Sex in the Sea - Environmental effects on sexual selection in copepods and its feedback on population dynamics
Final Report Abstract
In the project ”Sex in the Sea” we aimed to explore the influence of the biotic and abiotic environment on sexual selection in marine copepods and its potential feedback on population dynamics. Sexual selection is a context dependent process. Mate choice and mate competition depend on environmental factors like the mate encounter rate, the variation in the partner quality and the costs of matings. By combining laboratory and modeling techniques we examined the potential influence of these processes and conditions in pelagic copepods. We were able to gain new insights in the mating behaviour and the importance of sexual selection in pelagic copepods. Several experiments were necessary to gather missing information about the reproductive biology of copepods. We looked at (1) pheromone tracking, (2) population density and sex ratio, and (3) predator presence. Besides this, we assessed the theoretical potential for and direction of mate choice using a life history model, which takes into account important factors known to influence the strength of sexual selection. (1) In several copepod species, females use pheromones to attract males and thereby increase their detectability. In other organisms like fish, pheromones can be influenced by the pH value, which with regards to the current ocean acidification might have a large impact on copepod reproduction. However we couldn’t find any influence of changes in the pH on the mate tracking behavior of copepods. The number of pheromone tracking behaviors was constant within a range of big range of pH values. Due to potential problems with our experimental approach we however cannot exclude any influence of ocean acidification. Pheromones also convey information about the location, and sometimes state of potential mating partners. We show that males of Oithona davisae change their swimming behaviour depending on the reproductive state of the females. This suggests that they are able to distinguish between virgin and mated females, which would drastically reduce their costs of mate searching. (2) Mate encounter rate is driven by the size of copepods and the population density in combination with the adult sex ratio. The adult sex ratio depends on male mortality during the adult phase, which in copepods is mainly induced by predators. Data from our experiments and literature was used in a life history model to determine the strength and direction of mate choice in pelagic copepods. The model suggests that sexual selection in copepods follows the ‘traditional’ pattern of choosy females and competing males. Population density and the need for remating in females are the strongest determinants of the strength of female choosiness in copepods.We also tried to find out whether having the ability to choose between males has any fitness implications for females. However, an initial analysis of the data did not reveal any such effects. (3) The presence of predators should reduce choosiness and the strength of sexual selection. In a mesocosm experiment we followed the population and mating dynamics of Eurytemora affinis to test if this is also a relevant mechanism in copepods. The data we gained in this experiment is still being processed and analyzed. The analysis for most experiments is still ongoing, but in many cases promising and interesting patterns seem to arise. Follow up experiments are planned with regards to the influence of changes in pH on female tracking behaviour, and the development of the model about spermatophore size in copepods is ongoing.
