The relationship between functional shell morphology and nature of the bored substratum in the family Pholadidae (Bivalvia)

Abstract

Shell external sculpture of pholadid bivalves was examined for understanding their functional morphology and evolutionary history. It can be classified into the rasp, grater, rake, and spike types by its function. Shell external sculpture other than these four sculpture types is grouped as the "Angel Wing" type (Cyrtopleura type) because of resemblance of shell outline and arrangement of ridges where concentric ribs and radial ribs cross. The bored substratum by pholadids can be classified into volcanic rock, consolidated sedimentary rock, semiconsolidated sedimentary rock and unconsolidated sediment, based on kind of rocks and degree of consolidation of sediments. Shell length, height, outline, area of external sculpture and length from shell anterior margin to umbo were measured as parameters that were available in both fossil and living species. As a result, the following relationships are recognized among the shell size, the shell shape and the hardness of the bored substratum : 1. In pholadids boring into hard substratum, the shell tends to be robust and small oval in outline, and shell external sculpture becomes to be the dense rasp type on the anterior of shell. 2. In pholadids boring into soft substratum, the shell tends to be large, and the shell sculpture extends to nearly posterior-umbonal margin of shell. Two types of shell external sculpture are observed in these bivalves. One is the "Angel Wing" type; the pholadids with this sculpture type bore into soft substratum and their shells are elliptical in outline. The other is the spike type; the pholadids with this sculpture type burrow into the unconsolidated sediment and their shells are thin and rectangular in outline. To review the history of adaptive radiation of the Pholadidae, the fossil pholadids were examined on the basis of correlation between the shell external sculpture and the bored substratum recognized in living species. The result suggests that pholadids with the shell external sculpture of the rasp type appeared as wood-boring bivalves and they have adapted to various lives in hard substratum since Cretaceous. Finally, the pholadids with shell external sculpture of the "Angel Wing" type and the spike type appeared in the Neogene and they have adapted to the soft substratum.