Contrasting Population Status between the Planktonic and Direct-Developing Batillariid Snails Batillaria multiformis (Lischke) and B. cumingi (Crosse) on an Isolated Tidal Flat in Tokyo Bay

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  • 東京湾谷津干潟におけるウミニナとホソウミニナの対照的個体群状況

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Two closely related batillariid snails, Batillaria multiformis and B. cumingi, commonly occur together on tidal flats around Tokyo Bay. During the last decade, however, local populations of B. multiformis have declined precipitously or become extinct, while those of B. cumingi have remained stable or even increased. The results of this research, which consisted of population status surveys of the batillariid snails in an enclosed tidal flat in Tokyo Bay and a rearing experiment of B. multiformis, suggest that the differences in status between these two species could be explained in terms of their respective life cycles ; while B. multiformis produces egg capsules that hatch into planktonic larvae, B. cumingi spawns isolated eggs that hatch out as benthic juveniles. Since the late 1960s, extensive coastal landfill has eliminated most of the tidal flats and salt marshes that originally formed a continuous ring around the Tokyo Bay shoreline. Only a few suitable tidal habitats remain, and these are isolated from one another. The planktonic larvae of B. multiformis may thus be unable to drift to a suitable habitat for settling out. In addition, severe bottom hypoxia occurs in the bay during the summer months, when the larvae are released, and this may adversely effect their survival rate. In contrast, the benthic juveniles of B. cumingi are able to recruit directly on their parent tidal flats, which are not impacted by the coastal landfill or hypoxia, making it easier for them to maintain or expand their local populations.

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