Variable mate-guarding time and sperm allocation by male snow crabs (Chionoecetes opilio) in response to sexual competition, and their impact on the mating success of females

Two laboratory experiments investigated mate guarding and sperm allocation patterns of adult males with virgin females of the snow crab, Chionoecetes opilio, in relation to sex ratio. Although females outnumbered males in tre atments, operational sex ratios were male-biased because females mature asy nchronously and have a limited period of sexual attractiveness after their maturity molt. Males guarded females significantly longer as the sex ratio increased: the mean time per female was 2.9 d in a 2 male :20 female treatm ent compared to 5.6 d in a 6 male :20 female treatment. Female injury and m ortality scaled positively to sex ratio. Males that guarded for the greates t number of days were significantly larger, and at experiment's end had sig nificantly smaller vasa deferentia, suggesting greater sperm expense, than males that guarded for fewer days. In both experiments, the spermathecal lo ad (SL)-that is, the quantity of ejaculate stored in a female's spermatheca -was independent of molt date, except in the most female-biased treatment, where it was negatively related. The SL increased as the sex ratio increase d, mainly because females accumulated more ejaculates. However, similarly s ized males had smaller vasa deferentia and passed smaller ejaculates, such that, at a given sex ratio, the mean SL was 55% less in one experiment than in the other. Some females extruded clutches with few or no fertilized egg s, and their median SL (3-4 mg) was one order of magnitude smaller than tha t of females with well-fertilized clutches (31-50 mg), indicating sperm lim itation. Males economized sperm: all females irrespective of sex ratio were inseminated, but to a varying extent submaximally; each ejaculate represen ted less than 2.5% of male sperm reserves; and no male was fully exhausted of sperm. Sperm economy is predicted by sperm competition theory for specie s like snow crab in which polyandry exists, mechanisms of last-male sperm p recedence are effective, and the probability that one male fertilizes a fem ale's lifetime production of eggs is small.