The mechanisms and consequences of oviposition decisions in the European bitterling

Abstract

Oviposition-site decisions can have disproportionate effects on offspring survival and success, and while the effects of these decisions are frequently investigated, the processes underpinning these decisions and the cues used are often less well understood. The aim of this thesis was to understand the mechanisms associated with making oviposition decisions, and their consequences, using the European bitterling (Rhodeus amarus), a small freshwater fish that lays its eggs in the gills of freshwater mussels. Using an artificial mussel, females were shown to respond to dissolved oxygen, relating to offspring survival, while males attend to water flow velocity, with implications for sperm competition. Oviposition decisions by either sex were unaffected by olfactory cues associated with sperm release, though males and females were responsive to visual cues associated with ejaculation. An analysis of the placement of eggs within the mussel gills by females failed to show evidence of “handedness”, though there was a tendency to place eggs in a mussel’s inner gills, which may be adaptive in avoiding competition with the mussel’s embryos. Male response to oviposition sites showed inter-population variation, with males from a population with high levels of mean crowding showing an elevated frequency of mussel inspection and probability of ejaculation. Males also increased the frequency of mussel inspection in response to the presence of a gravid female, suggesting a role for sperm competition and fertility assurance. Social network analysis was used to investigate how male interactions affect oviposition decisions. This approach showed that male mating tactics could be understood by quantifying how males distributed their sperm among oviposition sites. Findings are discussed in the context of our understanding of the bitterling mating system and research on oviposition-site decisions.