Ecology and Evolution of the brittle star species complex Ophioderma longicauda

Brittle stars are a very diverse group of marine invertebrates. They are the most species-rich class of echinoderms, the phylum encompassing sea urchins and sea stars, amongst other. They colonized all marine environments, from very shallow to abyssal depths, and from the tropics to the poles.

My PhD used several approaches to understand the ecology and evolution of the cryptic species complex Ophioderma longicauda, occurring in the Mediterranean Sea and in the North Atlantic Ocean. This species complex is particularly interesting because it encompasses closely related species with contrasting reproductive strategies.

We first showed that sympatric brooding and broadcast spawning individuals of O. longicauda represent two distinct biological species (Weber et al, 2014) that are currently under description (Stöhr, Weber, Boissin and Chenuil, in preparation). We also showed that these two species display distinct ecological preferences as exemplified by different levels of thermotolerance (Weber et al, 2013).

Two central metrics in assessing a species’ persistence potential, namely the connectivity and genetic diversity, were also contrasted between the brooding and the broadcasting O. longicauda species (Weber et al, 2015). These results were part of a large-scale study on genetic diversity, showing that its main determinant is parental investment (Romiguier et al, 2014). Since these species are closely related, sympatric and therefore differing only in their level of parental investment, our dataset was particularly important to support this major result.

The comparison of transcriptomes from the brooding and broadcasting species showed that two genes involved in sperm motility were positively selected in the brooding species (Weber et al, 2017), highlighting a potential new avenue for species recognition and speciation. Finally, a large-scale analysis of all O. longicauda mitochondrial lineages showed that this species complex encompasses most likely six distinct biological species, with a complex diversification history including hybridization events (Weber et al, 2019).