Abstract

The stream and lake margin crayfish Orconectes virilis is nocturnal, intolerant of neighbors, and defends individual crevices in the rubble of the substratum. We report on the effect of enforced crowding on this behavior based on two experimental approaches. In laboratory aquaria, groups of four crayfish were increased in size, with additional animals at each half hour, to a density of 12. From initial spacing, the crayfish tended increasingly to form passive aggregations. During this period of increasing density, agonistic behavior immediately increased but subsequently decreased. Corners rather than space seemed to be the resource in short supply. In larger tanks, a paucity of 15 crevices was available for 45 animals, allowing single occupancy for only one third of the total. A control set was observed Where a plethora of 20 crevices was available to 15 animals, allowing single occupancy for all. In both experimental and control sets, these nocturnal animals were out wandering during the night but crevices were occupied at dawn. In the controls, single occupancy prevailed among the 96% in crevices, while in the experimental set, multiple occupancy prevailed among the 84% in crevices. Here again high density relative to crevices altered behavior and passive aggregation in crevices resulted. This laboratory demonstration of a behavior shift may reflect the seasonal periods in a stream where the stream bed is reduced in area and the carrying capacity is stretched by such a behavioral shift to passivity and multiple crevice occupancy.

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