Morbid behaviour of the commercial sand crab, Portunus pelagicus (L.), parasitized by Sacculina granifera Boschma, 1973 (Cirripedia: Rhizocephala)

. The behaviour of crabs Portunus pelagicus infected with the rhizocephalan barnacle Sacculina granifera is examined and compared with that of normal uninfected crabs. With regard to alarm reactions, feeding and locomotion, the infected crabs behave as normal crabs. Their stance, defaecation and burying behaviour differ in that they must accommodate to the physical presence of a large body, the parasite, suspended between cephalothorax and abdomen. General body care is increased as a result of the increased epizoic fauna on the exoskeleton which develops when moulting ceases. Infected crabs with an externa resemble ovigerous females in terms of their moderate daylight activity and particularly the grooming care and attention given to the sac—a surrogate egg mass. Parasitized crabs excavate a depression and using the third walking legs (and often second and first as well) attend the externa as an ovigerous female attends her egg mass. The grooming of the externa is of equal periodicity to egg mass grooming behaviour, but is less prolonged at maximum intensity. Sac grooming behaviour begins 18 days after emergence of an externa and declines progressively if the externa is removed and the interna degenerates. In interactions with other crabs parasitized individuals initiate fewer and dominate fewer encounters than normal crabs of similar size and sex; and parasitized males, at least, are less active when with other crabs than when alone. It is concluded the parasite secretes a hormonal mimic which induces ovigerous behaviour which maximizes the survival of the parasite population.