Synallactes 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 : 287-289

Genus Synallactes Ludwig, 1894

Diagnosis. (following SolÍs-MarÍn, 2003 for Synallactes sensu stricto). “Body cylindrical or sub-cylindrical. Tentacles 18–20. Stone canal attached to the body wall. Ventral surface flattened, without any marginal border. Ventral tube feet and dorsal papillae in longitudinal series and confined to ambulacra. On ventral surface, three zones of tube feet. Gonad in two tufts. Body wall ossicles comprise three or quadri-radiate tables, the distal ends of the arms with a larger or smaller number of perforations, and often lateral processes which may unite with similar processes of other arms to produce a complex latticelike network. Spire of a single pillar, which may be terminally divided or perforated, or both. Tube feet with supporting rods.”

Remarks. Of the 26 currently accepted species worldwide (WoRMS, 2024), none are currently recorded for Australia in AFD or ALA (2024), though species are present in Australian (see below) and Australian Antarctic waters (NMV catalogue, 2024). Three lots of Synallactes were recorded from IOT voyages at depths of 2156–2418 m, all identified as Synallactes cf. crucifera . Within the Synallactidae , Synallactes and Bathyplotes can have very similar external morphology, particularly when damaged or “stripped” during trawls. SolÍs-MarÍn (2005) noted that this has led to past confusion between the species (e.g. Koehler and Vaney, 1905; Heding, 1940) and the introduction of variations in ossicle description. Here we followed SolÍs-MarÍn’s diagnosis above for identification, which specifies single-pillared central spires for Synallactes , compared to the multi-pillared spires with crossbeams as currently known for Bathyplotes . Where the animal or skin is complete, midventral tube feet are also present for Synallactes but not Bathyplotes . Specimens sequenced from this taxon have identical COI and 16S haplotypes (fig. S5). For both genes, Synallactes cf crucifera is monophyletic within the genus Bathyplotes and sister to Bathyplotes samples collected in the southern Atlantic and Pacific oceans and Antarctica, indicating the need for a future revision of the group.

Synallactes cf crucifera Perrier R., 1898 sensu Rogacheva et al., 2013

Material examined. NMV F296862 * (2) [IN 2021 V04 031] ; NMV F308319 * (1) and NMV F312858 (1) [IN 2022 V08 187] .

Other material. Synallactes cf. crucifer a Perrier, 1898, sensu Rogacheva et al., 2013. – off eastern Australia, NMV F240992 [IN 2017 V 03 070 123]; NMV F241034 [IN 2017 V 03 090 106], identified by Mark O’Loughlin (2018).

Diagnosis of IOT material. Specimens elongated and sub-cylindrical, ventrally flattened to slightly curved, tapered at oral and anal ends. Body typically salmon-yellow to pink or orange (pre preservation). Where present, thick outer layer of bumpy skin with scattered white wart-like processes in some specimens, but can look flaky, fluffy, or vermiform as the thicker body wall tends to strip off. Where visible, single to double rows of retractable papillae along each dorsal radius, and additional row of shorter, thicker dorsolateral papillae. Papillae conical with whip-like tips. Ventrolateral tube feet and smaller midventral tube feet present, but skin often split along midventral and/or completely stripped. Mouth ventral or directed ventrally, with 18–20 tentacles. Anus subdorsal. Example of preserved size: 80 mm long, 25 mm wide, and 8 mm high ( NMV F296862 , larger specimen). Body wall with cross-shaped ossicles, typically four-armed (rarely 5 or 6) distally spatulated, and with a single solid, spinous, central spire. Spire of two typical types, short and thick cross type with spire height similar to arm length (56–110 μm in these specimens, fig. 50 d–h), and tall and thin cross type with spire approximately three times the arm length (up to 184 μm in these specimens, fig. 50i). Both types tapered at top, with vertically directed spines running up the spire. Each arm distally flattened and perforated, typically with 1–3 larger perforations and three or more smaller, but quite variable and often broken from taller ones. In these samples, dorsal and ventral body walls have predominantly the shorter cross type, papillae have both tall and short types (though longer papillae often dense with the taller version), tube feet have predominantly the shorter cross type, and one papillae sample also has curved rods and rare end plates. No “tall” cross types were found in NMV F308319 , but the short type agrees with other specimens. Tentacle ossicles are irregular curved rods, often bifid at ends, sometimes branching. Some smooth rods also observed in body wall .

Remarks. Ossicles with some similarities to those figured for Synallactes profundus (pl. 10: fig 19–20, Koehler and Vaney, 1905) but differs in body form. Closest to two eastern Australian Synallactes cf crucifera identified by Mark O’Loughlin in 2018 ( NMV catalogue; sensu Rogacheva et al., 2013 as per Mark O’Loughlin pers comm.) with mix of tall/thin and short/thick spires and variable numbers of perforations at spatulated ends of arms. Differs from Synallactes crucifera Perrier, 1898 and subsequent diagnosis by Deichman (1940) in having typically more perforations at the ends of arms and no perforations at the tip of the central spire. SolÍs-MarÍn (2003) noted that much Synallactes morphology is similar and more genetic work may be required to refine the species. See genus remarks for current genetic placement of these morphologically congruent Synallactes within Bathyplotes .

Distribution. These specimen lots: Indian Ocean, Australian IOT, Cocos (Keeling) Islands Territory, Santa Ridge Stn., Clara Marie Seamount Stn., 2156–2418 m.

Two comparative specimens noted above: off eastern Australia, New South Wales, Hunter Marine Park and off Byron Bay , 2474–2595 m .

Full bathymetric range. 2156–2595 m.

References. AFD (2024), ALA (2024), Deichman (1940), Koehler and Vaney (1905), O’Hara et al. (2020), O’Loughlin (notes and personal correspondence, 2018), Perrier R. (1898, 1902), SolÍs-MarÍn (2003), SolÍs-MarÍn and Laguarda-Figueras (2004), SolÍs-MarÍn (2005).