Molpadiodemas Heding, 1935

Genus Molpadiodemas Heding, 1935 View in CoL

Diagnosis (following O’Loughlin and Ahearn, 2005, amended for Miller et al., 2017 erection of Molpadiodemidae ). Cylindrical body with rounded terminal ends; pygal-furrowed; body surface covered in small discrete tube feet; large prominent tube feet and papillae absent from the paired radii; longitudinal muscles undivided and sit flat against the inner body wall, not cylindrical, broadly attached to the inner body wall; gonad tubules branch out from a common gonoduct base, not in series along the gonoduct; ossicles not found in body wall or tube feet; branched rod ossicles can be present in tentacles, frequently with ends intertwining and side branches fused to create mesh.

Remarks. Cosmopolitan genus with a large depth range of 103– 7086 m ( O’Loughlin and Ahearn, 2005). Sixteen species currently accepted worldwide, with only two previously known from Australia: Molpadiodemas crinitus and M. involutus . Others are known from Australian Antarctic waters (AFD, ALA and WoRMS, 2024). The group has traditionally been difficult to split from other pygal-furrowed genera such as Pseudostichopus due to similar external morphology and the lack of truly diagnostic ossicles, with no ossicles in the body wall or tube feet, and those existing in tentacles and gonads being highly variable. The clearest distinguishing feature is flat, broadly attached longitudinal muscles for Molpadiodemas , compared to rounded muscles for Pseudostichopus . O’Loughlin and Ahearn’s (2005) review remains the most comprehensive resource for morphological features, but we found that our species did not fall cleanly into their key as described. Here we report 30 specimen lots of Molpadiodemas from the IOT, with seven lots further diagnosed with genetic support to OTU level as follows: Molpadiodemas sp. MoV. 7329 (3 lots), Molpadiodemas sp. MoV. 7334 (3 lots), Molpadiodemas sp. MoV. 7335 (1 lot). Molecular data confirms the monophyly of Molpadiodemas , but at the time of this study molecular data is only publicly available for four of the 16 currently accepted species. Within Molpadiodemas , our new sequence data forms three distinct lineages. The first lineage, represented here as Molpadiodemas sp. MoV. 7334, contains previously published sequences from M. crinitus (only available for COI), M. morbillus (only available for COI), M. involutus (COI and 16S), and M. villosus (only available for 16S). The second lineage, represented here as Molpadiodemas sp. MoV. 7335 has only a single individual (NMV F296882) from genetically sampled material, and while differing in some internal characters, is superficially like specimens from the third lineage, represented here as Molpadiodemas sp. MoV. 7329 and comprising only IOT samples. While our data increases the available genetic information significantly, more sequencing is needed to assess whether these new lineages correspond to previously described species.