On the shape and posterior ornamentation of the third epimeral plates of gammaridean amphipods (Crustacea): decorative flamboyance or plain adaptation?

The functional significance of the widely divergent shapes and ornamentation of the third epimeral plates (E3) of gammaridean amphipods has been analysed statistically using the British fauna (supplemented by the Norwegian) as a model. Amphipod species have been categorized within a framework of 3 shape categories and 4 ornamentation types and correlations derived between the pattern of species allocated to these categories and species body size and life-style differences. Small species tend to have acute, unadorned E3 and lead active lives; medium-sized species tend to be covert (tubicolous, sediment infaunal), with rounded, unadorned E3. In this way tubicolous species avoid inadvertent tube damage. Large species tend to be free-swimming, or otherwise overt, in habit and to have quadrate E3 often distally excavate: perhaps as predator protection. Multi-excavate E3's are found in species of all sizes. This pattern though is particularly associated with epifaunal, fast-swimming species and may have selective value either in predator deterrence or in reduction of wake turbulence.