Functional morphology of the amphipod mandible

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Abstract
While several features of the amphipod body are used both to classify and estimate phylogenetic relationships of species, little is understood of the functional significance of most of these features. The amphipod mandible consists of a compact coxa bearing a toothed incisor orientated to cut in the transverse plane of the body, a row of lifting spines leading dorsally to a molar designed for crushing. The basic pattern is retained in those groups where microphagy is important. Modifications include reduction of the incisor, loss of the lifting spines, reduction or loss of the molar, or all of these. In several families the mandible is maintained unmodified; in others most genera possess mandibles of the basic pattern but one or two modifications can be seen in a small number of genera. In a few families the basic form is retained in only one or two genera, while several different modifications are seen in the majority of the others. Finally, several families have lost all vestige of the basic mandible pattern. In this latter group, however, the number of modifications is low. Most mandible modifications occur in response to predation and/or scavenging as a feeding strategy, although two independent pathways to this end are seen. In the first the reductions occur on a compact coxa, and seem designed for feeding on small prey, while in the second the mandible body (coxa) becomes elongate and the orientation of the incisor changes such that cutting is now in the vertical frontal plane of the animal's body. It is concluded that families with exclusively predatory/scavenging mandibles cannot be considered to be plesiomorphous.