THE BEHAVIOR OF LOBSTERS IN RESPONSE TO REDUCED SALINITY

Two experiments were conducted to measure the behavioral responses of lobsters, Homarus americanus (Milne-Edwards), to reductions in salinit y. In the first experiment animals were placed in a 3 ft diameter tank that was divided in half by plastic mesh. Spontaneously active lobste rs were able to move between the two halves of the tank by passing thr ough either of two conduits. The conduits were equipped with optical s ensors to monitor the passage of animals, and a perfusion system to co ntrol the salinity of the area in, and around, the conduit. When the s alinity in the vicinity of both conduits was the same (28-32 ppt), lob sters exhibited no preference for either conduit. However, when the sa linity in one of the conduits was lowered, lobsters preferred to pass through the high salinity (20-25 ppt) conduit rather than the one with low salinity (10-15 ppt). In addition, females appeared to be more se lective in their preference and exhibited higher overall activity than males when exposed to reduced salinity. In the second experiment, ind ividual lobsters were placed in a shelter at one end of a long seawate r table and exposed to seawater of gradually decreasing salinity. The salinity required to cause a movement out of a shelter, i.e. an avoida nce response, was recorded. On average, lobsters first ventured small distances (<one body length) out of their shelter when the salinity re ached a level of 18.4 ppt +/- 1.42 (SEM), and definitively moved away from their shelter (> one body length) when levels approached 12.62 pp t +/- 1.59. Although it was not statistically significant, females aga in seemed to be either more sensitive to salinity or found it more ave rsive, because they tended to initiate movements at salinities greater than those required to influence males. These behavioral data indicat e that: (1) adult lobsters are capable of detecting changes in salinit y which are comparable to the levels found during natural fluctuations in coastal bays and estuaries; (2) when exposed to low salinity of su fficient magnitude, they attempt to avoid it, and; (3) females appear to be more sensitive to drops in salinity and/or they find it more ave rsive. Previous studies have demonstrated that estuarine lobster popul ations are dominated by males and that there are seasonal migrations o f lobsters into, and out of, estuaries. We conclude that the behaviora l responses of male and female lobsters to low salinity may determine, in part, the distribution and movements of lobsters in estuarine habi tats.