Molecular and morphological systematics of Elysia Risso, 1818 (Heterobranchia: Sacoglossa) from the Caribbean region.

The Caribbean is a biodiversity hotspot for photosynthetic sea slugs, with about 27 described species in the genus Elysia Risso, 1818. However, many species are poorly known or have complex taxonomic histories, complicating assessments of regional biodiversity and impeding studies of plastid symbiosis, speciation, and larval biology. Using an integrative approach, we address the taxonomy and systematics of Caribbean elysiids by performing robust tests of existing species hypotheses, and describe six new species. Species delimitation included DNA barcoding of up to 189 nominal conspecific specimens; nuclear gene sequences were then used to confirm that divergent lineages were genetically distinct candidate species. New synonymies and species descriptions are based on external anatomy, penial and radular morphology, developmental characters, and host ecology of all species described from the region, plus a critical review of the literature. We synonymized three species (Elysia annedupontae Ortea, Espinosa & Caballer in Ortea, Caballer, Moro & Espinosa, 2005, Elysia clarki Pierce et al. 2006, and Elysia leeanneae Caballer, Ortea & Espinosa in Ortea, Espinosa, Buske & Caballer, 2013), transfered one species from Bosellia (Elysia marcusi), and described six new species (Elysia pawliki n. sp., Elysia zemi n. sp., Elysia christinae n. sp., Elysia hamanni n. sp., Elysia taino n. sp., and Elysia buonoi n. sp.). We resurrected the name Elysia velutinus Pruvot-Fol, 1947, a senior synonym of Elysia tuca Ev. Marcus & Er. Marcus, 1967. Based on a four-gene phylogeny of 76 Elysia spp., we identified shifts in host use and penial armature that may explain patterns of endemic diversification in Elysia, invoking both ecological and non-ecological mechanisms. Non-monophyly of stylet-bearing species rejects previous attempts to classify species based on presence of a stylet (i.e., the genus Checholysia Ortea, Caballer, Moro & Espinosa, 2005). Our findings show how integrative approaches can resolve the taxonomic status of problematic species (e.g., Elysia papillosa Verrill, 1901) for soft-bodied marine taxa.