Lovesick Crab Hot on the Odour Trail

With the antennae to the ground, the female shore crab hunts for prince charming. The olfactory organs are contained within the antennae of crabs. To the shore crab, odours and tastes form the letters of the language of love. The role of chemical communication in the love life of shore crabs has been charted in a new dissertation from Lund University in Sweden.

Mattias Ekerholm at the department of Cell and Organism Biology, Lund University in Sweden has studied the role of odours in shore crab mating. The female needs to find a male quickly, since she can only mate during a brief period directly after shedding her old shell ( i.e. moulting). She sniffs her way to a group of males competing for dominance. Following the scent trail of a dominant male, she also tries to avoid other males. Once near the male, she is overwhelmed by his odour, and rises up on tip-toe at the same time as she pumps her urine towards him. The female urine contains a pheromone that immediately makes the male respond. Mattias describes mating the following way:

– Now the male also starts walking around on tip-toe. With claws extended, he tries to look as impressive as possible. He has picked up the scent of the female. He then sniffs and tastes his way along her scent trail and, after a while he manages to find her.

– They circle around each other for a while, suddenly he rushes towards her and tries to grab and carry her beneath him. She first resists him to assess his quality. If he manages to hold her, she eventually stops resisting. He has passed the test and won the female.

The male then carries the female beneath him for between a few days to over a week. During this time, mating and insemination take place. To ensure that the male does not let her go while moulting – when she is most vulnerable – she releases another odour that reinforces the male’s determination to carry her.

Recently, the shore crab has become more common along the Swedish west coast. This is thought to be either due to decreased predation from cod, or increased abundance of filamentous green algae, the settling substrate of the larvae.

Unintentionally, the shore crab has been introduced to other parts of the world. In Canada, USA (where it is known as the green crab), and Australia, it is perceived as a pest species, where it eats or outcompetes native fauna, and damages aquaculture. Sexual pheromones, that is, chemical substances utilized for between-sex communication may be useful to manage these populations. This technique has been successfully applied to insect pest management.

– We still know very little about pheromones in marine animals compared to those used by insects. But in five years, there may be a reliable and effective means of utilizing pheromones in pest management. In that case I think it is better to direct interest to the male pheromone than the female, since it works to attract females over long distances. If you want to reduce the population, it is also more effective to trap the females, Mattias Ekerholm says.