Response by Rhizocephalan-Parasitized Crabs to Analogues of Crab Larval-Release Pheromones

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Abstract
During larval release in the crab Rhithropanopeus harrisii, the attached eggs release peptide pheromones that induce the female to undergo stereotypic larval-release behavior entailing vigorous pumping of the adbomen (pumping response). Rhithropanopeus harrisii is parasitized by the rhizocephalan Loxothylacus panopei, which develops a reproductive body (externa) in the position normally occupied by the egg mass. The pumping response by the host crab has been observed at the time of larval release by rhisocephalan parasites infecting other crab species. Similar behavior was observed in R. harrisii on one occasion. Parasitized R. harrisii show the pumping responses upon exposure to peptides that are similar to the pheromones released by eggs. Peptide sensitivity, however, is enhanced in the parasitized crabs in two ways: the threshold concentrations are lower and the types of active peptides expanded, from those having a neutral amino acid at the amino terminus and basic amino acid at the carboxy terminus (neutral-basic), to include acid-basic and basic-basic peptides. The preconditions for responsiveness to the pheromone peptides are apparently the presence of material on the underside of the abdomen and an internal sense (probably hormonal) that a crab is ovigerous, or pseudoovigerous as in the case of parasitized crabs. The peptide receptor site is not located solely on the antennules, dactyls, or chelipeds. The evidence suggests that peptides, similar to the normal egg-hatching pheromones, are transmitted from the parasite to the crab at the time of larval release.