SIZE-ASSORTATIVE PAIRING IN THE BIG-CLAWED SNAPPING SHRIMP, ALPHEUS HETEROCHELIS

Abstract The big-clawed snapping shrimp, Alpheus heterochelis, is found in size-matched male-female pairs. A common assumption is that a large size advantage in males, an extended pair bond, and a positive correlation between female size and fecundity promotes size-assortative pairing. Since all of these conditions apply to A. heterochelis, we investigated size-assortative pairing in the laboratory by designing experiments in which the pairs were size matched or mismatched using three size categories; large, medium and small. We found that snapping shrimps prefer to pair according to size and that such pairing is stable. We also found that, contrary to the initial assumptions, female-female competition and female choice for large males plays the main role in the pair formation process. Smaller, and hence less competitively successful, females will pair with smaller males, but in a male-biased environment, will preferentially pair with larger males. Large males provide the best protection from shelter eviction, which is crucial for female A. heterochelis, as their reproduction is closely associated with moult, and therefore requires shelter and a male present for successful reproduction.