Nest site choice in two-spotted gobies (Gobiusculus flavescens)

Many species, including the two-spotted goby, use natural cavities as nest sites, which they defend against competitors and predators. Studies of nest site preference in fishes can give valuable insights into their reproductive success, because the choice of nest can influence the fitness of the chooser and its mate. Even if there is an abundance of nest sites, high quality nest can be a scarce resource. Previous work on several goby species has shown that males often compete for high-quality mussels. However, two-spotted gobies are unusual in utilising two very different nest substrates: empty blue mussels (Mytilus edulis) and natural cavities (e.g. fronds) in the kelp vegetation (mostly Laminaria saccharina and L. digitata). Mussel beds and kelp co-occur many places, but not everywhere. To understand the reproductive dynamics of two-spotted gobies it is important to know whether there is a scarcity on preferred nests in nature and, accordingly, the potential for male-male competition over preferred nests. Two-spotted gobies live in areas where potential kelp nesting sites appear to far exceed numbers of breeding two-spots. If this is a preferred nest type there should not be much male competition. However, field observations suggest that blue mussels are locally a more scarce resource. If such nests are preferred over algae there should be more male competition. In this study we let male two-spotted gobies choose between two potential nest types: empty blue mussels and fronds/cavities in L. saccharina. The study is done in the lab, using tanks with a small blue mussel bank (5 live, 5 empty mussels) on one side and a suitably sized L. saccharina on the other side. On day 1, we let the male in and observe where he swims. On day 2, we again observe the position of the male when alone in the aquarium before we add a female confined in a jar as a nesting stimulus and perform a third observation of male behaviour and positioning. Thereafter, the female is let out of the jar and the behaviour and position of the fish, including the site of any egg-laying, is observed this and the following day.