Mating behaviour, female receptivity and male–male competition in the intertidal crab Hemigrapsus sexdentatus (Brachyura: Grapsidae)

Abstract

Female receptivity and male-male competition were studied in a laboratory population of the New Zealand rock crab Hemigrapsus sexdentatus. Mating only occurred in the few days prior to oviposition, when females had mobile gonopore opercula. After mating, males guarded females constantly and mated with them repeatedly until the females laid eggs. Isolated females remained receptive for significantly longer than females housed temporarily or constantly with males in the lab- oratory. The duration of female receptiveness can, therefore, vary in relation to the presence of males. Male-male competition was high with frequent attacks on pairs by other males. Large males mated significantly more often than medium and small males, and were more likely to take over a female from another male. However, large and small male H. sexdentatus both adjusted the ejaculate size according to the size of the female; i.e. transferred relatively large ejaculates to large females, and were equally able to transfer ejaculates to small and large females during the first mating in the sea- son. Post-copulatory guarding reduced the risk of sperm competition. Although male-male competi- tion appears to be the dominant factor in pair-formation, the ability of females to extend their recep- tivity in the absence of males, will have an impact on the operational sex ratio, and on the extent of male-male and sperm competition.