Our organisms

Holothuroids, or sea cucumbers, are elongate, soft-bodied “echinoderm worms” ranging from a few millimeters to 3 m and 5 kg in size. The ~1600 described extant species occur from the intertidal to the deepest oceanic trenches. Their ubiquity in the largest ecosystem, the abyssal plain, ostensibly renders them, albeit far from sight, one of the dominant large animals on Earth. Unique among echinoderms, adult holothuroids can be planktonic and even ectocommensals. Holothuroid diversity is highest on tropical coral reefs, where ~400 species are recognized, can reach great abundance and biomass, and are important members of coral reef ecosystems. Much of this tropical diversity is in the families Holothuriidae (~180 species) and Stichopodidae (~33 species) of the Aspidochirotida, an order comprised of >350 species in three families. We will concentrate our efforts on three main groups:

Stichopodidae Haeckel, 1896 As currently constituted, the Stichopodidae is comprised of 9 genera and ~33 recognized species. Until the end of the 19th century, aspidochirotids were recognized as a single family with a handful of genera now referable to the holothuriids and Stichopus. Synallactids entered following exploration of deep waters by the Challenger and other oceanographic expeditions. In early treatises Stichopus corresponded largely to the current Stichopodidae, with a few (or more, depending on author) holothuriids mixed in. Clark revised Stichopus, excluded most species inappropriately included, and erected three genera (Thelenota, Apostichopus & Parastichopus) for species not comfortably accommodated in Stichopus. Deichmann separated two additional genera (Isostichopus, Neostichopus), while Liao, Cutress & Miller, and Levin each added a genus. Rowe undertook the only serious revisionary look at the family since Clark and Deichmann, preparing a detailed and heavily annotated key, based largely on ossicle characteristics. Although unpublished, this document provides the best overview of the family. Uncertainties at the generic level are profound. As an example, consider Eostichopus, erected by Cutress & Miller for E. arnesoni, S. regalis and S. owstoni. Rowe relegated Eostichopus as a subgenus of Stichopus, restricted it to S. arnesoni, considered S. regalis to belong in Parastichopus, and placed S. owstoni in synonymy of S. nigripunctatus, a species assigned to Parastichopus by most authors, but considered a synallactid by Rowe. The pictured stichopodid is Thelenota rubralineata Massin & Lane 1991.

Actinopyga Bronn, 1860 The well-defined genus Actinopyga, currently with 18 recognized, large (20-50 cm) species, was most recently diagnosed by Rowe. While lacking the tumultuous nomenclatural history of Bohadschia, the genus nevertheless contains numerous ill-defined species. As in Bohadschia, these problems result from over-reliance on a few body regions to assess variation in ossicle form and ignorance of sometimes considerable differences in living appearance and habit. For example, ossicles typical of the genotype A. echinites are seen in at least two other morphs, distinct in body shape, papillae structure, color, behavior and habitat. Another widespread species that may consist of at least two species is the much studied and sought after A. mauritiana. Indian Ocean forms match the original description, while western Pacific populations are distinct, and match Muelleria guamensis, presently considered a junior subjective synonym. Even relatively subtle morphological differences may distinguish some species. Actinopyga miliaris is considered widely distributed in the Indo-west Pacific, but appears to consist of several, sometimes sympatric units that differ slightly in size, color, texture, “soapiness”, behavior, and habitat. Other species (e.g., A. carolinensis), cannot be diagnosed from their original descriptions and must be re-examined and redescribed. We are also aware of seven undescribed species known from photographs or specimens. Exploration of reefs in poorly sampled regions, will doubtless yield still more of these large and colorful animals. We estimate the genus to contain 30-35 species. The pictured Actinopyga is Actinopyga caerulea Samyn, VanDenSpiegel & Massin, 2006.

Bohadschia Jäger, 1833 and Actinopyga, together with the subgenera Holothuria (Microthele) and H. (Platyperona) appear to constitute a distinctive, if unrecognized branch of the Holothuriidae, characterised by mostly large, loaf-like bodies, prominent soles, and development of anal papillae. Bohadschia was diagnosed most recently by Rowe. With the exclusion of the distinct B. graeffei to Pearsonothuria, the genus consists of ~11 species of large-bodied (30-60 cm) holothuriids. While the genus is well defined, species are poorly delineated, and the number of species recognised has waxed and waned extensively over the years. The type species, B. marmorata Jäger, 1833, has especially been the subject of controversy. Semper added four similar species (Holothuria similis, H. koellikeri, H. tenuissima and H. vitiensis), while Ludwig added H. clemens. Théel suggested synonymizing these, as well as the distinctive B. argus, as varieties or juveniles of marmorata. Mitsukuri described H. bivittata, mentioned Théel’s proposed synonymy, but decided that marmorata, bivittata and argus could be separated on color alone. Koehler and Vaney felt that Semper’s species should be united under his tenuissima. Pearson, unaware of bivittata, combined koellikeri, similis, tenuissima, clemens and vitiensis under vitiensis. Panning agreed, but later placed vitiensis and bivittata under marmorata as subspecies. Cherbonnier separated marmorata and tenuissima and described B. cousteaui, and later added steinitzi, maculisparsa and mitsioenensis. Rowe and Doty recognized a single species, and suggested that ossicle differences were the result of ontogenetic variation. Field workers have generally followed Rowe and Doty’s synonymy, as did Rowe and Gates, although noting that additional study may overturn the single-species taxonomy. Other recent researchers have treated some of these morphs, sometimes provisionally, as separate species. These disputes are the result of substantial intra-specific variation, limited inter-specific differentiation, and the rather undiagnostic ossicle complements characterising the genus. The various nomina have often been distinguished by colour pattern, sometimes without recognised ossicle differences. Initial molecular scrutiny indicates that color pattern can be diagnostic. We will start the revision by testing the distinctiveness of all colour forms with molecular markers, then will morphologically characterise species so differentiated, and sort out available nomina through type specimens. In addition to these forms, numerous other distinctive colour variants remain undescribed. If these and established colour forms all turn out to be specifically distinct, as preliminary work suggests, the diversity of Bohadschia will easily reach ~25 species. The pictured Bohadschia is an undescribed species from the western Pacific.