Assessment games in shore crab fights

Theory on the sequential assessment model (SAM) has frequently been tested by using swimming crabs of the family Portunidae. Predictions on the effect of resource value and resource holding power were not met for all species and not even within a single species. They also were not tested simultaneously in a single study. Moreover, alternatives for SAM have been proposed and existing experimental tests fail to discriminate between them. Taken together, this calls for a reassessment of the assessment game in portunids. We staged fights between shore crabs (Carcinus maenas) because this portunid gave rise to the sharpest contrast with the sequential assessment theory. Pairs of shore crabs of different sizes were allowed to feed on mussels (Mytilus edulis) offered in different densities. We tested effects of mussel density, competitor size and absolute body size on contest duration, intensity of fighting, fighting rate (no. of displayed contests per min) and winning frequency. Fighting rate increased with increasing competitor size, and was not affected by mussel density and absolute body size. Winning frequency decreased with increasing competitor size and was not affected by mussel density. Contest duration increased when competitors were size-matched, independent of their absolute body size. Contest duration also increased with decreasing mussel density, suggesting that the value of a mussel item (resource value) increases with decreasing mussel density. Results were tested against predictions from different assessment games and were partly supportive of self-assessment and partly supportive of the mutual assessment game SAM. However, contrary to SAM, crabs engaged in pushing contests where they assessed resource holding power along a continuum, rather than in discrete sequential phases. (C) 2007 Elsevier B.V. All rights reserved.