Can the morphology of the integumentary spicules be used to distinguish genera and species of phyllidiid nudibranchs (Porostomata: Phyllidiidae)?

Abstract

Thirty-eight specimens belonging to four genera and 15 species of the nudibranch family Phyllidiidae were examined to investigate whether the morphology of their integumentary calcareous spicules and/or the occurrence of the spicules within the regions of the body could be used to distinguish genera and species. The spicules were studied separately from five regions of the body of each specimen—the foot, gills, mantle, dorsal pustules (or ridges in Reticulidia) and rhinophores. The mantle itself plus its pustules were found to possess the full complement of spicules in every individual. Four types of spicules were recorded overall—smooth diactines, centro-polytylote diactines, triactines and tetractines. Different regions of the body were found to possess different spicule types: (a) only smooth diactines in the gills, (b) both smooth diactines and triactines in the foot and (c) all of smooth diactines, centro-polytylote diactines and triactines in the mantle, dorsal pustules and the rhinophores. Among the genera, three types of spicules (smooth diactine, triactine, and tetractine) are present in Phyllidia, Phyllidiopsis and Reticulidia, but the form of the spicules is not diagnostic between these genera or between the constituent species. The fourth type of spicule (centro-polytylote diactine) is present exclusively in Phyllidiella, and is diagnostic for that genus. However, we failed to find any difference in spicule form, or composition, or location in the body between the three (closely related) species of Phyllidiella we investigated. Therefore, our key conclusion is that spicule morphology is an extremely important character to tell the genus Phyllidiella apart from all the other genera of the family, but it is not taxonomically informative at the level of species.