北西太平洋産Glycymerisyessoensis (SOWERBY) と北東太平洋産Glycymeris subobsoleta (CARPENTER) との類縁関係

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書誌事項

タイトル別名
  • Systematic relationship between Glycymeris yessoensis (SowERBY) and Glyc.ymeris subobsoleta (CARPENTER) (Mollusca : Bivalvia)
  • ホクセイ タイヘイヨウサン Glycymeris yessoensis So

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説明

Distributions of the Recent glycymeridid bivalves of the Northwestern Pacific are usually affected by warm currents, i.e. the Kuroshio and Tsushima currents. However, a warm temperate to subboreal species, Glycymeris yessoensis (SOWERBY, 1889), shows a unique distribution which has no relation to the warm currents. Glycymerissubobsoleta (CARPENTER, 1864) is also distributed far north than other glycymeridid bivalves of the Northeastern Pacific. It is suspected that an ancestor of these two species was continuously distributed from the Northwestern Pacific to the Northeastern Pacific in the warm early Miocene time, and then it migrated into two directions, i. e. the Northwestern and the Northeastern Pacific, and differentiated into C. yessoensis and C. subobsoleta. In order to verify this hypothesis, we inferred genealogy of the glycymeridids from the Northern Pacific mainly based on morphology of valves and soft parts. In this study, we used not only qualitative characters but also quantitative characters. We calculated the GOWER's similarity index and clustered the species with unweighted pair-group method by average linkage (UPGMA). Then the result is nearly consistent with a classification of subgenera adopted by former authors, but we think it doesn't show the real genealogy because it is inconsistent with the fossil records. So that, we tried phylogenetic systematics to get a classification that reflects the genealogy. There are several cladograms due to insufficiency of items of characters. Thereupon, a phylogeny is inferred by adding the order inferred from the fossil records of Glycymerididae and out-group comparison. According to cladistic consideration, C. subobsoleta is closely allied to G. yessoensis. This result supports the hypothesis that both species were derived from a common ancestor. Species of the subgenera Veletuceta, including C. albolineata, G. vestita and C. reevei, are distinguished from other glycymeridids by the characters of ligament, anus and adductor scars. The cladogram also suggests that both C. rotunda and G. imperialis are not allied so much to G. yessoensis and Tucetona species have derived from some species close to G. rotunda. The result is in opposition to the present classification of the Glycymerididae.

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