A Natural History of Shells by Vermeij Geerat J.;

Author:Vermeij, Geerat J.;
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2021-08-03T00:00:00+00:00

Fig. 5.3. Specialized right peeling claw of the box crab Calappa convexa from Panama (top). A long tooth on the outer face of the movable finger (bottom) breaks off pieces of the outer shell wall of prey snails. Top, Front surface of the articulated right claw. The height of the claw is 43 mm.

Although breakage is ecologically and geographically widespread, it reaches its greatest specialization in the marine tropics. Many predators living on rocky bottoms break shells; so do some sand-dwellers, such as calappid crabs and many fish. Shell-breaking equipment among temperate marine predators tends to be much less specialized than is that of tropical forms; the same is generally true for most freshwater predators. Only in Lake Tanganyika in East Africa, which is home to an extraordinary assortment of heavily armored gastropods, does one find freshwater crabs whose claws are as robust as those of specialized tropical marine shell-crushers.

Among tropical marine crabs, several genera display a geographic pattern in which species from the Indo-West Pacific region (the western Pacific and Indian oceans) have larger and more robust claws than do species in the eastern Pacific, which in turn have more robust equipment than species in the Atlantic. This pattern is evident in the genera Carpilius, Eriphia, and Ozius, all of which are known to rely at least in part on molluscs and hermit crabs as prey. Whether size and robustness of the claw translate into greater crushing force and pressure on shells is unfortunately not known.

Drilling is a specialized form of predation in which a hole is made either through the shell wall or, in the case of some bivalve prey, at the valve edge (figs. 5.5, 5.6). Naticid and muricid gastropods use an accessory boring organ, which secretes an enzyme (carbonic anhydrase) that softens the shell, in conjunction with the tonguelike radula to excavate a circular hole through which the predator’s proboscis is then inserted. Octopod cephalopods drill by means of the salivary papilla to produce a more irregularly shaped hole. Some land snails may also dissolve or scrape away the shell of their victims.

Fig. 5.4. Viselike jaws of the spiny puffer or porcupinefish, Diodon hystrix, from Guam. Top, Articulated jaws; bottom, separated jaws.

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