Identification and morphological characteristics of a fingernail clam Sphaerium nucleus (Mollusca: Sphaeriidae) newly recorded from Japan

Sphaerium nucleus is a freshwater clam species of the family Sphaeriidae, distributed from Europe to Central Asia. Through morphological and genetic analyses of recently collected specimens, the present study reports the first occurrence of S. nucleus in eastern Hokkaido, Japan. The species was morphologically examined from eight localities and genetically confirmed from seven of them. The characteristics of the shell and soft body of the Japanese specimens were largely consistent with those previously described for this species. The examined specimens represented the following character states of S. nucleus: closed nephridium, dense pores on the inner surface of the shell, separated scars of the siphonal retractor and posterior adductor and strongly bent cardinal teeth. However, polymorphism was observed in the nephridial types, which is the most effective for distinguishing S. nucleus from other congeners; one Japanese population represented the open type. The 16S haplotypes of the Japanese specimens closely resembled those of the Lithuanian and Russian specimens, whereas they also exhibited distinct haplotypes. The diversity of Japanese Sphaeriidae has not been thoroughly understood, necessitating taxonomic and faunal studies employing integrative approaches.