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Pattern of growth rate around aperture and shell form in Bivalvia: a theoretical morphological study

¹ Takao Ubukata. Institute of Geosciences, Shizuoka University, Oya 836, Shizuoka 422-8529, Japan. sbtubuk{at}ipc.shizuoka.ac.jp

Shell growth and morphogenesis were studied in nine species of Bivalvia from the viewpoint of theoretical morphology. The aperture map, or pattern of relative rate of shell accretion for each point around the aperture, received particular attention. Morphometric analyses indicate that the basic pattern of the aperture map is generally maintained throughout ontogeny, whereas both shell convexity and aperture shape commonly change with growth. Computer simulations show that posterior elongation of the aperture with growth cancels the effect of ontogenetic shell inflation to move the maximal growth point anteriorly. In the species examined, the coiling axis is generally inclined to the hinge axis toward the anterodorsal direction and is plunging to the dextral side of the valve. This condition allows ontogenetic shell inflation without modification of the basic pattern of the aperture map. The result indicates that ontogenetic change of shell form is architecturally constrained by a basic pattern of the aperture map, which is kept throughout ontogeny.

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