Redescriptions and Attachment Modes of Hypermastus peronellicola and H. tokunagai (Prosobranchia: Eulimidae), Ectoparasites on Sand Dollars (Echinodermata: Clypeasteroida) in Japanese Waters

Abstract

Two species of the eulimid genus Hypermastus are redescribed based on specimens recently collected from sand dollars caught in the Seto Inland Sea and the type specimens: Hypermastus peronellicola (Kuroda & Habe, 1950) from Peronella japonica, and H. tokunagai (Yokoyama, 1922) from Scaphechinus mirabilis. These two eulimid species are very similar in their shell morphology but are distinguished from each other based on characters such as the proportions of shell length to several dimensions of the shell, width/length ratios of each teleoconch whorl, the protruding part of the outer lip margin, and the coloration of the visceral mass that can be seen through the translucent shell in living specimens. H. peronellicola was attached to the host by inserting the proboscis into the host's body, whereas no proboscis penetration was observed in H. tokunagai.