Systematics and Evolution of Volachlamys from Japan (Preliminary Notes)

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  • 日本産ヤミノニシキ類の分類と進化(予察)
  • 日本産ヤミノニシキ類の分類と進化-予-
  • ニホンサン ヤミノニシキルイ ノ ブンルイ ト シンカ ヨ

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Abstract

Volachlamys Iredale, 1939 [type-species : Pecten cumingii Reeve 1853 (=Pecten singaporinus Sowerby, 1842)] is an Indo-Pacifiic pectinid genus (probably a monophyletic group), characterized by Chlamys-like deep byssal notch, persistent ctenolium, relatively large (commonly rectangularly or acutely truncated) posterior wing, undeveloped scales on disk and wings, and generally simple, rarely bifurcated and unornamented radial costae. Chlamys hirasei Bavay, 1904 is a solitary extant representative of this genus in west Japan and Yellow Sea. Every deme of this species seems to be dimorphic one phenon (var. ambigua) has strong radial costae, whereas the other phenon (var. ecostata) reveals much weaker ribs or nearly smooth surface. Most other shell characters are not significantly different between the two phena, though the ratio of height/length is somewhat larger in var. ecostata, and the shell is considerably heavier in var. ambigua. The two phena seem to be strictly sympatric at least in Osaka Bay and Ariake Bay in west Japan. Chlamys ambigua Bavay, 1904, Pecten awajiensis Pilsbry, 1905, Pecten pulchellimus Tokunaga, 1906, Pecten atsumiensis Yokoyama, 1926, and Pecten teilhardi Grabau and King, 1928 can be all regarded as specifically synonymous with C. hirasei. A middle Pleistocene fossil species, Volachlamys yagurai (Makiyama, 1924), from the environs of Kobe and west Kyushu is monomorphic, and morphologically resembles Volachlamys singaporina (Sowerby, 1842) now living in the tropical seas of southeast Asia and north Australia. Though much should be further studied, the following evolutionary history is hypothetically presumable : 1) V. singaporina (or its ancestral species) was once widely distributed in the western Pacific, and its northern limit of distribution reached Japan. 2) V. yagurai is an incipient species which was derived from a peripheral isolate of V. singaporina (or its ancestral species) adapted to some inland sea of Japan or its adjacent region (about 500, 000 years B. P. or a little earlier). 3) The number of radial ribs on the disk gradually decreased in the course of phyletic evolution from V. yagurai to V. hirasei. 4) Every deme of V. hirasei after 130, 000 years B. P. came to bear smoothish mutant individuals (i.e. var. ecostata) at a certain relative frequency. At present the frequency is as high as 50 percent both in Osaka Bay and Ariake Bay.

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