Pairing behavior, reproduction and diet in the deep-sea holothurian genusParoriza (Holothurioidea: Synallactidae)

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 pallens were about 350 µm in diameter,
whereas those of P. prouhoi were up to 450 µm. 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.