Does environment drive cryptic speciation in Paraonidae (Annelida, Polychaeta)? Two case studies

The family Paraonidae includes polychaete worms that typically occur on soft bottoms, where they gave rise to a wide adaptive radiation. Despite the high number of taxonomic works on Paraonidae, phylogenetic relationships as well as reproductive and ecological features within this family are poorly known. In polychaetes the occurrence of cryptic species is strikingly common; speciation events leading to cryptic species have been speculated for Paraonidae as well, although it has never been proved. Our molecular study has been carried out by means of 460 bp 16S rDNA sequences on Aricidea (Acmira) assimilis and Aricidea (Acmira) catherinae, two common paraonid species occurring in the Mediterranean Sea. Main results highlighted, within each species, two deeply divergent clades, which were consistent with the presence of cryptic species. Cluster analysis clearly separated shallow-water individuals of A. assimilis from deep-water counterparts. Moreover, in the shallow-water clade it was possible to identify, even though at a lesser extent, genetic divergence between marine and brackish-water individuals. In this last case, the genetic divergence we found was also consistent with the morphological differences observed between individuals from the two habitat types. On the other hand, in A. catherinae, cluster analysis clearly separated coarse-sand individuals from fine-sand ones, suggesting that granulometry can be an important factor driving cryptic speciation. This study highlighted the importance of the molecular ecological approach to clarify the role played by environmental features in evolutionary processes and, ultimately, to contribute to a deeper and deeper knowledge of marine biodiversity.