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We investigated the relationship between digestive function and ecological distribution for four related species of Gastropoda (Cerithidea rhizophorarum, Cerithidea cingulata, Batillaria multiformis, and Batillaria attra- mentaria) inhabiting the Tanaka River estuary (Mie Prefecture, Japan). We compared the digestive enzyme activities for four hard degradable carbohydrates, namely, cellulose, mannan, xylan, and laminarin. Among the investigated four species, Cerithidea rhizophorarum showed the highest cellulase, mannanase, xylanase, and laminarinase activi- ties, and was also dominantly distributed in the estuary reed bed. The results of CN stable isotopic analysis suggested that this species assimilated organic matter derived from reeds. Hard degradable carbohydrates, including plant com- ponents derived from the reed bed or from dry land, accumulate in the sediment of estuaries. Estuarine benthic ani- mals are assumed to consume these accumulated hard degradable carbohydrates, as well as microphytobenthos and particulate organic matter (including phytoplankton). Our present findings suggest that Cerithidea rhizophorarum is dominant in reed beds because it can more efficiently digest plant-derived carbohydrates than can Cerithidea cingu- lata, B. multiformis, and B. attramentaria. To the best of our knowledge, ours is the first study to demonstrate that the specific ecological distribution of related animal species with similar ecological traits can be explained by the effi- ciency of their digestive enzyme activity.

Buccinum striatissimum and Buccinum tenuissimum are commercially important whelks in the Sea of Japan. B. striatissimum is very similar to B. tenuissimum and their morphological variations make the identification of these species difficult, especially in case that these species are caught together because they are subject to the same environmental factors. In order to develop an easy and objective method to delineate these species, we utilized the difference of mitochondrial 16SrDNA of these two species. As a result, we could easily distinguish these two species by PCR-RFLP method using the difference of this region and AseI restriction endonuclease. In addition, we succeeded in distinguishing sympatric samples of these species, which suggests that these two species are likely to be reproductively isolated although the possibility of hybridization can not be completely ruled out.