Swarming behaviour in Aplysia

Dear Steven,
The basic question is not what makes them congregate but whether they congregate at all. My understanding of the literature is that there is no evidence that Sea Hares congregate or migrate - except as a chance larval swarm. Although at times it seems that large numbers of adults have 'just' arrived at a certain locality, this appears to be an illusion. Juveniles have very good colour camouflage, and grow very quickly. Unless an observer is meticulously surveying a locality on a regular basis, it is not unusual to not notice a large Aplysia population until they have reached adulthood. Observations can also be complicated by the phenomenon of mass mortality, where large numbers of animals can be seen washing up on beaches to die. These are usually from subtidal populations, usually near the end of their life-cycle being washed ashore because of prevailing weather conditions. Basically a 'swarm' of Sea Hares differs from a swarm of bees. In bees it is a pheromone driven part of their behaviour, in Sea Hares, it is a result of them growing up together in dense populations.

Have a look at the Mass Mortality Page for more discussion on this topic and relevant links.
Bbest wishes,
Bill Rudman