Zygothuria R. Perrier, 1898

Genus Zygothuria R. Perrier, 1898 View in CoL

Diagnosis. (following Gebruk et al. 2012). “Body ovoid, flattened, but with well-differentiated sole; integument often wrinkled. Tube feet arranged exclusively in single or double row along ventrolateral ambulacra, placed on sole margin, which often forms fringe. Tube feet widely spread, may be quite big; dorsal papillae minute, not numerous, irregularly distributed or arranged in two simple rows, sometimes totally absent. Twenty tentacles, rarely 13–19; no tentacle ampullae; madreporite placed close to body wall, without penetrating it. Mouth ventral or terminal, anus terminal, without special arrangement. Calcareous ring with triangular radial segments. Ossicles of integument three-pillar (rarely four-pillar) tables; close to middle of their length pillars are linked by transverse beams; pillars merge on top forming spire.”

Remarks. Deep-sea genus with six currently accepted species worldwide, two of which have been reported previously for Australia: Z. lactea from the south and east and Z. marginata from the northwest (WoRMS and ALA January 2024). Three lots of Zygothuria are recorded from the IOT voyages at depths of 1426–4766 m, and further identified to species OTU level as Z. lactea (1 lot), Z. thompsoni (1 lot, new species record for Australia), and Zygothuria sp. MoV. 7328 (1 lot). Distinguished from Mesothuria by flattened body shape with well-distinguished sole and fringe of ventrolateral tube feet, along with and typically tri-radiate compared to quadri-radiate table ossicles. With limited reviews of genera within Mesothuriidae available, we relied heavily on SolÍs-MarÍn (2003) and Gebruk et al. (2012) to identify specimens; both resources highlight the need for further study of the group. Based on both COI and 16S sequence data, Zygothuria is monophyletic (fig. S2). In the COI phylogeny, Z. thompsoni is sister to Z. oxysclera , but no 16S data is available. Without additional samples matching the morphological and/or genetic description of specimen NMV F296850, we are reluctant to identify this specimen past the current OTU level.