Pairing behaviour, reproduction and diet in the deep-sea holothurian genus Paroriza (Holothurioidea: Synallactidae)

Abstract

Deep-sea holothurians of the genus Paroriza (Family Synallactidae) have often been observed or photographed in pairs or triplets. Observations of tracks on the sea floor suggest that pairs may remain together for some time. We explored the reasons for pairing in a study of reproductive condition and diet, using animals collected by trawl from the bathyal and abyssal north-east Atlantic and by manned submersible from the bathyal slope of the Bahamas. Both species of Paroriza studied were found to be simultaneous hermaphrodites producing yolky oocytes. Oocytes of Paroriza pollens were about 350μm in diameter, whereas those of P. prouhoi were up to 450μm. Examination of oocyte size-frequency distributions suggests that reproduction is continuous for both species. Neither mean oocyte size, nor shape of the oocyte size-frequency distribution differed significantly between paired and unpaired P. pallens collected during the same season. These deposit-feeding holothurians appear to take food from the nutrient-rich phytodetrital layers on the surface of the sediment. Because their food resources are probably not patchy on small scales, the observed pairs are not likely to be feeding aggregations. We hypothesize that pairing increases the likelihood of external fertilization and that spawning in these species may be induced at any time by the presence of a conspecific rather than by seasonal cues.