860. LARVAL PALEOECOLOGY OF FIVE BIVALVE SPECIES FROM THE UPPER PLIOCENE OF SOUTHWEST JAPAN

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  • 860. 西南日本上部鮮新統産二枚貝 5 種の幼生古生態
  • 西南日本上部鮮新統産2枚貝5種の幼生古生態〔英文〕
  • セイナン ニホン ジョウブ センシントウサン 2マイガイ 5シュ ノ ヨウセイ

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Abstract

Juvenile shells of five bivalve species, Glycymeris rotunda, Limopsis tajimae, Oblimopa japonica, Crenulilimopsis oblonga and Venericardia panda from the Upper Pliocene deposits of Kakegawa, Miyazaki and Okinawajima regions, southwest Japan retain the prodissoconch in their initial shell portion. Based on comparison of their prodissoconch morphology and morphometry with those of modern species hitherto investigated, developmental types of the five species were presumed : direct development for V. panda having a relatively large (185-259μm in length) prodissoconch, and lecithotrophic with a short non-feeding pelagic stage for other four species characterized by relatively large (ca. 150-200μm in length) prodissoconch I and well-marked prodissoconch II. C. oblonga and L. tajimae both show remarkable intraspecific variation in prodissoconch II size, and the specimens from the northern region generally possess larger prodissoconch II than those from the southern region. Judging from the knowledge on larval history of some Recent bivalves in relation to water temperature and the paleooceanographic setting of the study areas in late Pliocene age, the latitude dependent variation of prodissoconch size observed in the two species may reflect the difference in durations of larval stage depending on the changes in temperature of marine environments.

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