Mesothuria Ludwig, 1894

Mackenzie, Melanie, Davey, Niki, Burghardt, Ingo & Haines, Margaret L., 2024, A report of sea cucumbers collected on the first dedicated deep-sea biological survey of Australia’s Indian Ocean Territories around Christmas and Cocos (Keeling) Islands (Echinodermata: Holothuroidea), Memoirs of Museum Victoria (Mem. Mus. Vic.) 83, pp. 207-316 : 243

Genus Mesothuria Ludwig, 1894

Diagnosis. (following Gebruk et al., 2012). Body nearly cylindrical, slightly tapered toward both ends, without marginal fringe. Ventral side somewhat flattened; dorsal side uniformly covered with small pedicles of small and equal size, or much smaller dorsally, where they can be hardly visible. Tube feet with sucking disks scattered over entire body. Usually 20 tentacles, very occasionally 18–22. No tentacle ampullae; stone canal attached to body wall without penetrating it. Mouth terminal, anus ventral or subventral. Calcareous ring with rectangular radial segments. Gonads only on left side of dorsal mesentery, in single tuft. Ossicles of body wall quadri-radiate or tri-radiate tables with central primary cross elevated from disk.

Remarks. Cosmopolitan genus with 26 species worldwide, 11 of which have been reported previously for Australia: Mesothuria abbreviata , M. bifurcata , M. carnosa , M. lactea , M. marginata , M. murrayi , M. norfolkensis , M. parva , M. regularia , M. sufflava and M. verrrilli (WoRMS and ALA January 2024). Two new species for Australia are recorded here: M. cathedralis , and M. gargantua . Twenty-four lots of Mesothuria were recorded from the IOT voyages at depths of 754–3345 m, with all further identified to OTU species level as follows: M. cathedralis (2 lots), M. gargantua (19 lots), and M. murrayi (3 lots). Distinguished from Zygothuria by semicylindrical body shape, full covering of papillae and typically quadri-radiate compared to tri-radiate table ossicles. In both the COI and 16S datasets (fig. S2), Mesothuria is paraphyletic with M. cathedralis , M. gargantua , and M. oktaknemus forming a well-supported monophyly and M. murrayi in a separate clade. For COI, M. murrayi and Holothuria hilla form a single, well-supported lineage but the 16S phylogeny supports M. murrayi as monophyletic.