The role of vermetid gastropods in the formation of Mediterranean and Atlantic reefs.

Vermetids significantly contribute to the features of certain intertidal reefs. In the eastern Mediterranean, notably along exposed shores of Israel, these reefs are large eolianite platforms divided into rimmed flat basins. Their surfaces are incrusted by a dense cover of vermetid shells cemented by coralline algae-the rims by a crust of Dendropoma petraeum, 10-15 cm thick, and the surrounded basins by Vermetus triquetrus, usually less than 5 cm thick. These two species live in very different environments-the rims are incessantly surfswept and often exposed to air while the basins are usually watercovered and relatively calm. Accordingly, Dendropoma differs from Vermetus in certain morphological and anatomical characters, as well as in feeding mechanisms. There is circumstantial evidence that their crust has a protective effect on the underlying rock and is the controlling factor in the development of these platforms.Less common and smaller vermetid-covered reefs of the Israeli shore are circular limestone formations, in which an outer rim incrusted by D. petraeum surrounds a single shallow basin incrusted by V. triquetrus. These small reefs, resembling miniature atolls, show superficial similarity to intertidal reefs in Bermuda, where certain environmental conditions are rather similar to those prevailing in the eastern Mediterranean. As in Israel, a species of Dendropoma, D. irregulare, is an important component in the composition of the reefs, though there is no Vermetus. Also, coralline algae do not function just as cement but are a significant component.Both in Israel and in Bermuda such reefs develop when erosion, brought about by the encroaching sea, is hindered by the protective biogenetic crust. In Israel this crust never increases in thickness beyond a certain level because by the time the rising sea-level permits further upward growth, the reef has already collapsed under the combined action of boring organisms and waves. In Bermuda, voids and borings within the reef are rapidly filled-in and cemented, mainly by transported calcareous debris. Instead of borers and waves weakening it, the Bermudan reef is consolidated and does not readily collapse, and the thickness of its organic crust is remarkable. Thus, the Bermudan "microatolls" are genuine wave-resistant reefs. Their growth is initiated and perpetuated by vermetids and coralline algae. Though a large portion of their framework is removed, it is replaced by a secondary deposition and cementation of other lime-secreting organisms and debris.