Rate and Pattern of Shell Growth of Glycymeris fulgurata and Glycymeris vestita (Bivalvia: Glycymerididae) in Tosa Bay as Inferred from Oxygen Isotope Analysis

Abstract

The subtropical species Glycymeris fulgurata and the temperate species G. vestita are both mainly distributed around the Japanese islands. These species have shells that are so similar in form that their taxonomic treatments have been rather confused to date. To evaluate potential differences between these two species in the rate and pattern of their shell growth, their oxygen isotope profiles were determined using dead specimens collected from dredged sediments at a depth of approximately 40 m in Tosa Bay off the coast of Shiwa, Shimanto-cho, Kochi Prefecture. The clear growth rings located on the shell surface of both species correspond to the isotopically lighter or higher temperature peaks of the oxygen isotope profile. This characteristic suggests that the clear growth rings are formed annually. Based on this established association, we estimated the annual growth increments for both species living in Tosa Bay. The maximum shell height was approximately 43 mm for G. fulgurata, and approximately 62 mm for G. vestita, and the maximum age was 15 years for G. fulgurata and >30 years for G. vestita. The early shell growth rate was greater for G. fulgurata than for G. vestita. However, the rapid growth period lasted longer for G. vestita than for G. fulgurata, which corresponded with the longer life span of G. vestita. Thus, G. fulgurata and G. vestita found in Tosa Bay exhibit quite different shell growth rates and patterns. These observations suggest that they represent well-established, independent species with an overlapping geographic distribution originating after speciation presumably sometime in the Pleistocene Epoch.