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.

Bits of Cucumber in the Fossil Record

Aggregation of Eocaudina septaforminalis sclerites from Boczarowski (2001). Scale bar = 200 µm.

Echinoderms are a dream group of animals for invertebrate palaeontologists (that's palaeontologists who study invertebrates, not palaeontologists who are invertebrates). Their calcified skeletons mean that their fossil record is extensive and detailed. When most of the body is covered in plates, looking at the fossil can give you an instant idea of what the animal looked like when alive. But as with all things in biology, there are notable exceptions. The sea cucumbers are one group of echinoderms that has significantly reduced the original skeleton, sacrificing a hard outer skeleton for increased flexibility. Instead of solid plates, the sea cucumber skeleton is made up of many minute sclerites embedded in the skin. And while this may be all well and good for the sea cucumber, it is not so convenient for the palaeontologists. In the fossil record, these minute sclerites become separated, and one separated sclerite does not tell you much about the appearance of the sea cucumber as a whole.

As a result, palaeontologists looking at sea cucumber remains have found themselves presented with a conundrum. The classification of modern sea cucumbers is largely based on features of the soft body that are usually not preserved in the fossil record, making comparison of living and fossil cucumbers difficult. Also, the skeleton of a single sea cucumber may include different forms of sclerite, performing different roles. If two different types of sclerite are found close together in the fossil record, did they come from a single sea cucumber or from two different sea cucumbers that died close together? To bypass these barriers, palaeontologists have often used what are referred to as 'parataxa'. A single type of sclerite is treated as a single 'parataxon', with the recognition that there may not be a perfect correlation between the parataxon and the theoretical taxon that it originally came from.

Individual calclamnine sclerite, Priscocaudina crucensis, from Boczarowski (2001).

The Calclamnidae has been recognised as one such 'parafamily' of sea cucumber sclerites. As defined by Frizzell & Exline (1966), the Calclamnidae grouped together rounded or polygonal sclerites that are perforated with holes like a sieve, and that don't have any sort of stalk or other ventral protrusion. This is a very common sort of sclerite for echinoderms: 'calclamnid' sclerites have been identified as far back as the Ordovician (Boczarowski 2001), and sclerites of this sort are still found in sea cucumbers today (just to confuse matters, the skeletons of some brittle stars also include very similar sclerites, raising the spectre of misidentification). Boczarowski (2001) recognised two subfamilies of Calclamnidae: in one, the Eocaudininae, the perforations of the sclerite are all more or less even in size, while in the other, Calclamninae, the pores towards the centre of the plate are larger and arranged in a cross-shape. The eocaudinines include the earliest calclamnid plates, with the calclamnines appearing during the Devonian.

Recognition of parataxa is a convenient tool for keeping records of things like biostratigraphy without getting bogged down, but what sort of sea cucumber did calclamnids actually come from? The calclamnids resemble sclerites found in the group of modern sea cucumbers called the Dendrochirotacea, so they have often been classified with this group. However, a number of features of the dendrochirotaceans, including perforated calclamnid-like sclerites, have been suggested to be primitive for sea cucumbers, so similarities between calclamnids and dendrochirotaceans may represent shared ancestral features rather than true affinities. Haude (1992) commented on a number of cases of sclerites found preserved in assemblages that he believed represented original life associations, including some containing calclamnids. One of these contained sclerites that Haude identified as similar to Calclamna germanica, the type species of the family, in association with large hook-shaped sclerites. Hooks are not characteristic of dendrochirotaceans, but of Apodacea, a different group of sea cucumbers characterised by the loss of tube feet (with the hooks working to provide mobility in their place). Haude suggested the possibility that Calclamna might represent a stem-group apodacean that retained some primitive sclerite features. In other fossil groups such as conodonts, the identification of preserved assemblages has allowed palaeontologists to progress beyond the use of parataxa and integrate more recognition of evolutionary relationships. Hopefully we get the same opportunity with sea cucumbers.

REFERENCES

Boczarowski, A. 2001. Isolated sclerites of Devonian non-pelmatozoan echinoderms. Palaeontologia Polonica 59: 1-219.

Frizzell, D. L., & H. Exline. 1966. Holothuroidea—fossil record. In: Moore, R. C. (ed.) Treatise on invertebrate Paleontology pt U. Echinodermata 3 vol. 2, pp. U646-U672. The Geological Society of America, Inc., and The University of Kansas Press.

Haude, R. 1992. Fossil holothurians: sclerite aggregates as 'good' species. In: Scalera-Liaci, L., & C. Canicatti (eds) Echinoderm Research 1991, pp. 29-33. Balkema: Rotterdam.

Hemiaster: An Echinoid with Heart

The Upper Cretaceous Hemiaster (Hemiaster) bufo, in (a) aboral, (b) oral, (c) lateral and (d) posterior view. From Fischer (1966).

For today's post subject, I've drawn the echinoid genus Hemiaster. Hemiaster is a member of the group of echinoids known as heart urchins, in reference to their overall shape when viewed from above. Species of Hemiaster are also fairly deep, so their overall shape when viewed from the side is somewhat reminiscent of a hoof. Heart urchins mostly live burrowed into sediment (mud, in the case of Hemiaster). One notable feature compared to other echinoids is that they have lost the Aristotle's lantern, the 'jaw' structure found in regular echinoids. Heart urchins are detritivores feeding on organic matter either buried in sediment or deposited on the surface of their substrate. The specific habits of living Hemiaster species seem to be poorly known, due to their living in deep-water habitats, but an Atlantic specimen of H. expergitus has been found living in a 12 cm deep burrow with a narrow funnel opening to the surface (Gage 1987).

In order to maintain their burrows, heart urchins have exceedingly long and well-developed tube feet, the openings for which in the test are visible as a petal-shaped pattern (and I must expose my ignorance, here: before I started looking up stuff for this post, I had always assumed that the petaloid pattern on heart urchins was on the underside. It is, in fact, on the aboral side). Also characteristic of heart urchins are fascioles, bands of closely-crowded tiny spines covered with cilia, that are believed to function in respiration by increasing water flow over themselves (a necessary process when the respiratorily available surface of the animal has been mostly buried by mud) (Fischer 1966). In Hemiaster, the only fasciole present runs around the space occupied by the petaloids; other heart urchins may have different patterns of fascioles on different parts of the body.

Cretaceous Hemiaster whitei, from here.

Fossils attributed to Hemiaster date back as far as the Cretaceous, and it appears to be better known as a fossil than a living animal. This is not entirely unusual for echinoderms: I have a vague recollection of Chris Mah, who works on living echinoderms, complaining about this very point, but I can't recall exactly where/when he did so (sorry, Chris!). Still, in some justification, Hemiaster was more diverse in the past than it is now: of the seven subgenera recognised in Hemiaster by Fischer (1966), only the nominotypical subgenus survives to the present, and none of the others postdates the Palaeocene.

REFERENCES

Fischer, A. G. 1966. Spatangoids. In: Moore, R. C. (ed.) Treatise on invertebrate Paleontology pt U. Echinodermata 3 vol. 2, pp. U543-U628. The Geological Society of America, Inc., and The University of Kansas Press.

Gage, J. D. 1987. Growth of the deep-sea irregular sea urchins Echinosigra phiale and Hemiaster expergitus in the Rockall Trough (N.E. Atlantic Ocean). Marine Biology 96: 19-30.

Mystery Animal for Today

Take a close look at the photo above. What kind of animal do you think this is? [The photo comes from here, but don't look there just yet, because that would be cheating.]

Some of the more observant among you may have noticed the five rays visible on the animal, and so you would have correctly decided that this is an echinoderm, seen from the underside. Echinoderms are the phylum of marine animals that includes crinoids (sea lilies and feather stars), asteroids (sea stars or starfish), ophiuroids (brittle stars), echinoids (sea urchins) and holothuroids (sea cucumbers). The five rays are the ambulacra - furrows lined with the tube feet that the echinoderm uses for walking on, or for passing food particles to the central mouth. Before I reveal exactly what kind of echinoderm this is, though, I'll show you another photo of the same specimen (from the same site) seen from the side:



By now, it should be pretty obvious which of the five living classes of echinoderms this is. So if you guessed "starfish"* - you're absolutely right. This specimen is, in fact, the type specimen of Podosphaeraster toyoshiomaruae Fujita & Rowe, 2002. Podosphaeraster is an extremely unusual asteroid known from the western Pacific and north-east Atlantic that has abandoned the typical star-shape of most members of its class, and adopted a near-spherical form much more similar to that of an echinoid. If you were to look closely at the specimen, you would be able to see a difference from a typical echinoid in that the ambulacral furrows only go halfway up the side of the sphere, rather than all the way up as in echinoids.

*Kevin Zelnio is going to kill me for calling it a starfish instead of a sea star. Tough.

The way in which Podosphaeraster has evolved its unusual form is relatively simple. The development of the plates that normally form the dorsal (aboral) surface of the flattened star has been greatly reduced relative to those that form the ventral (oral) surface. The reasons why this unusual morphology has evolved in Podosphaeraster, however, are unknown. Though five species have been described to date, specimens of Podosphaeraster are few and far between. All species are small (the largest specimens are little over a centimetre in diameter) and there is evidence that they live in habitats that are not conducive to easy collecting - among sponges or rocky ground in depths of 85 - 615 m. It may be adapted to living in cracks or crevices in these habitats.

For all its unusualness, Podosphaeraster is not unique. A fossil family of asteroids, the Sphaerasteridae, also developed a similar globose form by the reduction of the aboral surface. Also, Smith (1997) suggested that echinoids could have also evolved from a star-like ancestor in just this way. If true, what might seem an interesting but inconsequential oddity in the asteroid world could actually be very significant in understanding how another of the major modern animal groups came into being.

REFERENCES

Fujita, T., & F. W. E. Rowe. 2002. Podosphaerasteridae fam. nov. (Echinodermata: Asteroidea: Valvatida), with a new species, Podosphaeraster toyoshiomaruae, from southern Japan. Species Diversity 7: 317-332.

Smith, A. B. 1997. Echinoderm larvae and phylogeny. Annual Review of Ecology and Systematics 28: 219-241.