Sex and violence in lobsters - a smelly business

Abstract: The aim of this thesis was to study the chemical communication involved in aggressive and reproductive behaviours in the European lobster (Homarus gammarus). Both male and female H. gammarus established and maintained dominance, but the sexes used different strategies for dominance maintenance. Male losers recognised individual fight opponents and avoided them but fought actively against unfamiliar dominants. In contrast, female losers avoided both familiar and unfamiliar dominants, indicating that they react to the dominance status of the opponent. Unexpectedly, females used more high-level aggression than males. Blocking of the urine release in male lobster pairs with established dominance led to increased fight duration and increased aggression in a subsequent encounter, de¬monstra¬ting the importance of urine signals for dominance maintenance in male H. gammarus. Intruding American lobsters (H. americanus) have repeatedly been caught in European waters. Since the two species are closely related and have similar food and shelter require-ments, aggressive and reproductive behaviours and communication signals may be similar and result in both competition for resources and possibly hybridisation. Aggressive interac-tions between male European and American lobsters showed that interspecific communi-cation and dominance maintenance indeed occurs between the two species. Lobsters often reproduce when the female is newly moulted, but mating can occur at any time during the female moult cycle. Intermoult courtship and mating behaviours were common in European lobsters, unless the sense of smell (olfaction) was blocked in the male, indicating the presence of a female pheromone that induces mating. Female olfaction was not important for these behaviours. A morphological study of the European lobster antenna demonstrated unique sex differ-ences in size and distribution of the olfactory aesthetasc hairs. Females had more antenna segments with aesthetascs than males, and also had longer aesthetascs. In contrast, males had more aesthetascs per antenna segment, possibly compensating for the fewer number of segments with this type of sensory hair.