Reproductive biology of two hydrothermal vent Cocculinidae species (Mollusca: Gastropoda) from the Arctic and Southern Ocean

The ecology of hydrothermal vents in polar oceans is less known than other regions, as a consequence of the logistical challenges of deep-sea fieldwork at high latitudes, and particularly under permanent ice cover in the Arctic. This paper describes the reproductive biology of the first two cocculinid gastropods sampled from hydrothermal vents: Cocculina enigmadonta from Kemp Caldera, in the Southern Ocean, and Cocculina aurora (Chen et al., Royal Soc Open Sci, https://doi.org/10.1098/rsos.220885 , 2022) from the Aurora Vent Field, Gakkel Ridge, in the Central Arctic Ocean. Histological analysis was used to describe oogenesis, size at first reproduction and fecundity, whilst individual size-frequency distributions from morphometric measurements were produced to investigate demographic population structure. Both species exhibited similar gametogenic patterns with all but three individuals possessing all oocyte developmental stages. Cocculina enigmadonta and C. aurora had similar reproductive strategies, exhibiting low fecundities. For C. enigmadonta the mean fecundity was 16.1 ± 9.3 mature oocytes and for C. aurora 4.1 ± 2.4 mature oocytes. Gametogenic maturity, mean oocyte diameter and instantaneous fecundity were statistically significantly different between species. Overall, both species presented asynchronous reproduction, quasi-continuous gametogenesis and discontinuous recruitment. Importantly, our results provide the first account of the reproductive biology of cocculinid species from hydrothermal vents and discuss their adaptive life-history traits to inhabiting unstable vent environments.