In earlier posts on this site, I've presented snippets of the diversity of fossil crinoids, so perhaps it was about time I finally worked up to the modern taxa. Today's Taxon of the Week is a family of feather stars, the Charitometridae.
Despite including the vast majority of modern species (and the best-studied of modern species), the feather stars (the order Comatulida) are in fact somewhat odd creatures within the main scope of crinoid historical diversity. The main point of oddness, of course, is their massively reduced stem (other strange features, which they share with other living crinoids, include the reduction of plating on the adoral side of the animal). When they first settle down from their free-swimming larval stage, feather stars are attached to the substrate by a stalk as in more typical crinoids, but before they reach maturity they once again break free. Technically, however, adult feather stars are not completely stemless - the proximalmost part of the stem is retained, and this becomes expanded and fused with the infrabasals (the lowermost ring of plates in the main body of the crinoid) to form the large basal plate known as the centrodorsal (Breimer, 1978a - it may seem odd to have something called the "centrodorsal" on the underside of the animal, but the thing is that, compared to other living echinoderms, crinoids are upside-down). The centrodorsal is the point of attachment for the cirri, tendril-like outgrowths of the underside. The cirri are used by the feather star for moving about, like something out of a Japanese cartoon.
Not that they necessarily do much moving about. Though feather stars are capable of a surprising amount of motility when the mood takes them (some even using their arms to become active swimmers), the mood does not often take them. Like their permanently attached ancestors, feather stars are still filter feeders, a lifestyle that is best achieved in a sedentary manner. Crinoids will only move if the local conditions become unfavourable, and then only as far as they must to find a more suitable location. Once there, they will fix themselves onto any available piece of substrate - Austin Clark provided a brief but disturbing description of the consequences of comatulids being denied a suitable attachment site (quoted in Breimer, 1978b):
If a dozen specimens of Antedon were thrown at night into a large basin of water and were left without any means of attachment they were all found dead in the morning, conglomerated at the bottom of the basin, clinging to each other with their cirri and having their arms intertwined in such a manner as to suggest the idea that they had died of the asphyxia produced by overcrowding after exhausting themselves in efforts to find suitable attachment...
The majority of studies on modern comatulids seem to relate to two families, the Antedonidae and Comasteridae - particularly the former. The Charitometridae, in contrast, have been much more neglected. As far as I can tell, they seem to have been pretty much untouched since being monographed in 1950 by Austin Clark, who recognised 32 species divided between eight genera, distributed pretty much world-wide but with the main centre of diversity in the Pacific (only a single genus, Crinometra, seems to have made it into the Atlantic*). Clark distinguished the Charitometridae from related families by the presence of distinct covering plates at the bases of the pinnules (the side-branches of the arms), by the lack of differentiation between pinnules at the bases and more distally on the arms, and by the relatively undifferentiated cirri. In general, Clark regarded the Charitometridae as a more generalised form than the closely related Thalassometridae, though of course in those pre-cladistic days its a little difficult to know exactly what he meant by this - whether or not he was actually saying that the charitometrids were ancestral to the thalassometrids, or whether he was just making a comparison.
*Clark recognised only a single species in this genus, Crinometra brevipinna, but a large number of varieties within that genus. Whether some of those varieties might be recognised as species were the genus to be revised, I couldn't say.
This lack of specialisation is perhaps part of the reason for the lack of study of charitometrids - Clark (1950) writes at length about the difficulties of distinguishing taxa within the family, and one gets the distinct impression that he was not particularly satisfied even with the system he himself ended up using. Even more of a factor, probably, is that charitometrids seem to be mostly inhabitants of deeper waters - Clark gives a depth range of 55 - 2194 metres. The ecology of the group, not surprisingly, seems to be completely untouched - we know that they're down there, but we don't really know what they're doing with their time.
Breimer, A. 1978a. General morphology: recent crinoids. In Treatise on Invertebrate Paleontology pt. T. Echinodermata 2. Crinoidea (R. C. Moore & C. Teichert, eds.) vol. 1 pp. T9-T58. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).
Breimer, A. 1978b. Ecology of recent crinoids. In Treatise on Invertebrate Paleontology pt. T. Echinodermata 2. Crinoidea (R. C. Moore & C. Teichert, eds.) vol. 1 pp. T316-T330. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).
Clark, A. H. 1950. A monograph of the existing crinoids. Volume 1. The comatulids. Part 4c.-Superfamily Tropiometrida (the families Thalassometridae and Charitometridae). Bulletin of the United States National Museum 82 (4c): 1-383.