Field of Science

Scheloribates

Over the years, I've put up several posts about the diversity of oribatid mites. It's time for another one.

Scheloribates laevigatus, copyright R. Penttinen.


One of the largest genera of oribatids out there is the genus Scheloribates, for which well over 200 species have been described. Their distribution is pretty much worldwide; they are found in a range of microhabitats, such as in leaf litter, in pastures or marshes, or among rocks. Distinguishing features of the genus from other oribatids include well-developed, immobile pteromorphs, tridactylous (three-clawed) legs, and a notogaster with ten pairs of setae and three pairs of sacculi (little sac-shaped glandular openings) (Ermilov & Anichkin 2014).

Considering their abundance in soil habitats, Scheloribates probably have a significant role to play in decomposition and nutrient cycling. Studies on the diet of one of the better-known species, S. laevigatus, have found that it will eat almost any type of vegetable or fungal matter, though its preferred diet is microscopic algae (Hubert et al. 1999). Indeed, they are most abundant in damper habitats that would provide good conditions for the growth of such algae.

Scheloribates species may impact on human lives in other ways too. They are an intermediate host for the larvae of anoplocephalid tapeworms that infect livestock when the mites are accidentally ingested during grazing. S. laevigatus is a known host for at least eight tapeworm species in North America. Rates of tapeworm infestation in Scheloribates can be quite high; over 60% of the individuals of one species at a particular locality in Australia were infected (Lee & Pajak 1990). Scheloribates species are also noteworthy as a likely source of the toxic alkaloids found in the skin of arrow-poison frogs. The alkaloids are likely to be synthesised by the mites (as suggested by their presence in adults but not in juveniles, despite no known difference in diet between the two life stages) and then sequestered by the frogs after they eat the mites (Saporito et al. 2011). And if they eat enough mites, they end up becoming dangerous even to something the size of a human.

REFERENCES

Ermilov, S. G., & A. E. Anichkin. 2014. A new species of Scheloribates (Scheloribates) from Vietnam, with notes on taxonomic status of some taxa in Scheloribatidae (Acari, Oribatida). International Journal of Acarology 40 (1): 109–116.

Robert, J., V. Šostr & J. Smrž. 1999. Feeding of the oribatid mite Scheloribates laevigatus (Acari: Oribatida) in laboratory experiments. Pedobiologia 43: 328–339.

Lee, D. C., & G. A. Pajak. 1990. Scheloribates Berlese and Megascheloribates gen. nov. from southeastern Australia, with comments on Scheloribatidae (Acarida: Cryptostigmata: Oriopodoidea). Invertebrate Taxonomy 4: 205–246.

Saporito, R. A., R. A. Norton, N. R. Andriamaharavo, H. M. Garraffo & T. F. Spande. 2011. Alkaloids in the mite Scheloribates laevigatus: further alkaloids common to oribatid mites and poison frogs. Journal of Chemical Ecology 37: 213–218.

A Parasitic Eel?

The following post was inspired by an e-mail that I was sent recently by Sebastian Marquez. He told me about a friend of his catching a trevally when fishing, then cutting it open to find a snake eel inside the body cavity (but outside the stomach), wrapped around the trevally's internal organs. According to Sebastian, the lead suspicion for what had happened was that the eel had somehow burst out of the trevally's stomach before it was caught, and he wanted to know if I'd ever heard of anything similar. I didn't have an explanation for him, but his story did get me thinking about the snub-nosed eel.

Snub-nosed eel Simenchelys parasitica, from Jordan (1907).


The snub-nose eel Simenchelys parasitica is a small deep-sea eel, about 20 to 35 centimetres long. It has attracted note by being found a number of times burrowed into the body cavity of larger fishes with perhaps the most renowned case being two juveniles that were found nested inside the heart of a mako shark. This lead to the description of S. parasitica as an endoparasite (hence the species name). However, acceptance of this tag has been far from universal. The snub-nosed eel has been caught free-living more regularly than it has been found in other fish and because of its deep-sea habitat it has never been observed in life. An alternative suggestion has been that Simenchelys is normally a scavenger; because many of its recorded 'hosts' have been collected through non-targeted methods such as trawls, it is not impossible that the snub-nosed eels may have burrowed into their body cavity after they were already deceased.

It was with this conundrum in mind that the cranial anatomy of the snub-nosed eel was described by Eagderi et al. (2016). The jaws of Simenchelys are relatively short and muscular (hence its 'snub nose'). It also has teeth arranged in such a way that they form an even cutting edge (in contrast to the more spaced and uneven teeth of other eels). Eadgeri et al. came to the conclusion that the snub-nosed eel probably feeds by biting out plugs of flesh, in a similar manner to a cookie-cutter shark. Simenchelys also resembles a cookie-cutter in having large, fleshy lips that are probably used to form a seal between jaws and food source. A large hyoid ('tongue') apparatus probably works to provide suction to maintain the seal. The snub-nosed eel may also rotate while biting, a behaviour known from both cookie-cutters and other eels.

So is Simenchelys a parasite? It is probably not a habitual endoparasite, lacking as it does any clear adaptations to the endoparasitic lifestyle. There are fish that could be described as ectoparasites, in that they habitually feed on live animals larger than themselves in a manner that does not normally lead to the host's death. The cookie-cutter is one such fish; another is the candiru Vandellia cirrhosa, a small freshwater catfish from the Amazon basin that feeds on blood from the gills of other fish. It is possible that the snub-nosed eel could have a similar lifestyle to one of these. However, recorded evidence of its habits is even more consistent with scavengers such as hagfish and the candiru-açu Cetopsis candiru (another South American catfish) that tear flesh from the submerged bodies of dead animals, and may often burrow their way into the corpse's body cavity as they do so.

Of course, the two modes of feeding are not mutually exclusive. The only difference between predator and parasite in this scenario is whether the attacked animal is alive or dead, and the thing about flesh-feeders is that they're not always picky. A habitual scavenger may easily choose the opportunity to take a nibble from a still-living host, especially is said host is in some way incapacited (as a result of being swept up by a trawl, for instance). The snub-nosed eel may not be a habitual parasite, but it may be an opportunistic one.

REFERENCE

Eagderi, S., J. Christiaens, M. Boone, P. Jacobs & D. Adriaens. 2016. Functional morphology of the feeding apparatus in Simenchelys parasitica (Simenchelyinae: Synaphobranchidae), an alleged parasitic eel. Copeia 104 (2): 421–439.

Of Shrimp Plants and Bear's Breeches

For today's semi-random post, I drew the plant subfamily Acanthoideae. As recognised by Scotland & Vollesen (2000), the Acanthoideae is the largest of the subfamilies of the Acanthaceae by a considerable margin, including about 95% of the family's 2500+ species. Though perhaps not hugely familiar to readers in more temperate climes, the Acanthoideae are one of the dominant groups of herbs and shrubs in tropical parts of the world.

Golden shrimp plant Pachystachys lutea, copyright Dryas.


The Acanthoideae have been recognised as a morphological group since the late 1800s and their integrity has been confirmed by more recent molecular studies. They are distinguished from related plants (within the Lamiales, the order that also includes such plants as the mints and snapdragons) by having capsular fruits that dehisce explosively when mature to scatter their seeds. The seeds are attached within the capsule by hook-shaped stalks called retinacula that presumably play a role in determining how the seeds are released. A classification of Acanthaceae published in 1965 by Bremekamp restricted the family to species with explosive fruits and retinacula, dividing them between two subfamilies, the Acanthoideae and Ruellioideae, based on the absence or presence, respectively, of cystoliths. These are outgrowths of the epidermal cell walls that are impregnated with calcium carbonate. They are visible in the stems and leaves, at least in dried specimens, as hard white streaks. As phylogenetic studies have supported division of Acanthoideae in the broad sense between a cystolith-possessing and a cystolith-lacking clade, the decision whether to recognise 'Ruellioideae' as a separate subfamily comes down to a ranking choice only. At lower levels, the classification of Acanthoideae is less straightforward. Over two hundred genera of Acanthoideae are recognised but just three of those—Justicia, Strobilanthes and Ruellia—account for about half the total number of species. Each of these mega-genera is morphologically diverse and likely to be para- or polyphyletic with regard to related taxa, raising the distinct likelihood of future revisions.

Spiny bear's breeches Acanthus spinosus, copyright Magnus Manske.

Economically, few of the Acanthoideae are of great significance except for a number of species being grown ornamentally. One such species is Acanthus mollis, which goes by the vernacular name of 'bear's breeches' (why, I have absolutely no idea). Acanthus was a popular decorative motif in classical Greece and forms the basis for the design of Corinthian columns. Its use as an ornamental has lead to it becoming regarded as an invasive weed in some regions, largely because this is one of those garden plants that Just Will Not Die, spreading easily from seeds and tubers. We've got some in a pot outside that is currently flourishing despite having been burnt down to a nub by the searing Perth summer sun, metaphorically shouting its defiance at an uncaring world.

REFERENCE

Scotland, R. W., & K. Vollesen. 2000. Classification of Acanthaceae. Kew Bulletin 55 (3): 513–589.

Magnificent Eurhins

Eurhinus festivus(?), copyright Andreas Kay.


Weevils are one of the most incredibly diverse of beetle groups, coming in an incredible array of shapes and structures, but they are not usually renowned for their bright colours. Nevertheless, in a group of this size, there is always scope for surprise: witness the image above. Eurhinus is a genus of absolutely stunning metallic-coloured weevils native to Central and South America; one species, E. magnificus, was first recorded in Florida in 2002 and has since become established there. The photo above was identified on Flickr as E. magnificus but looking over the descriptions in Casey (1922) I suspect it is more likely to be the closely related E. festivus. Eurhinus magnificus differs in having patches of red on the pronotum and elytral humeri (the 'shoulders'); see photos here, for instance.

Eurhinus species feed on vines of the Vitaceae, the grape family. Eggs are inserted into young stems where the larvae cause distinct galls as they develop. It does not look like they are known to cause significant damage to economically important species though studies on whether it can successfully attack grapes are inconclusive.

REFERENCE

Casey, T. L. 1922. Studies in the rhynchophorous subfamily Barinae of the Brazilian fauna. Memoirs on the Coleoptera 10: 1-520.

Finches in Drag

Green-headed tanager Tangara seledon, copyright Dario Sanches.


In many parts of tropical South America, it is common to see small flocks of brightly coloured small birds foraging among vegetation, plucking off berries or hunting for insects. In many cases, these flocks may contain individuals of multiple or even several species. These are the tanagers, one of the Neotropical region's most characteristic bird families.

Tanagers are members of the bird clade known as the nine-primaried songbirds (so-called because their wings have nine functional primary feathers rather than the ten of other songbirds) that also includes the finches, buntings and cardinals. The largest genus of tanagers, and indeed one of the larger genera of birds in general, is Tangara. This genus includes about fifty species found in various parts of the neotropics. In their overall structure, they are fairly uniform: small, sturdy birds with a stout, moderate-length bill and an average-length tail (Hilty 2011). In other words, they have a fairly unremarkable, finchy-type appearance. In colour and patterning, however, they are considerably more varied, to the extent that I am at a loss to know where to begin. There are species of a rich, deep blue and of a bright, emerald green. There are species with bold, contrasting patterns of blues, blacks, greens or golds; there are species of a solid, uniform brilliance. There are species with caps or chests of orange or black. There are even a few, such as the plain-coloured tanager Tangara inornata, that eschew the gaudy pigments of their congeners entirely in favour of more restrained patterns of greys and beiges. In many species, males and females show little or no difference in appearance; however, in the black-capped group (including species such as the black-capped tanager T. heinei), the males have contrasting patterns of black and blue or yellow whereas the females are largely green and grey.

Golden tanager Tangara arthus, copyright Alejandro Bayer Tamayo.


As noted above, tanagers feed on a mixed diet of fruit and insects. The fruit part is dominated by small berries that they can either swallow whole or mash with their bills before swallowing them piecemeal. Studies on the mixed-species flocks formed by Tangara species have found that while different species show very little variation in how they obtain the fruit component of their diet, they usually show very distinct specialisations in how they forage for insects. Some hunt for insects along branches, others prefer to look on leaves. Branch-hunting species may differ in the thickness and density of branches preferred, or in the mode of searching employed. For instance, the golden tanager T. arthus and flame-faced tanager T. parzudakii can both be found foraging on moss-covered branches, but the flame-faced tanager usually catches insects by probing directly into the moss whereas the golden tanager usually either focuses on the moss-free sections or catches insects sitting on the moss surface without probing. A few species catch insects aerially, making short sallies from a perch.

Blue-grey tanager Thraupis episcopus, indicated by phylogenetic analysis as a species of Tangara, copyright Mdf.


Somewhat unexpectedly for a genus of this size and diversity in a group as taxonomically challenging as the tanagers, molecular phylogenetic studies have largely corroborated Tangara's monophyly. They have also supported the monophyly of most of the species groups recognised within the genus of the basis of similarities in plumage patterns (Sedano & Burns 2010). The only exception has been the discovery that many of the species previously included in the genus Thraupis form a clade nested within Tangara, leading to the suggestion that these two genera should be synonymised (apart from in informal discussions online, I'm not aware of anyone suggesting the alternative that Tangara be split). The 'Thraupis' species are larger and plainer in coloration than most other Tangara species. A few taxonomists have also suggested that the colourful green tanagers of the genus Chlorochrysa should be included in Tangara, but this relationship has not been supported by molecular data. Chlorochrysa species are glossier than the often more matt-coloured Tangara, and they have an acrobatic mode of foraging involving postures such as regularly hanging upside-down that differ from any Tangara species.

REFERENCES

Hilty, S. L. 2011. Family Thraupidae (tanagers). In: del Hoyo, J., A. Elliott & D. Christie. Handbook of the Birds of the World vol. 16. Tanagers to New World Blackbirds pp. 46–329. Lynx Edicions: Barcelona.

Sedano, R. E., & K. J. Burns. 2010. Are the northern Andes a species pump for Neotropical birds? Phylogenetics and biogeography of a clade of Neotropical tanagers (Aves: Thraupini). Journal of Biogeography 37: 325–343.

The Wingless Penguin

A couple of weeks ago, I put up a page on the 'terrestrial penguin' Cladornis pachypus, described from the Oligocene of Patagonia by the Argentine palaeontologist Florentino Ameghino. As it happens, Cladornis wasn't the only unusual penguin recognised from the Patagonian fossil record by Ameghino nor was it even necessarily the most unusual. That title should probably go to another species, the wingless Palaeoapterodytes ictus.

Anterior (left) and posterior view of humerus of Palaeoapterodytes ictus, from Acosta Hospitaleche (2010). Scale bar = 10 mm.


Like Cladornis, Palaeoapterodytes was based on only a single bone, in this case a humerus (upper wing bone) from the Early Miocene. And also like Cladornis, Ameghino's description of this bone indicated a truly remarkable bird. The distal part of the humerus lacked any sign of the facets that would normally articulate with the succeeding wing bones and, as a result, Ameghino concluded that the wing skeleton had been reduced to the humerus only. The crest and pits on the humerus marking the attachment of the wing muscles were also reduced. Ameghino's Palaeoapterodytes presumably had wings reduced to the merest nubs, effectively functionless and probably of little mobility. Nevertheless, the humerus of Palaeoapterodytes remained relatively robust, its breadth little less than that of other penguins.

I am not aware of any other bird with a wing structure anything like this. In other birds without functional wings, the entire wing skeleton becomes reduced, not simply truncated. Perhaps the closest approximation I have found is the wing of Hesperornis, which also lacks known wing bones beyond the humerus. However, the Hesperornis humerus is slender and gracile, and even without direct indication of the presence of more distal bones, it still looks to retain some remnant of the ancestral articulation. Also, the whole concept of a wingless penguin is decidedly problematic. Hesperornis derived its main propulsion in swimming from its feet and so its wings became reduced because they served little function. Penguins, on the other hand, get most of their propulsion from their wings, swimming in a manner that has been compared to flying underwater. Despite being flightless, penguins retain a wing skeleton that is, if anything, even more well developed than that of their flying relatives. For Palaeoapterodytes to have lost functional wings, it would have somehow had to change its mode of propulsion.

Reconstruction of the Palaeoapterodytes humerus with missing sections restored, from Acosta Hospitaleche (2010).


As a result, even while authors were cautiously considering Ameghino's interpretation of Cladornis, they treated Palaeoapterodytes with more scepticism. This scepticism was eventually concerned when the humerus was re-examined by Acosta Hospitaleche (2010). The reason for the lack of structure at its distal end was very simple: the original distal end had been broken off. The apparent lack of development of the muscle attachment structures was the result of erosion, not any indication of the bone's original appearance. When alive, Palaeoapterodytes had probably been very similar to, if not identical with, one of the several other penguin species known from around the same time and place. Unfortunately, the state of preservation of the type humerus is so poor that its exact identity cannot be determined, and Palaeoapterodytes ictus has been cast into the taxonomic limbo of nomen dubium. Ameghino's Cladornis may remain an intriguing mystery, but his Palaeoapterodytes is just a red herring.

REFERENCES

Acosta Hospitaleche, C. 2010. Taxonomic status of Apterodytes ictus Ameghino, 1901 (Aves; Sphenisciformes) from the Early Miocene of Patagonia, Argentina. Neues Jahrbuch für Geologie und Paläontologie—Abhandlungen 255 (3): 371–375.

Brittle Stars, Brittle Taxa

Amphiura arcystata brittle stars extending their arms above the sediment, copyright James Watanabe.


The brittle stars are something of the poor cousin among echinoderm classes. Their tendency to relatively small size and cryptic habitats means that they do not attract the level of attention given to starfish, sea urchins or sea cucumbers. Despite this, they are perhaps the most diverse of the living echinoderm classes, with more recognised species around today than any other.

It should therefore come as no surprise that the internal classification of brittle stars remains decidedly up in the air. The basic framework of the surrent system was established over a hundred years ago by Matsumoto (1915) and changes to this arrangement since have been fairly cosmetic. However, a significant challenge to Matsumoto's system has been arisen following the input of molecular data to the mix: many of Matsumoto's higher groupings have not been supported by moleculat analyses. Perhaps the nail in the Matsumoto system's coffin has come from a recent publication by Thuy & Stöhr (2016) who found that a formal analysis of morphological data also failed to support the pre-existing classification. At this point in time, we know that a new classification of brittle stars is needed but we don't yet know what form it will take.

Excavated specimen of Amphiuridae, copyright Arthur Anker. The radial plates are visible as a pair of bars alongside the base of each arm; I don't think that the genital plates are visible externally.


Perhaps one of Matsumoto's groupings that will survive the transition is the Gnathophiurina. Notable features of this group include a ball-and-socket articulation between the radial shields (large plates that sit on the aboral side of the central body on either side of the insertion of each arm) and the genital plates (sitting below and alongside the radial shields), with the socket in the radial shield and the ball on the genital plate. The genital plates are also firmly fixed to the basal vertebra of each arm. I haven't been able to find what the functional significance of this arrangement is, such as whether it renders the body more flexible that in other groups where the radial-genital plate articulation is more fixed. At least one of the families of Gnathophiurina, the Amphiuridae, includes species that commonly live in burrows with the tips of their arms extended into the water column, using their tube feet to capture food particles (Stöhr et al. 2012). In contrast, some Ophiotrichidae are epizoic, living entwined around black corals and the like. The Gnathophiurina as a whole seem to be most diverse in relatively shallow waters.

Matsumoto's (1915) original concept of the Gnathophiurida included species that are now classified into four families, the Amphiuridae, Ophiotrichidae, Amphilepididae and Ophiactidae, and recent analyses have returned results not inconsistent with this association. In Thuy & Stöhr's (2016) morphological analysis, Gnathophiurina species all belong to, and make up the bulk of, their clade IIIc. In the molecular analysis presented by Hunter et al. (2016), the families belong to two separate clades but the branch separating them is very weakly supported. Further research is needed, of course, but it may turn out that Matsumoto was on to something when he focused on that ball-and-socket joint.

REFERENCES

Hunter, R. L., L. M. Brown, C. A. Hill, Z. A. Kroeger & S. E. Rose. 2016. Additional insights into phylogenetic relationships of the Class Ophiuroidea (Echinodermata) from rRNA gene sequences. Journal of Zoological Systematics and Evolutionary Research 54 (4): 269–275.

Matsumoto, H. 1915. A new classification of the Ophiuroidea: with descriptions of new genera and species. Proceedings of the Academy of Natural Sciences of Philadelphia 67 (1): 43–92.

Stöhr, S. T. D. O'Hara & B. Thuy. 2012. Global diversity of brittle stars (Echinodermata: Ophiuroidea). PLoS One 7 (3): e31940.

Thuy, B., & S. Stöhr. 2016. A new morphological phylogeny of the Ophiuroidea (Echinodermata) accords with molecular evidence and renders microfossils accessible for cladistics. PLoS One 11 (5): e0156140.