Nest building in a Mediterranean wrasse (Symphodus ocellatus): are the algae used randomly chosen or actively selected?

To increase their reproductive success, fish species have evolved various strategies, including both simple processes and more complex mechanisms that involve parental care by nest-building. In Symphodus ocellatus, a species that lives in rocky infralittoral zones of the Mediterranean Sea, during the reproductive period, the male builds a nest within a hole or crevice using fragments of algae and/or sand particles. Data on the nesting activity of wrasses are rare, and previous descriptions regarding the algal composition of nests have provided conflicting results. In this regard, it is unclear whether territorial males actively select algal species for nest construction or algal choice is random and depends exclusively on the algal species available in the surrounding habitat. To clarify this, several S. ocellatus nests and the algal community near the nests were sampled. The specific algal composition of the nest, the Strauss selectivity index of the used algae, the mechanical strength and the ability of the algae to accommodate eggs were investigated to examine overall nest performance. Our findings show that the dominant male ocellated wrasse deliberately selects algae when building the nest. The mechanical strength and ability of the algae to accommodate eggs is of particular importance. Among the species commonly used for nest-building, Jania rubens performed the best. Our results show that J. rubens provides resistance to biochemical decomposition and wave action.