COEXISTENCE AND EXCLUSION AMONG GAMMARUS SPECIES - BEHAVIORAL AVOIDANCE OF INTERSPECIFIC PRECOPULATION BY MALE-G PULEX (AMPHIPODA)

The significance of interspecific mate choice behaviour in explaining distributional patterns of Gammarus species is investigated. Male G. pulex from a pure population and from mixed G. pulex IG. duebeni populations were given a choice between G. pulex and G. duebeni females. Males discriminate between conspecific and congeneric females during grabbing and holding of the females, and this behaviour increases in frequency towards G. duebeni females the closer those females are to moulting. Male G. pulex with no prior experience of congenerics appear to require a more thorough period of assessment in order to determine species. However, G. pulex males rarely enter interspecific precopula guarding and appear not to copulate with G. duebeni females, thus avoiding the deleterious effects this would have on both male and female fitness. We conclude that, with G. pulex, proximate mechanisms of species discrimination preclude interspecific pairing and mating, which cannot therefore bc invoked to explain the superior competitive ability of this species over G. duebeni. The results are discussed in terms of the general applicability of the sterile interspecific mating hypothesis to explain patterns of coexistence, exclusion, habitat choice and displaced breeding periods among Gammarus species. We propose an alternative explanation based on mutual but differential predation by males on moulted females in precopula.