The effects of temperature on the physiology and locomotory behavior of the American lobster, Homarus americanus
Temperature has a pervasive influence on the physiology and behavior of all lobsters, and especially those found in habitats where temperature fluctuates the most. One population of lobsters undergoes a seasonal migration in the Great Bay estuary system in NH, entering the estuary in late spring and returning to the coast in the late summer and fall. This migration may be triggered by physiological or behavioral responses to seasonal temperature shifts. The first set of experiments examined the effect of temperature on heart rate in order to further understand the influence of temperature on lobster metabolism. Heart rate was measured in intact lobsters, lobsters in which the cardioregulatory (CR) nerves had been lesioned and isolated lobster hearts. The heart rate of intact lobsters increased significantly and linearly with increasing temperature. Results from lobsters with lesioned nerves were not significantly different from the trend observed in intact lobsters. Isolated hearts did not show a significant increase in beat rate with increased temperature and ceased beating between 20 and 25°C. These results indicate that, although some exogenous input must be involved in the heart rate response to temperature, the CR nerves are not the source of this input. Temperature avoidance behaviors were examined for both summer- and winter-acclimated lobsters. Summer-acclimated lobsters avoided water that was warmer than 22.3°C +/- 0.5 and there was a significant effect of sex, with male lobsters tolerating higher temperatures. Lobsters of all sizes exhibited similar responses. Winter-acclimated lobsters exhibited avoidance at 14.26°C +/- 1.1 with no influence of sex but a significant effect of size, with larger lobsters tolerating higher temperatures. Regardless of season, all lobsters exhibited avoidance after an approximately 8°C increase in ambient temperature. This suggests that an increase of this magnitude is aversive, possibly due to the simultaneous increase in rate of physiological processes that could place the lobster outside of its aerobic scope. Activity levels were monitored in juvenile lobsters under both light and dark conditions over a range of ambient temperature conditions from 2.5--17.8°C. Lobsters traveled significantly farther in the dark than in the light and there was also a positive correlation between distance traveled and water temperature. There seem to be two distinct "activity states", with consistently low activity below 12°C and higher but more variable activity above 12°C. There was no significant effect of sex or size on activity. These results correlate well with behaviors observed in the migratory population of lobsters in the Great Bay estuary system. Spring migrations generally occur when temperatures are entering the range associated with the higher activity state. Movement out of the estuary could represent an avoidance response to high temperatures in the upper estuary in late summer. It appears as if lobsters may be avoiding warm water because the associated increase in metabolism may cause them to exceed their aerobic scope.
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