Changes in sexual selection resulting from novel habitat use in the sand goby

Resource availability may determine local breeding systems and may also vary locally
between different habitats, affecting the way individuals distribute themselves between
these habitats. For nest-breeding fishes, nest site availability is a crucial
resource that has been found to affect local sexual selection regimes and thus
breeding systems. In this study, we compared the availability and size distribution of
nest sites and their consequences for habitat preference, fish distributions and the
breeding system in sand gobies (Pomatoschistus minutus) breeding in two different
habitats. The usual breeding sites of sand gobies are shallow sandy beaches, but here
we report their breeding in a novel environment, on rocky bottoms. We found
obvious differences between the two habitats. The density of occupied nests was
nearly 50 times higher in rock habitat than on sand bottoms. However, competition
for nest sites was stronger on sand; nearly all natural nest sites found were in use and
empty nest sites were occupied at a much higher rate on sand than in rock habitat.
In addition to the numerical differences, nests were larger and contained more eggs
in rock habitat than in sand habitat. Moreover, we found significant habitat-specific
differences in fish size distribution. The intensity of intrasexual competition as a
result of the degree of nest site availability explains the observed patterns within the
habitats, but not between them. These habitat-related differences are better explained
by the larger average size of nests on rocky bottoms. The effect of nest size on habitat
preference was confirmed experimentally by laboratory experiments. When nest size
did not differ between the habitats, gobies originating in the two environments
showed an equal preference for the sand habitat.