The effect of habitat composition on sexual conflict in the seaweed flies Coelopa frigida and C. pilipes

Despite the recent explosion of interest in sexual conflict, the effect of environmental conditions on the intensity of sexual conflict within populations has been largely ignored. Reproductive encounters within coelopids are characterized by sexual conflict in the form of intense harassment by males, usually resulting in a vigorous premating struggle. We investigated the effect of habitat composition and duration of exposure to oviposition sites on the level of sexual harassment by males and mating success in two species of European seaweed flies, Coelopa frigida and C. pilipes. The wrack beds inhabited by these two species are dominated by two genera of brown algae, Fucus and Laminaria, the relative proportions of which can vary considerably between wrack beds. Fucus is known to stimulate harassment by males, increase copulation duration and induce females to oviposit in both species. In this study Laminaria stimulated a higher level of harassment by male C. frigida than Fucus did. However, a similar effect was not observed in C. pilipes, with the main additional factor affecting harassment in this species being the age of the male. Our study highlights the potential importance of environmental conditions on the intensity of sexual conflict within a population. We discuss the evolutionary significance of these observed effects in seaweed flies.