Madreporites of Ophiuroidea: are they phylogenetically informative?

Abstract

The madreporite of brittle stars is poorly studied, and the features of its structure are rarely used in the taxonomy. However, it is known that there is diversity in the madreporite structure. But are the ophiuroid madreporites phylogenetically informative? To check this hypothesis, we investigated the structure of the madreporite of 33 species of brittle stars from 4 families of Euryalida and 12 families of Ophiurida. The fixed specimens were processed with sodium hypochlorite using the standard procedure and then studied using SEM. If we combine our results with the modern phylogenetic data about brittle stars (O’Hara et al. in Curr Biol 24(16):1874–1879, 2014), we will find wide morphological diversity of the madreporites present in each of the three clades of Ophiuroidea. The madreporites with numerous pores, the well-developed oral shields in other interradii instead of irregularly arranged plates and the multiple madreporites occur in the representatives of all three clades. Only in Euryalida, which belongs to the clade A as well as the sister clade Ophiuridae + Ophiomusium, the definite oral shields are absent in all interradii except CD. Whereas in the family Ophiuridae (as in the clades B and C), the oral interradial shields are regularly formed. Contrary to this, the multiple madreporites and numerous madreporic pores appear to have evolved several times in different clades. Hence, the hypothesis that madreporite morphology is phylogenetically informative must be rejected since madreporites are highly homoplasious.