Frequent fissiparous asexual reproduction in Kolga hyalina (Holothuroidea:Elpidiidae) in the abyssal central Arctic.

In the deep sea, holothurians often dominate the megafauna biomass of abyssal plains. Their role as deposit feeders is significant, in organic matter recycling as well as in deep-sea food webs. As with their shallow water relatives, abyssal sea cucumbers show substantial temporal and spatial dynamism in abundance; however, rarely reaching such massive accumulations due to much lower energy supplies. With a sexual reproduction dependent on extracorporeal fertilization, the generally low population densities observed in the abyss have long presented an enigma to deep-sea biology. Two expeditions with the research icebreaker RV Polarstern to the Central Arctic Ocean have collected images of the abyssal Arctic seafloor indicating that fissiparous, asexual reproduction in adults of the deposit feeding holothurian Kolga hyalina species was occurring in the summer months of both 2012 and 2023 at depths in excess of 4000 m. A substantial proportion of holothurians were observed to develop transverse incisions in their middle sections, in a manner reminiscent of fissiparous holothurians previously observed in some tropical and shallow reef environments. In the summer of 2012, 12.3% (SD=8.28) of K. hyalina individuals observed were in the process of splitting, whereas in 2023, 7.9% (SD=4.79) of observed individuals were undergoing fissiparity. These expeditions surveyed contrasting regions of arctic seafloor, differing in food supply, including areas with fresh food falls of the ice algae Melosira arctica, and areas without such food falls. Though K. hyalina individuals were found to be clearly attracted to algal food falls and fed from them, the presence or absence of this material apparently did not impact on the ratio of holothurian individuals exhibiting asexual reproductive behaviour. Interestingly, many individuals observed in the process of splitting were also developing gonadal cells, indicative of active sexual reproduction. Possibly the capacity to employ both reproductive strategies is an advantage in a region where benthic food availability has become very dynamic. These observations are the first evidence of fissiparity occurring in abyssal holothurians globally, and within polar holothurians at any depth. The ability to carry out both sexual and asexual reproduction in the changeable Central Arctic environment is a trait which may explain the dominance of Kolga hyalina in the Central Arctic abyss.