A review of taxonomy and phylogenetic relationships in the genus Costasiella (Heterobranchia: Sacoglossa), with a description of a new species

The history of the genus Costasiella is summarized. A new species of Costasiella, C. arenaria n. sp., from Lake Worth, Florida, USA is described. For comparative purposes specimens of C. formicaria (Baba, 1959) from Japan and serial sections of C. pallida Jensen, 1985 from Hong Kong have also been examined. These three species, like the type species C. virescens Pruvot-Fol, 1951 and C. coronata Swennen, 2007 from Thailand are not associated with the green alga Avrainvillea, which is the food of most known species of the genus. They share several morphological characters, i.e. pharynx without muscular pouches, radular teeth with narrow, smooth, blade-shaped cusps and short bases, unarmed penises and branches of albumen gland entering the cerata. A molecular phylogeny of 14 species of Costasiella based on four genes (mitochondrial COI and 16S rRNA and nuclear 28S rRNA and H3) supports morphological findings that the species not associated with the green alga Avrainvillea form a monophyletic group within the genus.