Insights on the reproductive strategies of two new Uristidae amphipods from hadal depths

The reproductive strategies of amphipods living deeper than 6000 m are poorly characterised, mainly due to the challenge of sampling brooding females. This study explores the oxygen-temperature hypothesis using micro computed tomography as an influential factor for the evolution of two distinct reproductive strategies by Uristidae Hurley, 1963 females from two geographically discrete hadal features. Uristidae amphipods are geographically widespread scavengers found globally from shallow waters to hadal depths. This study further utilises integrative taxonomy to describe two Uristidae species and raise a new genus. Galathella ovaimmensus sp. nov. is placed into the genus based on key morphological features including the tapering coxa 1 and the rounded sub-acute lateral cephalic lobe. Profundum solomoni gen. nov. sp. nov. did not morphologically correspond to any genera within Uristidae prompting the raising of the new genus Profundum gen. nov. The oxygen-temperature hypothesis describes a phenomenon whereby embryo size increases with decreasing water temperatures and increasing oxygen content. The embryos of Galathella ovaimmensus sp. nov. collected from ~7400 m in the cold, well oxygenated South Sandwich Trench (Southern Ocean) ranged in volume from 0.402 to 0.884 mm3 and had a mean fecundity of 4.3. The embryos of Profundum solomoni gen. nov. sp. nov. from ~7400 m in the Santa Cruz and San Cristobal trenches (Southwest Pacific Ocean) where oxygen content at hadal depths is lower, ranged in volume from 0.102 to 0.255 mm3 and had a mean fecundity of 23.0. The differences in embryo size and fecundity appear to align with the oxygen-temperature hypothesis, although other influencing factors, such as fluctuations in primary productivity are also discussed. ZooBank ID: urn:lsid:zoobank.org:pub:C242EAF9-8DC2-441E-8D8F-326050873365