SEXUAL SELECTION AND DIMORPHISM IN TWO DEMES OF A SYMBIOTIC, PAIR‐BONDING SNAPPING SHRIMP

Abstract

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, 1972). 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.