Mya japonica, IS A VALID

MYA JAPONICA IS A VALID View in CoL SPECIES

Although the morphological differences between the clams identified as M. arenaria and M. japonica in the present study were minor, the ABGD delimitation, reciprocal monophyly, genetic divergence and spermatozoan ultramorphology of soft-shell clams from the western Pacific strongly support the recognition of M. japonica as a valid species. Within Mya , we found two clades, one comprising M. arenaria and M. japonica and the other containing both other Mya species examined. While M. arenaria and M. japonica were most related to each other, there was deep divergence between M. arenaria from the Northeast Pacific, Atlantic, Mediterranean and Barents Sea (Arctic) and M. japonica from the Pacific, which was further supported by ABGD. Furthermore, we found significant differences in spermatozoan ultramorphology between

SHELL MORPHOLOGICAL EXAMINATIONS AND INTRASPECIFIC VARIATION OF M. JAPONICA

Although individuals identified as M. arenaria and M. japonica were similar in appearance, there were recognizable differences in general shell shape, shell texture and appearance of the pallial line. Although the shape, size and orientation of the left valve chondrophore are often important diagnostic characters among Mya and related bivalves ( Bernard, 1979), we found considerable variation in the characteristics of the chondrophore among all the material examined and it was not useful in discriminating between species. Given the comparative nature of the characteristics reported here (e.g. less/more impressed, elongated/ shorter etc.) and overall degree of intraspecific variation, it may be difficult to identify individual clams using the descriptions contained herein, particularly for those who have not had the benefit of studying many specimens. Thus, we strongly recommend molecular or other analyses to confidently separate M. arenaria and M. japonica .

Makiyama (1934, 1935) reported two forms of soft-shell clam in Japan, a northern form described as M. japonica and the southern form described as M. oonogai . The latter was referred to as M. japonica oonogai by Habe (1955) and Fujie (1957, 1962) and M. arenaria oonogai by Kwon et al. (2001), Kwon, Park & Lee (1993) and Yoo (1967). Fujie (1957) not only agreed that northern forms were M. japonica but also described a forma ‘α’, which was shorter and more ovate. Nagao & Inoue (1941) described differences between northern and southern Japanese forms and noted many intermediate specimens. The northern group, which possessed a thick, relatively short and nearly equilateral shell that was somewhat well rounded or slightly truncated posteriorly, was considered M. arenaria (due to synonymy with M. japonica ). The southern group, which was characterized by its generally thin, long and somewhat inequilateral shell with a narrowly rounded or frequently acuminated posterior extremity, was considered an unnamed species. Nagao & Inoue (1941) also reported that the chondrophore in the northern group was nearly perpendicular to the antero-posterior diameter of the shell, and was narrower and more deeply excavated, with its inner border less convex than in the southern group. The posterior ridge was also not well differentiated from the chondrophore.

Similarly, Scarlato (1981) noted two morphological forms of M. japonica in Russian Far Eastern seas. One form is distributed in low-boreal (temperate) waters (Aniva Bay in southern Sakhalin, southern Kurile Islands and Peter the Great Bay), where this species reaches its maximum size and possesses the typical shape with a smooth surface and fine, regular growth lines. The other form was found in the northern part of its distributional range (Bering and Chukotsk seas, eastern Kamchatka and northern Sea of Okhotsk), where M. japonica never reaches its maximum length and often has an atypical shape, sometimes appearing misshapen or deformed. However, our examination of M. japonica from Peter the Great Bay indicates that these variations also occur in the same geographical locality and are related to different environments ( Fig. 2 View Figure 2 ) along with intermediate specimens. The cooccurrence of different morphological forms and intermediate specimens in a single geographical region strongly suggests that these forms are ecophenotypes. It should be noted that the ‘typical’ thin and regular form reported here fully complies with ‘ M. japonica oonogai ’ collected from Hokkaido and Nagasaki, Japan, and our ‘thick-shelled’ form is very similar to M. japonica forma ‘α’, as illustrated by Fujie (1957). In addition, the M. japonica of Fujie (1957) is intermediate between the two forms we reported here. The results of our phylogenetic and morphological analyses indicate that M. japonica is a single species, which displays a high degree of phenotypic plasticity in shell morphology, and many previously reported forms/species combinations probably reflect ecophenotypic variation or represent natural intraspecific variation.