The reproductive and population biology of the deep-sea elasipodid holothurian families Psychropotidae and Deimatidae from the North Atlantic is described. In the psychropotid species the ovary is a large thick-walled nodular structure containing oocytes with an interspecific maximum size of 1.2 mm to 3 mm. In the deimatid species the ovaries consist of thin-walled elongate tubules containing oocytes up to a maximum diameter of 900 µm. In the psychropotids all the oocytes undergo vitellogenesis whereas in the deimatids some could act as nurse cells for those oocytes continuing to grow. Fecundity is low in both families. The soluble lipid content of the ovary in all species is high when compared to other tissues. The calorific content of the whole ovary varies from 25.46 to 28.08 Jmg-1 AFDW. There is no evidence for any reproductive periodicity in any of the species examined.
In the psychropotids the testes are well-developed structures packed with spermatozoa except in those specimens that are infested with protozoan parasites. In the deimatids functional males were never observed. About 50% of the specimens from any population are female but the other half of the population are sexually inactive and are assumed to be predominantly males.The population structure of the species examined is very variable with single modes skewed towards small or large sizes and in some cases a bimodal distribution. Very small specimens were taken rarely in benthic samples although juveniles of Benthodytes sordida and Psychropotes longicauda have been taken in rectangular midwater trawls at distances of 17 to 1000m above the seabed. This suggests the possibility of a pelagic juvenile capable of dispersal by deep-ocean currents and may account for the cosmopolitan distribution of many of these species.