Mesothuriidae Smirnov, 2012

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

Family Mesothuriidae Smirnov, 2012

Diagnosis. (following Smirnov, 2012 and amended for Miller et al., 2017 erection of Holothuriida ). Body elongated, rounded, or flattened. Tube feet present along entire ventral side (genus Mesothuria ) or only along ventral ambulacra (genus Zygothuria ). Papillae virtually evenly spread on dorsal surface. Twenty (13– 22) tentacles with no free-hanging tentacle ampullae present. Stone canal attached to the body wall without penetrating it. Rete mirabile not developed. Radial muscle bands undivided. Gonads in a single tuft of tubules to the left of the mediodorsal mesentery. Calcareous ring well developed. Table ossicles with large, laced disc perforated with large holes, and spire composed of three or four pillars surrounding the central hole.

Remarks. This family exhibits primarily table-shaped ossicles and gonads in a single tuft. The only two accepted genera worldwide, Mesothuria and Zygothuria , are distinguished in external morphology by tube foot arrangement, body form, structure of calcareous ring and ossicles ( Gebruk et al., 2012). Both genera have been reported widely from Australia at depths of 373–4250 m (AFD and ALA, 2024), and are again represented here in IOT material. Miller et al. (2017) maintained the Smirnov (2012) erection of Mesothuriidae , moving it from its former position in Aspidochirotida to their new order Holothuriida . See Smirnov (2012) for further comments on distinguishing traits between families.