviparity.

Northern Trichosternus species found in tropical Queensland are all inhabitants of rainforest (hence the restriction of the genus to east of the Great Dividing Range: on the western side of the range, rainforests are absent and the arid zone begins). Southern species are found in upland temperate rainforests or in savannah woodland (Darlington 1961). Some species have very restricted ranges: T. montorum, for instance, is known from two mountains on the Spec Plateau, Mts Bartle Frere and Bellenden Ker.

REFERENCES

Darlington, P. J., Jr. 1961. Australian carabid beetles VII. Trichosternus, especially the tropical species. Psyche 68 (4): 113–130.

Moore, B. P. 1965. Studies on Australian Carabidae (Coleoptera). 4.—The Pterostichinae. Transactions of the Royal Entomological Society of London 117 (1): 1–32.

The Lonely Life of the Cave Collembolan

For a few weeks last year, I had the job of sorting and identifying a collection of Collembola, springtails. Prior to doing this work, I had only the vaguest of understandings of springtail diversity: I knew that there were the round blobby ones, the long thin ones, and the ones that look a bit like sausages, but that was about as far as it went. Needless to say, there's a bit more to it than that.

Pseudosinella immaculata, copyright Andy Murray.


Pseudosinella is the largest genus of Collembola currently recognised, with over 280 described species. The greater number of those species are in Europe and North America, but various Pseudosinella have also been described from other regions of the world (there don't appear to be any from South America, but then I don't know how thoroughly anyone's looked). Pseudosinella species are mostly associated with subterranean habitats, from soil and litter to deep caves, with the highest diversity in the latter. According to a key at collembola.org, Pseudosinella are distinguished from related genera by having reduced eyes (with six or fewer ommatidia, as opposed to the eight ommatidia of other genera), and a bidentate mucro lacking a projecting lamella (the mucro is the claw-like structure at the end of the furcula, the posteroventral prong that forms a springtail's 'spring'). The key also distinguishes Pseudosinella from the similar genus Rambutsinella by it's not having the fourth antennal segment swollen as in the latter, but Bernard et al. (2015) described the species Pseudosinella hahoteana as also having the fourth antennal segment swollen so I'm not sure how reliable that feature is. Pseudosinella is very similar to another genus Lepidocyrtus, the main difference between the two being Pseudosinella's reduced eyes, and more than one author has raised the possibility that Pseudosinella may be a polyphyletic assemblage derived from Lepidocyrtus adapted for life underground.

As well as the reduced eyes, Pseudosinella tend to show a number of other features commonly associated with a subterranean lifestyle, such as a pale coloration and relatively elongate appendages. The claws of the feet also tend to become modified, with the larger of the two becoming longer and progressively narrower (Christiansen 1988). This latter feature is probably an adaptation to movement on the wet surfaces that predominate in caves. At a moderate length, the claws dig into the substrate surface more than those of surface-dwelling forms, allowing greater grip. At longer lengths, the claws are suited to allow the springtail to walk over the surface of the water itself (most springtails float on water surfaces due to their small size and low density, but not all can move with purpose in this position).

Pseudosinella hahoteana, from Bernard et al. (2015). Scale bar = 200 µm.


The aforementioned Pseudosinella hahoteana is worthy of extra attention, as it is one of a half-dozen springtail species endemic to caves on Rapa Nui, the landmass previously known as Easter Island. Many of you will be aware of the ecological catastrophe that beset Rapa Nui following human settlement, as its entire forest covering was cleared away. As a result of this clearing, the native fauna was also all but wiped out; no vertebrates survive, and of about 400 arthropods known from the island only about twenty are indigenous (Bernard et al. 2015). As such, the handful of minute animals clinging to survival in patches of ferns and moss at the entrance to caves represent a significant proportion of Rapa Nui's surviving native fauna.

REFERENCES

Bernard, E. C., F. N. Soto-Adames & J. J. Wynne. 2015. Collembola of Rapa Nui (Easter Island) with descriptions of five endemic cave-restricted species. Zootaxa 3949 (2): 239–267.

Christiansen, K. 1988. Pseudosinella revisited (Collembola, Entomobryinae). Int. J. Speleol. 17: 1–29.

Define 'Trichostomum'


The moss in the above photo Icopyright Hermann Schachner) generally goes by the name of Trichostomum crispulum. Trichostomum is a cosmopolitan genus in the Pottiaceae, the largest recognised family of mosses with about 1500 species overall. But with great diversity comes great difficulty of identification. Pottiaceae tend to be small mosses that are common in harsh habitats. Features of pottiaceous mosses are often hard to distinguish and may be quite variable, making it difficult to confidently define taxa. As a result, Pottiaceae is a prime example of what I like to call 'taxonomic blancmange': something that tends to just get prodded nervously then backed away from when it wobbles ominously.

Characteristic features of Trichostomum as it is commonly recognised tend to include symmetric leaves with more or less plane margins, and with the basal cells of the leaf differentiated straight across the blade or in a U-shape. The peristome of the capsule also tends to be short and straight, and the sexual system is usually dioicous (with separate male and female plants) (Flora of North America). However, none of these features are entirely reliable, and some species have been the subject of extensive disagreement about whether they should be placed in Trichostomum, or in a related genus such as Weissia or Tortella.

To date, only a selection of Pottiaceae species have been subject to molecular analysis, but these analyses have confirmed the unsatisfactory nature of the current system. A molecular phylogenetic analysis of the pottiaceous subfamily Trichostomoideae by Werner et al. (2005) did not identify Trichostomum species as a monophyletic clade; instead, various representatives of the 'genus' were scattered throughout the subfamily. The type species of Trichostomum, T. brachydontium, was associated with a few close relatives such as T. crispulum in a broader clade containing numerous species of the genus Weissia. As a result, it has been suggested that the two genera should perhaps be synonymised, in which case the name Trichostomum would be absorbed by the older Weissia. But first, someone would need to work out just how such a genus could be recognised...

REFERENCE

Werner, O., R. M. Ros & M. Grundmann. 2005. Molecular phylogeny of Trichostomoideae (Pottiaceae, Bryophyta) based on nrITS sequence data. Taxon 54 (2): 361–368.

Cryptophytes: Four Genomes for the Price of One

Sometimes, the little things really do make a difference. Cryptophytes (or cryptomonads) are one of the many groups of minute flagellate protists to be found around the world whose role in our lives tends to get dismissed because of their microscopic size. Nevertheless, cryptophytes make up a large part of the photosynthetic phytoplankton in both freshwater and marine habitats and so ultimately are a starting point for many of the food chains that we depend on. They also had an important role to play in our developing understanding of how modern eukaryote cells have evolved.
Structure of a typical cryptophyte, from here.


As well as occurring in the phytoplankton, cryptophytes have also been found in damp soil and snow. They have a distinctive, slightly lop-sided cell morphology with two haired flagella of unequal length inserted in an invaginated gullet towards the right side of the front of the cell. This invagination is also lined on the ventral side by organelles called ejectosomes (sometimes spelled 'ejectisome'). When the organism is threatened, these ejectosomes shoot out a proteinaceous ribbon that propels the cell rapidly away from the source of irritation. Some of the references to ejectosome function that I've found seem to imply that the expelled ribbon is itself toxic, but I'm not sure if I've understood correctly. Smaller ejectosomes may also play a role in capturing bacteria and the like for the cryptophyte to feed on. Cryptophytes have a distinctive way of moving through the water column, resulting from the uneven lengths of their two propellent flagella, that has been reffered to as 'recoiling'. Essentially, they move in a series of circular tumbles while the cell itself corkscrews around its axis. This movement is distinctive enough that cryptophytes have been dubbed with the Dutch vernacular name of 'rekylalger', 'recoiling algae' (Novarino 2003).

Diagram of typical cryptophyte movement, from Novarino (2003).


The majority of cryptophytes are heterotrophic: one or more large chloroplasts provide much of the cell's energy, but they are also capable of ingesting particulate matter through the gullet. As alluded above, the cryptophyte chloroplast has been significant in the study of how chloroplasts evolved. The 1960s and 1970s saw an increasing acceptance of the concept that some organelles, most notably mitochondria and chloroplasts, had originally appeared through a process of endosymbiosis: bacteria had become intimately associated with eukaryote cells, becoming embedded in the host cell and eventually ceding enough of their vital functions to the host to be unable to function as independent organisms. The chloroplasts of the ancestors of land plants arose in this manner from cyanobacteria, as indicated by the presence of a remnant but reduced bacterial genome within the chloroplast itself, and the presence of a double membrane around each chloroplast (corresponding the cyanobacterium's original cell membrane, plus the vacuoule membrane in which it had been enclosed by the host eukaryote). In the early 1970s, however, it was found that cryptophyte chloroplasts have not two but four surrounding membranes. What is more, wedged between two of those membranes was a tiny remnant cell nucleus, dubbed the nucleomorph. The nucleomorph was a crucial piece of evidence in demonstrating that cryptophyte chloroplasts had arisen by a process of secondary endosymbiosis. A eukaryote cell containing a chloroplast that had arisen in the manner described above was itself engulfed and converted to a chloroplast by another eukaryote. The four membranes around the cryptophyte membrane were therefore, from the inside out, the original cyanobacterium cell membrane, the vacuole membrane containing the cyanobacterium, the cell membrane of the primary host cell (with the nucleomorph between this and the last), and the vacuole membrane in which that had been contained in turn. Other groups of eukaryotes also have chloroplasts that arose in this way, such as brown algae and dinoflagellates, but in these the nucleus of the captured eukaryote cell has entirely disappeared.

Another cryptophyte structural diagram of the species Guillardia theta, showing the arrangement of the chloroplast, from here. This also shows the sites of the four genomes contained in the typical cryptophyte cell.


Exactly when the cryptophyte chloroplast arose remains a contentious subject. Various lines of evidence point to the captured chloroplast donor being a red alga, as is also the case with the aforementioned brown algae and dinoflagellates. As such, some have argued for the chloroplasts of all such algae being descended from a single capture event. However, there are also a number of protists related to such taxa that lack chloroplasts. In the case of cryptophytes, there is strong evidence that the sister clade to the the photosynthetic cryptophytes is the chloroplast-less genus Goniomonas. The subsequent sister to these two clades together is less certain but a number of recent studies have pulled forward another chloroplast-less group, the katablepharids. If the cryptophyte chloroplast shares an origin with that of brown algae, then it must have somehow been lost in the ancestors of both Goniomonas and katablepharids. So far, an author's preference for a single or multiple origins of red alga-derived chloroplasts tends to come down to whether they think it is easier for chloroplasts to be lost or gained, a question whose answer is still unclear.

The diversity within cryptophytes is still not that well understood, largely due to difficulties in observing significant characters. Prior to the advent of scanning electron microscopy, some authors had gone so far as to dismiss cryptophytes as essentially unclassifiable. Nevertheless, not everything was as bleak as the pessimists would have it. Cryptophyte taxa may differ from each other in overall size and shape. They may also differ in cell colour, due to the presence of various accessory pigments in addition to chlorophyll. The primary accessory pigments found in cryptophytes are known as phycocyanin and phycoerythrin; species containing the former are a blue-green colour whereas those containing the latter are reddish, golden or a greenish yellow. The use of scanning electron microscopy has led to the discovery of other useful features such as those relating to the periplast, a protein envelope that covers the inside and outside of the cryptophyte cell membrane. Electron microscopy has shown that the outer periplast layer is often ornamented, such as by being divided into scales. And even more recently, of course, researchers have recognised the value that molecular tools may have to offer cryptophyte taxonomy, though said tools have also complicated matters by, for instance, giving hints that previously recognised 'taxa' may represent different life cycle stages of a single organism. Whatever the eventual result, there is no question that we still have a lot to learn about cryptophytes.

REFERENCE

Novarino, G. 2003. A companion to the identification of cryptomonad flagellates (Cryptophyceae = Cryptomonadea). Hydrobiologia 502: 225–270.

Sweepers

It's time to meet the sweepers.

Smallscale bullseyes Pempheris compressa, copyright John Turnbull.


Sweepers, Pempheridae, are a group of moderately sized marine fish (usually about fifteen to twenty centimetres in length) found around tropical reefs in the Indo-Pacific and western Atlantic. I don't know why they're called sweepers, but in some areas they may be among the most abundant fish on the reef. Distinctive features of the group include a short, high dorsal fin and a long anal fin. The lateral line is also distinctively long, extending past the end of the tail right onto the caudal fin. Perhaps the feature that most stands out about sweepers is their large eyes. The eyes are so big because sweepers are nocturnal; during the day they retreat into protected crevices and caves, emerging at night to feed on minute crustaceans and other small animals (Mooi 2001).

Pygmy sweeper Parapriacanthus ransonneti, from here.


Sweepers are divided between two quite distinct genera. Members of the genus Parapriacanthus have a more 'average fish-like' elongate profile with the body less deep than the head is long. The other genus, Pempheris, has a distinctively deep profile, deeper than the head is long. The exact number of species of pempherid appears to still be uncertain. Pempherids lack the striking markings of other tropical fish and species can appear very similar to each other. What is more, they have two layers of scales on the body, with the outer scales being larger than the inner and deciduous (easily shed), and loss of the outer scales has the potential to change an individual's superficial appearance. Early descriptions of pempherid species are often inadequate for their reliable identification, and new species continue to be described at a quite rapid pace. A recent publication by Randall & Victor (2015), for instance, described no less than thirty-four new species of Pempheris from various locations in the Indian Ocean, close to doubling the number of species in the genus at a stroke. The genus Parapriacanthus is much less diverse, with only about five recognised species.

Orange-striped bullseyes Pempheris ornata in hiding during the day, copyright Peter Southwood.


Because of their relatively small size and retiring habits, sweepers are mostly not that significant economically. At least one species, Pempheris xanthoptera, is fished off the coast of Japan and mostly eaten as fish paste; it is supposed to be quite tasty. Some have appeared in aquaria.

When foraging at night, sweepers communicate with each other by producing popping noises through muscular flexing of the swim bladder wall. Noise production increases in the presence of potential threats, perhaps to warn other members of the school. At least some pempherid species also have bioluminescent glands associated with the posterior part of the gut. The bioluminescent compound is not directly produced by the fish itself but obtained by consuming bioluminescent ostracods. I haven't found whether the function of this bioluminescence is specifically known for pempherids, but similar ventral glows in other fish provide camouflage by breaking up the fish's silhouette when seen from below.

REFERENCES

Mooi, R. D. 2001. Pempheridae. Sweepers (bullseyes). FAO Species Identification Guide for Fishery Purposes. The Living Resources of the Western Central Pacific vol. 5. Bony fishes part 3 (Menidae to Pomacentridae) pp. 3201–3204. Food and Agriculture Organization of the United Nations: Rome.

Randall, J. E., & B. C. Victor. 2015. Descriptions of thirty-four new species of the fish genus Pempheris (Perciformes: Pempheridae), with a key to the species of the western Indian Ocean. Journal of the Ocean Science Foundation 18: 77 pp.

Long-legged Harvestmen of Southern Africa

New paper time!

Rhampsinitus conjunctidens, a new species of harvestmen from north-east South Africa, from Taylor (2017).


Taylor, C. K. 2017. Notes on Phalangiidae (Arachnida: Opiliones) of southern Africa with description of new species and comments on within-species variation. Zootaxa 4272 (2): 236–250.

When I first started research for my PhD thesis, *cough* years ago, I asked a number of museums if they could loan me their collections of monoscutid (now neopilionid) harvestmen. The species that I was interested in are found in Australia and New Zealand but when I opened a package of specimens sent to me from the California Academy of Sciences, I found a number of specimens from Africa in the mix. I immediately recognised what they were: not neopilionids, but representatives of another harvestment family, the Phalangiidae.

It seemed an easy enough error to make. Many species of southern African Phalangiidae resemble a lot of neopilionids in that the males have over-sized, elongate chelicerae. I referred to some of these species in the genus Rhampsinitus in an earlier post. To those not familiar with harvestmen diversity (which, let's face it, is the majority of people out there), the two groups can look very similar. True, the phalangiids are all distinctly much spikier than the neopilionids, but that doesn't seem that major a difference. To really see where they diverge from each other, you need to reach underneath the males' genital opercula and pull out their todgers.

Anywho, the specimens sat in storage for much longer than they should have, until I finally got around to looking them over in the latter part of last year. I then decided that it was worth writing them up into a short paper. Not only was there at least one new species among the specimens, they told me some very interesting things about variation within the species. Not only do Rhampsinitus species resemble Australasian neopilionids in their enlarged chelicerae, they resemble them in that individuals of a species vary in how enlarged the chelicerae are.

Major (left) and minor males of Rhampsinitus nubicolus, from Taylor (2013).


Now, I was not the first person to observe this point. Axel Schönhofer (2008) had already provided some detailed examples of variation in males of Rhampsinitus cf. leighi. I did, however, observe that the variation was even greater than Axel seemed to have recognised. Some of the least developed males of the species I was looking at had chelicerae that were pretty much no more developed than those of females. In some ways, the variation was even more remarkable than what I was familiar with in neopilionids. In most of the latter, major and minor males tend to be pretty similar to each other in features other than cheliceral development. In Rhampsinitus, we can see variation in almost all the features related to sexual dimorphism. In the species pictured immediately above, R. nubicolus, major males have massively long pedipalps as well as the long chelicerae; minor males have short, stubby pedipalps like those of a female. We can tell that they are the same species because they are found in the same location and have matching genitalia, but on the outside you would be hard pressed to pick them as such. Just to confuse matters even more, major males of two species may look very different to each other whereas minor males are externally almost identical. Without looking at the genitalia, it is all but impossible to identify which species a minor male belongs to.

As with the neopilionids, we can't yet say for sure what this variation means for the species' behaviour. In many other animal species with comparably varying males, large males will fight to protect and contain females while small males adopt a sneaking behaviour and try to spot females that are not being watched by large males. It seems quite possible that a similar thing is going on with Rhampsinitus. If you're a keen natural historian or behavioralist, there's something here that is crying to be looked into.

REFERENCE

Schönhofer, A. L. 2008. On harvestmen from the Soutpansberg, South Africa, with description of a new species of Monomontia (Arachnida: Opiliones). African Invertebrates 49 (2): 109–126.