Chemical communication in mating shore crabs Carcinus maenas

Abstract: Locating and evaluating a suitable partner for mating is one of the most important events in any sexually reproducing organism. To achieve this, an animal must be able to locate its partner over both long and short distances, and finally evoke the proper mating behaviour. This is made possible by use of conspecific signals picked up by one or several sensory systems, the most well investigated being visual, acoustic, tactile and olfactory.

This thesis presents evidence that the shore crab Carcinus maenas utilize pheromones and other chemical signals for mating. Briefly, I have studied male and female responses to odours and urine from both sexes in long-range, near-range and at contact range.

Males are attracted to female urine pheromones. When concentration and spatial scale are varied, the same chemicals (urine) evoke a chain of different behaviours. At close distance and high concentrations pairing-related behaviours are evoked, and search-related behaviours and display are abandoned. Conversely, low concentrations on longer distances show more search and display and no pairing behaviour.

Response variability plays a significant role in modifying behaviour in this species. I have shown that much of this can be attributed to a novel primer pheromone, which increases male receptivity to the female pheromone. Female moult stage also affects male responses at contact-range, but not at near-range. Male status also affect his search, but not pairing behaviour.

The female pheromone has due to the lack of long-range properties not been able to explain how males locate females over a distance. Instead, present evidence suggested that the pattern may be reversed, with females of the right moult stage locateing males over long distances. The females are able to discriminate between male and female odour and search preferentially for males. The pheromone in male urine evokes several pairing-specific behaviours in females, similar to the case of males and female pheromones.

As a final step in linking pheromone communication to the lek mating system, we show that females are able to identify male status by odour, and preferentially search for the dominant male. Females previously exposed to a fight between the dominant and subordinate male also showed higher preference for the dominant, than females that were unfamiliar with the males.

This dissertation shows that chemical communication during the mating period is of utmost significance for a marine species, the shore crab. Apart from this I have shown that complex behavioural chains can be evoked by the same cue, presented at different spatial context and concentration. I have also shown how important it is to know the mating system of the species to be able to identify the steps where pheromones and other chemical signals affect behaviour such as search and choice of a partner.