Functional significance and characterization of sexual dimorphism in holothuroids

Sexual dimorphism has been reported in all extant echinoderm classes except crinoids, but has never been examined from a phylogenetic, biogeographical, or life-history perspective. This review provides a literature survey of sexual dimorphism in holothuroids and uses this dataset to analyze putative drivers. Sexually dimorphic genital papillae were found in Persiculida and Dendrochirotida but not in other holothuroid taxa. No planktotrophic species (feeding larvae) had known genital dimorphism, though many lecithotrophs (non-feeding larvae) displayed clear morphological differences between male and female papillae. Males with genital dimorphism had digitate or extensible papillae while females had unbranched papillae. Extensible papillae were common among males in brooding species, suggesting an adaptive advantage for certain reproductive strategies such as sperm transfer or pseudo-copulation. Digitate papillae bearing many gonopore openings were common among free-spawning, lecithotrophic males (~ 80% of species) and may serve to disperse sperm into the water column. Finally, we found that external dimorphism of the genital papillae in a case study of the dendrochirotid Cucumaria frondosa (Gunnerus, 1767) provided a reliable method of sex determination. These results suggest that genital dimorphism among dendrochirotid sea cucumbers is widespread and may facilitate determination of sex in the field where sacrificing animals is not practical.