Two Cryptic Sympatric Species of Costasiella in the Bahamas Evolved Allopatrically
Mentor:Ángel Valdés, Assistant Professor of Biology, California State Polytechnic University Pomona
Costasiella ocellifera, Simroth 1895, is a species of sea slug (Mollusca: Gastropoda; Sacoglossa) that is found throughout the Caribbean. This species feeds exclusively on Avrainvillea green algae upon which is highly camouflaged. A preliminary phylogenetic analysis was constructed from nuclear (H3) and mitochondrial (16S, COI) gene sequences. It shows that several specimens identified as being C. ocellifera and collected from the Bahamas are genetically distinct from other specimens of C. ocellifera from the Bahamas and other western Atlantic locations. These genetically different specimens are considered to belong to a cryptic, undescribed species. The new species looks very similar (almost identical) to the typical C. ocellifera but a few differences in coloration and radular morphology provide additional support for molecular results.
Costasiella ocellifera and the new species are sister and sympatric in the Bahamas. Sister species with overlapping ranges, inhabiting areas in which there is no evidence of present or past biogeographic barriers to dispersal, could constitute potential cases of sympatric speciation. In this case, however, both species feed upon the same species of Avrainvillea and were collected in the same locations during the same time of the year. This seems to reject any possibility niche partition or allochrony, which are common pre-requisites for sympatric speciation. Thus, we hypothesize that these two species evolved allopatrically. Lack of evidence of hybridization (no nuclear alleles of the new species are found in C. ocellifera or vice versa) suggests reinforcement.
This case is similar to other reported new cryptic species in the Bahamas (Chelidonura, Philinopsis, Spurilla) and provides further evidence of a partial or complete interruption of gene flow between the Bahamas and other Caribbean areas at some point in the past.