Behavior of lobsters (Homarus americanus) in a semi-natural environment at ambient temperatures and under thermal stress

Abstract

In January, 1974 we established semi-natural habitats in two 10ft. diameter, octagonal aquaria, with five lobsters (Homarus americanus) each, and several Cancer irroratus, Anguilla rostrata, Pseudopleuronectes americanus and Tautoglabrus adspersus. The lobsters, with respect to size and sex, were identical as possible between tanks, as were the numbers of other species. The aquaria, which received ambient seawater, were arranged identically with an oyster shell substrate, and cement blocks, rocks and ceramic pipes to provide a surplus of shelters. Observations, spanning from February through August, were made both during the day, following feeding, and (using red light) just after sunset, when lobsters are active under natural conditions. Types of behavior we were able to quantify included occupation of specific shelters, feeding, activity and social behavior. In our large aquaria the lobsters appeared to be much less aggressive than generally has been reported. Aggression was most frequent during feeding. Observations at night revealed few encounters, and these were usually either one sided avoidance without pursuit, or mutual ritualized displays. Neither an animal's size nor sex seemed to determine its relative dominance. Dominance shifted somewhat between different animals during the study, and complicating this picture was possible territorial behavior in the larger individuals. In one tank, only the two adult females were territorial from February through mid May, following which no lobster showed stability of residence. In the second tank, only one animal, a female, was territorial for more than several weeks, until early June, when the largest male established a reproductive territory lasting until the end of August. Even in our large aquaria space may have been too limited for all animals to be territorial. Lobsters appeared to lose their position in the hierarchy just prior for up to a month or more following the molt. Such animals were often observed on top of shelters, in exposed locations, where other lobsters apparently did not harass them. Although captive lobsters are considered quite cannibalistic, we lost only one animal, a juvenile female, out of six molts. In our large aquaria, female lobsters about to molt sought out, took up residence, and actively courted the tank's largest male. The males were very non-aggressive toward these females, and yet during this period made violent attacks against other males as well as fish. In each case following mating, the males retired to the shelter and fed on the cast shell. Cohabitation, in or around the males' residences, continued for several days following mating. Diurnal activity, which was evoked by the presence of food, showed little change over the range of 5-28°C. Nocturnal activity, vihich was more spontaneous, was similar in both tanks through mid June (temp. range 5-18°C). The level of activity was as high in late February - early March as in late May, with a dip in activity in late March - late April, a period marked by storms. From mid June on, the nocturnal activity in tank I increased with the increasing temperature, leveling off approximately vihen the peak temperature of 28°C was reached. In contrast, activity in tank II did not increase at temperatures above 20°C, and remained at a much lower level than in tank I. Although patterns of residence and dominance in the lobsters changed seasonally, the direction of change was rather different in each tank and did not seem correlated with temperature. Other factors, such as molting and loss of dominance prior to mating in previously aggressive females, were probably more important than temperature effects. The frequency of temperature range 22-28°C was similar to levels at ambient temperatures. Interspecific relations between lobsters and the other species were mainly pacific, although predation on Cancer by Hi. americanus may have occurred. The response of the eels (Anguilla rostrata) to temperature increases was consistent between tanks. Swimming was first observed at 8°C, and feeding at 10°C. Further, the eels in both tanks became markedly aggressive when the temperature reached 26°C.