Molpadia cf blakei (Théel, 1886)

Molpadia cf blakei (Théel, 1886) View in CoL

Material examined. NMV F296851 About NMV * (3), [IN 2021 V04 024] , NMV F308219 About NMV * (1) [IN 2022 V08 120] .

Diagnosis of IOT material. Cylindrical, mud-filled specimens with pronounced posterior tail and constricted “neck”. Body length up to 77 mm long, 32 mm wide and 35 mm high, semi-retracted tail 6 mm long ( NMV F296851, largest specimen, preserved). Brown to light violet or grey. Skin thin and soft, with flat divided longitudinal muscles clearly visible. Tentacles all retracted in these specimens. Body wall ossicles tables with fairly uniform perforated base disc and three-pillared spires, typically over 130 μm high and twisted or fused from halfway. Most tables ~100 μm in diameter with six holes (3 large, 3 small) though tables with fewer or greater number of holes also present. Where not broken, spires are topped by a crown of five or more hooks (fig. 29c). Some shorter spires ~60 μm with less-developed hooks (fig. 29e) also present. Yellow and brown phosphatic deposits seen in tail sample of NMV F308219. Tail crowded with fusiform ossicles with perforated base disc branching into rods at end, and a three-pillared spire typically shorter than in body wall. Base plate ~270 μm with typically six (3–8) holes of irregular size. Crown with up to ten hooks but not as curved as in body wall ossicles (e.g. fig. 29j). Tail ossicles only broken discs and pillars in NMV F296851.

Remarks. TheIOTspecimensweredifferentiatedmorphologically using the key from Pawson (1977). Differentiated from Molpadia musculus by ossicles not being “almost exclusively” fusiform tables (including in the body wall), and from those fusiform tables (where present in the tail) being much smaller than the 500–1300 μm noted for M. musculus . Some ossicles, particularly from NMV F296851 are like those in M. liska ( Pawson, 1977) , but distinguished from that species by having divided muscles and more than four hooks on ossicle spires, and NMV F308219 also has additional distinguishing characters of phosphatic bodies, fusiform ossicles in tail, and typically more than three perforations in plates. A close match to the body shape, size, colour and skin thickness for the type specimen for M. blakei (68 mm with a 7 mm tail), though while ossicles are visible to the eye, skin in IOT specimens is soft rather than rough. Differs from M. blakei sensu stricto in that six perforations are more common than three perforations, and more than “a few” hooks are typical in all pillars including in the tail. Morphologically congruent with Molpadia aff blakei (Théel, 1886) as described by Rogacheva et al. (2013) from the mid-Atlantic ridge, where body wall ossicles with six holes were more prevalent than the discs with three holes as noted in the original description and descriptions of Deichmann (1930, 1940) and Pawson et al. (2001). For COI, the two IOT Molpadia cf. blakei samples have identical COI haplotypes and group with M. musculus (fig. S3). 16S sequence data was only available for NMV F296851 and it again groups with M. musculus . Without additional material for molecular and morphological comparison, we retain our specimens in Molpadia cf. blakei for now, noting the clear morphological differences from M. musculus as diagnosed by Pawson (1977).

Distribution. These specimen lots only: Indian Ocean, Australian IOT, Christmas Island and Cocos (Keeling) Islands Territories, Abyss S of Christmas Island Stn. and Rudist Seamount Stn., 3780–4766 m.

Full bathymetric range. 3780–4766 m.

References: AFD (2024), ALA (2024), Gebruk et al. (2014), Miller et al. (2017), O’Loughlin et al. (2013, 2015), Pawson (1965), Pawson (1977), Pawson et al. (2001), Rogacheva et al. (2013), Samyn and VandenSpiegel (2016), Smirnov (2012), Théel (1886).

Order Persiculida Miller, Kerr, Paulay, Reich, Wilson, Carvajal and Rouse, 2017 View in CoL

Diagnosis (see Miller et al., 2017)

Remarks. Persiculida was erected in 2017 when Miller et al. undertook a major revision of Holothuroidea after molecular evidence showed that three of the five previous orders were nonmonophyletic. Persiculida includes many genera formerly assigned to Synallactidae and Gephyrothuriidae ; morphological characters include lack of body wall ossicles and presence of a pygal furrow for which the clade is named (Latin persica = peach, + culus = posterior) ( Miller et al., 2017). All three currently accepted families – Gephyrothuriidae , Molpadiodemidae and Pseudostichopodidae – are found in Australia, along with four genera not currently sitting under a family, of which Benthothuria is the only one currently reported from Australia (ALA and WoRMS, 2024). Molpadiodemidae , Gephyrothuriidae and Pseudostichopodidae are reported from the IOT voyages.