Impact of parasites on the reproduction and fecundity of the blue sand crab Portunus pelagicus from Moreton Bay, Australia

The rhizocephalan Sacculina granifera had a marked effect on the gonad development and growth of its host Portunus pelagicus (L.).Crabs were frequently castrated by the parasite, but, in some cases, castration was incomplete.That is, the reproductive potential of the ~nfected host is not necessarily zero.Infected hosts were also capable of n ~a t i n g and, in a few cases, actually produced egg clutches.The impact of mating between uninfected and castrated crabs has not been previously established; sterile matings represent a further cost to the host population.Crabs were susceptible to infection by the parasite at any size/age, but juvenile crabs were more frequently infected with sacculina interna than mature and last instar adults.The abundance of the e g g predator Carcinonemertes mitsukurii was correlated with salinity and temperature, but not with egg mortality on the host.Hence, the worms had little impact on host fecundity.The nicothoid copepod Choniosphaera indica was the major source of egg mortality on P. pelagicus.Mean mortality d u e to the copepod was 2.4 % with a range of 0 to 19.6 %.While the copepod may b e capable of rapid development and infection of crab clutches, it had a negligible effect on host fecundity because it occurred at relatively low intensities (1 to 1821 copepods host-').The parasite and egg predators exhibited seasonal patterns in their infections of the host population.To assess the impact of the parasites on the reproductive biology of P. pelagicus, we examined crab maturation, mating, molting, gonad development, fecundity, and brood mortality.Crabs mature at 79 to 90 mm CW (carapace width, inside the epibranchial s p ~n e s ) .The molt increment and percentage change in s u e between molts did not vary between sexes for the carapace width but did vary between sexes for the chelar merus length.Seasonal patterns were found in the molting, mating, gonad development, breeding, and sex ratios of crabs.