Ovarian Development in the Class Holothuroidea: a Reassessment of the "Tubule Recruitment Model"

The "tubule recruitment model" for the development of the holothurian gonad was proposed (a) to connect the stages of oogenesis with ovarian morphology in holothurians throughout the reproductive season and (b) to emphasize the potential for the holothurian ovary as a model system for cytological and biochemical study of echinoderm oogenesis. To reassess the evidence for this model, we have examined published accounts and unpublished observations on gonad development in holothurians from both temperate and tropical habitats, in shallow water and in the deep sea. A very limited number of species were found to conform to the predictions of the tubule recruitment model. The patterns of gonad development vary substantially in holothurians, even at the individual level, and with taxonomic position, geographical location, and habitat. The tubule recruitment model can be applied to only a small subset of holothurians, specifically those in the families Stichopodidae and Holothuriidae that have gonad morphology similar to that of Parastichopus californicus. However, the tubule recruitment model is invalid for many other aspidochirotes, and does not have wider applicability within the class Holothuroidea.