Causes of a non-random pairing by size in the brine shrimp, Artemia salina: (Crustacea: Anostraca).

Brine shrimp (Artemia salina) males and females entered precopula assortatively by size in the laboratory; large males also had a pairing advantage over smaller males. We investigated the causes of such nonrandom pairing to test hypotheses on size-assortative mating.We found precopulatory biases with respect to male size in the absence of direct competition among males (which produces pairing biases in other species). Large males encountered females significantly more often than did small males. Similarly, large females encountered males more often than did small females, but showed less 'willingness' than small females to enter precopula when housed with small males. Consequently, large females took longer than small females to enter precopula with small males. Although large males entered precopula readily with small females, such size-mismatched pairs appeared short-lived.We conclude that non-random pairing by size in A. salina is determined by several factors including: encounter rates between males and females of different sizes, female behavior, and time following initial pair formation. Our results are likely applicable to other species and can help explain variation for selection on size or other traits.