Psychronaetes sp.

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 : 227-229

Psychronaetes sp. MoV. 7326

Material examined. NMV F308286 * (1) [IN 2022 V 08 151] .

Diagnosis of IOT material. Preserved specimen light violet to dark purple, medium-sized, elongated and dorsoventrally flattened ~ 130 mm long, 25 mm wide and 8 mm high. Very damaged specimen with a covering of mud and bryozoans (fig. 13a). Pronounced anterior constriction forming “neck” and ventral mouth with 15 tentacles on long stems terminating in dark purple elongate oval discs (fig. 13b). Anus appears dorsal and terminal. Body wall soft but suspect outer skin has been stripped. Evidence of dorsal papillae (row numbers not clear), and papillae modified to form a fused anterior brim. Bare midventral radii, but series of tube feet with dark purple discs at tips along each ventrolateral radii, appearing to fuse into a brim in some areas and torn off in others. Ossicles are wheels, all one type though varying in size, concave with smooth curved rim and partially or fully closed central nave with slightly raised central cross. Wheels with typically 12 (11–17) short outer spokes and four (3–4) inner struts as central cross (occasionally branching). Graduating in size but not always related to spoke number, though more common that large ones have 12 spokes and smaller ones ~15. Tube feet have some wheels slightly smaller again than in body wall. In this specimen, body wall wheels 64–240 μm (typically 120– 240 μm), tentacle wheels 104–208 μm, and tube foot wheels 56–152 μm. The tentacle sample also had irregular rods of variable size (152–676 μm), often curved and with slightly spinous ends.

Remarks. The calcareous membrane closing the central nave of the ossicle wheels can be difficult to detect under some microscopes but is clear in SEM images. Many morphological features are not available for full comparison, but this IOT specimen is placed in Psychronaetes rather than Laetmogone morphologically through the presence of an anterior neck, fused brim of dorsal papillae, and larger ossicle wheels. The typical leathery body and fusiform shape for Psychronaetes genus is not obvious in this specimen, presumably due to damage and twisting mid-body. The neck is more obvious in the preserved specimen. It is distinguished from the only accepted species in Psychronaetes , P. hanseni – by typical ossicle size and spoke number, which, along with tentacle number (15 vs. 18) also distinguish it from Psychronaetes sp. CCZ_101 (see Table 1). While the IOT material is likely to represent a new species, it has not been formally erected here due to the damaged nature of the single specimen found. Further material and more morphological features would be optimal before describing past Psychronaetes sp. MoV. 7326. There is insufficient genetic resolution to determine how this species is related to those within the order Elasipodida (fig. S1).

Distribution. This specimen lot only: Indian Ocean, Australian IOT, Cocos (Keeling) Islands Territory, Raitt Ridge North Stn., 3053–3144 m.

References. AFD (2024), ALA (2024), Bribiesca et al. (2022), Hansen (1975), Pawson (1983).