The Adverse Effects of Shell Ingestion for Molluscivorous Cichlids, a Constructional Morphological Approach

Abstract

Among the many food resources exploited by cichlids, molluscs are exceptional because they are enclosed in a shell of hard, dense and indigestible material. This study investigates some of the implications of this for prey processing in Astatoreochromis alluaudi, one of many cichlid species which crush the shell of their prey between their pharyngeal jaws. These so-called pharyngeal mollusc crushers were believed to ingest together with the molluscs flesh the entire, crushed shell. However, my study revealed that they eject shell fragments before ingesting their prey. The amount of mollusc shell ejected by A. alluaudi depends on the type and size of the mollusc and the size of the fish. The percentage of shell ejected increases with prey size and decreases with predator size for all prey species. More shell of gastropods than of bivalves is ejected. Shell fragments are ejected because shell ingestion affects (1) the buoyancy of the fish and (2) the space available for food storage in the guts. One model illustrates to what extent (1) the swimbladder volume has to increase to compensate for loss of buoyancy and (2) the storage space for mollusc flesh in the intestines is decreased when shell is ingested. A second model, which is based on the change of swimbladder volume in vertical movements, illustrates how prey ingestion may affect vertical movements and vice versa. Ingestion of more shell will decrease the upward vertical movement of the fish on the one hand and increased allowance for upward vertical movements will decrease the intestinal storage space on the other. The relevance of the results to foraging models and to theories on the evolution of shell form is indicated.