Molecular phylogenetics reveals the common species ofTrochus(Vetigastropoda: Trochidae) found on Japanese temperate subtidal reefs

Abstract

AbstractFive species of the marine gastropod genus Trochus are found on the Japanese coast. It was considered that Trochus rota was the most northerly species, whereas subtropical species, including T. histrio, were limited to the southern region. In recent years, however, several researchers have reported shells of T. histrio at higher latitudes than the previously assumed northern limit of this species. For the identification of the Japanese Trochus species, traditional shell morphological characters are insufficient. To provide better identification criteria, we performed a detailed morphological investigation and molecular analysis for Trochus individuals (n = 55) collected from a subtidal reef in Nagai, on the temperate Pacific coast of Japan. Our phylogenetic analysis, which was based on the mitochondrial DNA genes cytochrome c oxidase subunit I and 16S ribosomal RNA, shows that T. histrio, despite its relatively low genetic diversity, exhibits substantial variation in the morphology of the shell base (i.e. in the extent of red spotting on the inner part of the aperture and in the prominence of the knobs protruding along the base of the shell periphery). A shell size comparison indicates that the variation in the shell base reflects growth-associated morphological change. We found significant differences between T. histrio and T. rota in two shell characters (the number of knobs along the base of the shell periphery and shell height/width ratio) with size as a covariate. We conclude that these two species can usually be distinguished from each other by the combination of two characters: presence/absence of red spots on the inner aperture and the extent (i.e. number and prominence) of knob-like structures on the shell base. In addition, field sampling in Nagai shows that T. histrio is currently much more common than T. rota at two sites having contrasting kelp forest conditions. The diagnostic criteria presented here provide a useful basis for research on the ecology of Japanese Trochus species.