Sex on the Beach: Ecology, Sexual Conflict and Evolution in New Zealand Seaweed Flies

Abstract

This thesis investigates several aspects of seaweed-associated Diptera ecology, reproduction and evolution. Wrack is composed of washed up seaweed, kelp and other items of jetsam. It is rapidly colonised and utilised by a combination of marine and terrestrial invertebrates of which Diptera are a dominant component. I sampled the Diptera community at several beaches around the coast of New Zealand. Community composition and individual species were described and their diversity, presence and abundance was investigated in relation to different environmental variables. Large wrack piles had more species than small piles. Differences were found in the Diptera assemblages between sites on the North and South island and on differing seaweed types. The greatest effect on the presence and abundance of Diptera species was pile size. Seaweed flies (Diptera: Coelopidae) breed in the ephemeral and dynamic wrack environment and all show considerable conflict over mating. Sexual conflict occurs when males and females utilise differing tactics to increase reproductive success owing to the differing investments of each gender in reproduction due to aisogamy. I observed the lab rearing ecology of Coelopella curvipes, Chaetocoelopa littoralis and Baeopterus philpotti. The mating behaviours performed by these 3 previously unstudied species of Coelopidae were documented as well as the effects of male and female size on mating interactions. Also reported is the successful rearing procedure for C. curvipes and C. littoralis along with descriptions of the mating behaviors observed in these 2 species as rearing attempts for B. Philpotti failed. As reproduction is more energetically costly for females, they may resist male mating attempts resulting in sexual conflict. Several members of the Coelopidae family exhibit strong conflict over mating where females perform a range of resistance responses involving a combination of Kicking, shaking and abdomen curling. I used both Parsimony and Maximum likelihood analyses to explore the evolution of resistance behaviours in the Coelopidae. There was strong evidence for phylogenetic signal associated with these behaviours suggesting they arose in closely related species. Maximum likelihood reconstructions suggest a plesiomorphic (ancestral) origin for kicking and shaking in this group and an apomorphic (derived) origin for upward abdomen curling, suggesting it is a novel behaviour to this group. The findings of this thesis are unique in that they investigate and document aspects of seaweed Diptera ecology, reproduction and evolution previously not studied and lay the foundations from which further research can be undertaken.