Bathymetric and interspecific variability in maternal reproductive investment and diet of eurybathic echinoderms

While conditions in shallow-water and deep-sea environments differ markedly, it remains unclear how eurybathic species adapt their life histories to cope with these changes. The present study compared indicators of maternal reproductive investment of three common echinoderms collected shallower than 20 m and deeper than 850 m: Cucumaria frondosa (Holothuroidea), Solaster endeca and Henricia sanguinolenta (Asteroidea). Depth-specific and species-specific differences were found in gonad indices (GI), potential fecundity, oocyte size frequency, as well as lipid classes and fatty acids measured in gonad tissue and oocytes. The asteroids, S. endeca and H. sanguinolenta, exemplified the interspecific trade-off between size and number of oocytes: the former had fewer larger oocytes than the latter, with higher total lipid proportions in them. However, intraspecifically, larger oocytes found in deep specimens of S. endeca did not translate into lower fecundity but rather into a seemingly higher GI, indicating greater investment per oocyte without reducing fecundity. Oocytes were absent in specimens of C. frondosa sampled in deep water, suggesting delayed or impaired maturation at the limit of their depth tolerance. Analysis of S. endeca sterol proportions in gonads and oocytes across depths showed higher sterol input into oocytes in females from the deep. Gonads of S. endeca and H. sanguinolenta contained similar essential fatty acids, but showed significant differences in major fatty acids and some of the less dominant ones, indicating diet specificities. Analyses within S. endeca showed evidence of similar feeding mode (carnivory) at both depths, but suggested shifts in the diet or synthesis of fatty acids, presumably reflecting differences in available food sources and/or adaptations to their respective environments.