PRECOCIOUS MATURITY OF THE MAJID CRAB, PUGETTIA PRODUCTA, PARASITIZED BY THE RHIZOCEPHALAN BARNACLE, HETEROSACCUS CALIFORNICUS

¹ Department of Biological Sciences, University of California, Santa Barbara, California 93106

Mean size of adult kelp crabs, Pugettia producta, parasitized by the rhizocephalan barnacle, Heterosaccus californicus, was significantly less than the mean size of unparasitized crabs. This size difference resulted from parasitized animals having passed through fewer instars before the molt of puberty. Laboratory and field evidence indicated that parasitized crabs matured earlier than unparasitized ones. Precocious maturity of parasitized crabs fitted a model which postulated that size at maturity was determined by the value of a size-threshold. Crabs would not undergo the molt of puberty unless larger than the size-threshold. The value of the size-threshold would be influenced by environmental factors which could vary seasonally within one locality. Precocious maturity can be explained as a greater sensitivity of parasitized crabs to the stimuli that induce maturation above the size-threshold. The implication of this study is that size differences among populations of adult crustaceans may be accounted for by variation in the number of juvenile instars as well as size-selective predation and differential growth.