Reproductive isolation in temperate reef fishes

Opportunity and recognition isolation can lead directly to reproductive isolation, the former via divergence in the location and timing of breeding, and the latter via differential mate preferences. We describe the potential significance of these factors in the maintenance of reproductive isolation in a clade of triplefin fishes that occur sympatrically around coastal New Zealand. Specifically, we investigate the roles of spawning time and nesting habitat in promoting opportunity isolation, and of interspecific variation in male body length and breeding colouration in promoting recognition isolation. The triplefin species investigated are reproductively active over several months and show high overlap in breeding times, thus rejecting temporal isolation as a mechanism. Differences in nesting habitats resulted in a reduced probability of encounter between some species, especially between sister-species pairs. Interspecific colour differences generally decreased during the reproductive period, and males of sister-species pairs showed no interspecific colour differences in the ultraviolet light spectrum, thus mate selection based on male colour patterns is unlikely to lead to premating isolation. Finally, males of closely related triplefin species differed in body length, a secondary sexual trait often involved in assortative mating. Thus, spatial differences in nesting habitats reduce the chances of encountering allospecific mates, which may facilitate opportunity isolation and differences in male length, possibly related to species-specific female selection on male body size, may lead to recognition isolation. The combination of limited spatial overlap in nesting habitat and differences in male body size may facilitate species assortative mating in sympatry or parapatry.