Sexual Selection and Dimorphism in two Demes of a Symbiotic, Pair-Bonding Snapping Shrimp

Knowlton, Nancy

Highly polygamous and promiscuous species have been the subjects of most of the published studies on sexual selection (Bateman, 1948 [Drosophila]; Wiley, 1973 [sage grouse]; Le Boeuf, 1974 [elephant seals]; Parker, 1974 [dung flies]; Trivers, 1976 (Anolis]). Among monogamous species only the Arctic skua (O'Donald, 1972) and the feral pigeon (Burley. 1977) have been intensively studied with respect to the operation of sexual selection. Neglect of more monogamous species stems from the less spectacular differences between the sexes (Selander, 1972; Ralls, 1977) and from uncertainty as to whether sexual selection can operate in the absence of the potential for multiple matings (Mayr, 197 2 ). This neglect is unfortunate as monogamous species offer valuable opportunities for examining sexual selection when the costs and benefits of mating adaptations are more closely balanced. This paper provides evidence for sexual selection in the snapping shrimp Alpheus armatus (Caridea, Alpheidae), an obligate symbiont of the Caribbean anemone Bartholomea annulata (Clarke, 1955). Adult shrimp are typically found in male-female pairs, a fact which suggests a primarily monogamous mating system. Data on sexual dimorphism in behavior and morphology. on differences between two areas in sexual dimorphism, and on the natural history needed to interpret these patterns will be presented. The overall purpose is to show how differences between the sexes in the importance of mate choice and in the potential for multiple matings interact with environmental constraints to produce sexual dimorphism. The data presented here formed part of a larger study of A. armatus (Knowlton. 1976, 1978) done between 1974 and 1977 in Discovery Bay, Jamaica, West Indies. This work indicated that the species is territorial; juveniles defend their anemone against all conspecifics while adults generally tolerate the presence of an adult or the opposite sex. The shrimp are typically found near the column of the anemone under the shelter of the tentacles where they feed on detritus. Growth rates of the shrimp and both male and female size were correlated with the size of the host anemone. Large, unoccupied anemones were rarely seen, and were experimentally shown to be a resource limiting the density of pairs of shrimp. 0vigerous, paired females were found year round. Females are apparently unable to store sperm. as eggs laid without the presence of a male never developed.

Sexual Selection and Dimorphism in two Demes of a Symbiotic, Pair-Bonding Snapping Shrimp

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