Three-dimensional visualisation of brooding behaviour in two distantly related brittle stars from South African waters

Abstract

Brooding in ophiuroids (Echinodermata: Ophiuroidea) is a highly-developed reproduction strategy and has evolved independently several times. In South African waters, brooding occurs both in the very small amphiurid Amphipholis squamata, which is hermaphroditic, and in the large ophiodermatid Ophioderma wahlbergii, which is gonochoric. For the first time, microcomputed tomography was applied to visualise and compare brooding and brooded young of these two brittle star species from two distantly related families. The scanned A. squamata brooded six young, whereas O. wahlbergii brooded 13 young. Volume measurements of the adults revealed that A. squamata dedicated about 25% of its disc volume to brooding and O. wahlbergii about 14%. Although the brooded young of O. wahlbergii are thought to be among the largest of all known ophiuroids, young of A. squamata were four times larger in relation to adult body disc volume. Other than differences in the sizes of brooded young, these distantly related species have evolved very similar brooding adaptations. All juveniles were found lying with the mouth upward, pressed against the mother’s bursal wall, strengthening the hypothesis that the bursal wall acts as a nutritive organ. In both species, the large size of the young can only be explained by maternal nutrient supply. Both brittle star species, therefore, are regarded as truly viviparous.

Keywords: convergent evolution, fecundity, microcomputed tomography, μCT, Ophiuroidea, ophiuroids