No abstract

The feeding responses of decapod crustaceans to chemical stimuli have most often been evaluated in terms of one defining act, ignoring the organization of the behavior. To gain greater insight into foraging behavior, we considered the organization of food-search behavior in evaluating the responses of two species of crayfishes to a feeding stimulant. We also examined the effects of food deprivation on the behavioral organization and whether a behavioral dichotomy exists between food search and feeding behavior in these species. Individual crayfish of the species Procambarus clarkii and Orconectes rusticus were presented with infusions of a feeding stimulant consisting of a supernatant leachate of 100 ml water and 1 g of fish flakes. The stimulant was injected with a syringe and small-bore plastic infusion tubing into the center of a behavioral arena 25 cm square and 15 cm deep. Total injection time was 20 s. Experimental groups were presented with either the full-strength leachate (100%) or one of five dilutions: 75%, 50%, 25%, 10%, or 0% (controls) of full-strength. The feeding stimulant was presented either the day after the crayfish were fed or after one week of food deprivation. We analyzed three components of food-search behavior--detection, probing (near-field search), and locomotion (far-field, or distant, search)--recording the order of occurrence and the latency time to initiation for each behavior. When presented with the stimulus following regular feeding, both species responded to concentrations {ge}50% full-strength with probing behavior (near-field search) prior to locomotion, and to concentrations <50% full-strength with locomotion prior to, or even in the absence of, probing. Detection always occurred first. These results indicate that chemical stimuli preferentially activate distant food search in both species and that a behavioral dichotomy exists between food search and feeding behavior. One week of food deprivation had no effect on the organization of food-search behavior in P. clarkii; however, groups of unfed O. rusticus presented with 25% and 10% full-strength concentrations probed prior to locomotion, indicating a change in behavioral organization.

Sublethal behavioral effects of heavy metals on crayfishes may adversely affect their survival in the environment. They may experience increased predation due to changes in their ability to remain in or under shelter. The effect of sublethal cadmium exposure on the ability of juvenile Orconectes rusticus crayfish to remain in shelter was evaluated. Four different treatment groups were used (N=12 juveniles each): a control group not exposed to cadmium, and three experimental groups exposed to 1, 2, or 3 mg Cd/L for 7 days. Crayfish were placed, individually, into small glass aquaria containing 3L of laboratory water, with continuous aeration. Each crayfish was provided with a dark, thigmotactic shelter. Cadmium was introduced into the aquaria on days 1 and 4 to establish and maintain the nominal concentrations. On days 5 through 7, observations were taken on each crayfish five times per day, with a minimum of 30 minutes between observations (15 observations per crayfish). The locations of the animals were recorded as in cover or in the open area of an aquarium. Repeated measures ANOVA was used to examine the effects of sublethal exposure to cadmium on shelter use by the juveniles. Juveniles in the control groups were in the shelters for 92.7% of the observations. Results from the experimental groups indicate a significant decrease in shelter use compared to the control group, suggesting hyperactivity induced by the cadmium exposure. Over the 3 days of observations, those in the 1,2, and 3 mg Cd/L exposure groups used the shelters 57.7%, 56.3%, and 32.0%, respectively, indicating hyperactivity induced by cadmium exposure. Looking at the daily results, those juveniles in the 1 mg Cd/L group increased shelter use from 52.0% to 64.0%, then decreased to 57.0%, while those in the 2 mg Cd/L group increased shelter use from 45.0% to 79.0% of the observations by day 3 of the observations indicating some habituation to the cadmium or recovery from the cadmium exposure. Those in the 3 mg Cd/L group spent progressively less time in shelter over the 3 days of observation, from 39.0% to 25.0% then to 32.0% by the third day, indicating sustained hyperactivity due to the sublethal cadmium exposure.

No abstract

The cover-seeking behavior of juvenile and adult crayfish, Orconectes rusticus, was examined in relation to the relative importance of darkness and thigmotactic cues. Crayfish (N = 12 per experiment) were observed in individual aquaria 5 times per day, with at least 30 min between observations, for a 3-day period. Their positions were recorded as either within or outside the provided cover(s). When presented with a choice between a clear thigmotactic cover or open area (Experiment Ia), adult crayfish were observed in the cover significantly more often. Juvenile crayfish were observed in the open area significantly more often, but burrowed into the gravel to provide a measure of darkness (shadow) and tactile sensation. Experiment Ib presented crayfish with a choice between a dark thigmotactic cover and an open area; both adults and juveniles were observed in the cover significantly more than in the open area. In Experiment II, crayfish were given a simultaneous choice among a clear thigmotactic cover, a dark thigmotactic cover, and open area. Both age groups were observed in the dark cover more often than the other 2 conditions and were never observed in the clear cover. Although a clear cover was acceptable to adult crayfish (Experiment Ia), it was not acceptable to juveniles. Adults appeared to prefer dark cover to clear cover; juveniles altered their environment to provide shadowed cover, rather than utilize a clear cover. Thus, juveniles also appear to prefer "darkness" and will alter their environment accordingly. In Experiment III, crayfish were given a simultaneous choice among: a reduced-thigmotactic dark cover; a clear thigmotactic cover; and open area. Adults chose the large dark cover significantly more than the open area, but not significantly more than the clear thigmotactic cover. Adults that used the clear thigmotactic cover pulled gravel into their shelters, thus providing a measure of darkness. Juveniles chose the large dark cover significantly more than either the clear thigmotactic cover or the open area. Darkness appears to be the controlling factor in cover-seeking of juveniles; adults will modify cover providing only thigmotactic cues when dark thigmotactic cover is absent. Darkness plus thigmotactic cues, however, is the most preferred cover for both age groups.