Functional Morphology of the Mouthparts and Gastric Mill in the Hermit Crabs Clibanarius taeniatus (Milne Edwards), Clibanarius virescens (Krauss), Paguristes squamosus McCulloch and Dardanus setifer (Milne-Edwards) (Anomura: Paguridae)

Abstract

The structure of the maxillipeds and mouthparts is generally similar in C. taeniatus, C. virescens, P. squamosus and D. setifer, but differences exist in the setation of these limbs and in the form of the crista dentata of the third maxilliped endopods. The differences in setation are not sharply defined and are not indicative of different feeding processes. The crista dentata are weakly developed in C. taeniatus and C. virescens, strongly developed in P. squamosus and D. setifer, reflecting dietary differences.

C. taeniatus is a soft food detritivore, using the chelipeds and third maxillipeds to scoop up food. C. virescens is detritivorous and macrophagous, using the chelae and the crista dentata for triturition. P. squamosus is mainly macrophagous, employing similar methods. D. setifer is macrophagous and predatory, using the chelae to grasp and break up its food and the crista dentata for food transfer. Sorting and ingestion of the food material by the maxillipeds and mouthparts occurs in a similar manner in all species.

P. squamosus also filters particulate food from a water current produced by a unilateral action of the exopods of the second and third maxillipeds. Particles filtered from the current by the antennules, antennae, maxilliped exopods, chelipeds and pereiopods are cleaned off by the endopods of the third maxillipeds and passed to the inner mouthparts. No evidence was obtained of filter feeding in the other three species investigated.

The structure of the proventriculus, including the detailed arrangement of the ossicles and the general arrangement of the musculature, is similar in C. taeniatus to that described by Meiss and Norman in Pagurus pollicaris. A similar proventriculus structure obtains in C. virescens, P. squamosus and D. setifer as a distinctive feature of pagurids. The gastric mill has a common grinding action in all species, but the development and denticulation of the lateral accessory teeth, lateral teeth, dorsal tooth and cardiopyloric valve are positively correlated with the degree of macrophagy. The filtratory system leading via the ampullae to the digestive glands, in contrast, is generally similar in the four species, irrespective offood type, indicating that the usable part of the food is reduced to a similar consistency in all species before it leaves the cardiac stomach.

Present knowledge of feeding mechanisms in pagurids is reviewed. The basic adaptation in this group is to a generalized omnivorous mode of feeding on benthic, particulate, detrital and moderately macroscopic material, variously displayed in different species. Two major trends in feeding specialization have arisen from this. One is macrophagy and predation on large food masses. The other is filter feeding, using either specialized antennae or a number of anterior limbs. All modes of filter feeding in pagurids appear to be modifications of basic cleaning processes involving the endopods of the third maxillipeds. Each of the two specializations, macrophagy and filter feeding, has evolved several times independently within the Paguridae.