Cryptic speciation in a high gene flow scenario in the oviparous marine sponge Chondrosia reniformis

Sponge systematics has been traditionally based on the study of the skeleton (spicules and spongin fibres). However, sponges of the genus Chondrosia are devoid of those skeletal features, making it difficult to distinguish between different species in the genus. Chondrosia reniformis Nardo, 1847, the type species of the genus, was described from the Mediterranean Sea. The lack of distinguishing morphological features may have been responsible for the widespread assignment of specimens of the genus to this species; as a result C. reniformis is considered to be a cosmopolitan species. In this work, populations of C. reniformis from the western Mediterranean (France) and the West Atlantic (Bermuda and Brazil) were analysed using allozyme electrophoresis for 13 enzyme loci. Levels of mean heterozygosity were high (Bermuda and Brazil H=0.27 and W Mediterranean H=0.12), as is often observed in sponge species. Gene identities observed between West Atlantic and Mediterranean populations were low (I=0.40-0.52, typical values for congeneric species), including the presence of four diagnostic loci. This level of divergence clearly shows that they are not conspecific. Hence, a worldwide or cosmopolitan distribution of C. reniformis would seem improbable. However, the West Atlantic samples (Bermuda and Brazil) were genetically similar (gene identity, I=0.88-0.95) over a distance of 8,000 km. This is the first report of genetic homogeneity in a sponge species over such a large geographical distance.