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 skeleto n (spicules and spongin fibres). However, sponges of the genus Chondrosia a re devoid of those skeletal features, making it difficult to distinguish be tween different species in the genus. Chondrosia reniformis Nardo, 1847, th e 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 wo rk, populations of C. reniformis from the western Mediterranean (France) an d the West Atlantic (Bermuda and Brazil) were analysed using allozyme elect rophoresis for 13 enzyme loci. Levels of mean heterozygosity were high (Ber muda and Brazil H = 0.27 and W Mediterranean H = 0.12), as is often observe d in sponge species. Gene identities observed between West Atlantic and Med iterranean populations were low (I = 0.40-0.52, typical values for congener ic 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. Howeve r, 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 fir st report of genetic homogeneity in a sponge species over such a large geog raphical distance.