Stress and growth response of juvenile crayfish to rhythmic and arrhythmic temperature fluctuations.

Growth, molting, and survival of juvenile crayfish, Procambarus a. acutus, were recorded under constant and variable temperature regimes. Crayfish, were exposed to four rhythmic regimes: Constants temperatures of 10, 17.5, and 25 degree C, and a daily square-wave cyclic regime of 10 to 25 degree C (mean = 17.5 degree C); and to one arrhythmic temperature regime: A daily square-wave acrylic regime varying within 10 to 25 degree C limits (mean = 17.8 degree C). The direction and magnitude of temperature changes in the acyclic variable temperature chamber were selected randomly to make temperature changes unpredictable within the specified range. Rhythmic and arrhythmic temperature regimes had different effects on growth, molting, and survival of crayfish. Growth under both variable regimes was similar to growth at a constant temperature (17.5 degree C) equal to the mean of the variable regimes. Molt frequency was directly related to temperature. Growth rates approximately paralleled changes in the adjusted molt rate for each regime. Mortality under variable regimes approximated mortality at a constant temperature (25 degree C) equal to the upper extreme of the variable regimes. The cyclic regime tended to be less optimal for growth and survival than the acyclic regime, possibly because of differences in average daily temperature fluctuations and relative time spent at temperature extremes.

Laboratory experiments to determine if crayfish can communicate chemically in a flow-through system.

The importance of chemical cues for transmitting information concerning sexual identity, agonistic state, and stress-related condition in the crayfishProcambarus clarkii (Girard) was examined in a flow-through system. Experiments tested the effects of "conditioned water" from stimulus tanks on the behavior of solitary male or female crayfish. Twenty males and 20 females were subjected to a random sequence of five treatments: unconditioned water (control), conditioned water flowing through tanks containing a solitary male or female, and conditioned water from tanks holding either two males or two females. Durations of the following behaviors were recorded: chelae up, chela(e) in baffle hole, chela waving, climbing, digging, grooming, gross body movement, and meral spread. Results indicated that crayfish chemically detected another animal within 0.25 m without additional visual or tactile stimuli; however, crayfish apparently did not "communicate" information on sexual identity, agonistic state, or stress condition, nor does this detection necessarily imply discrimination between stimuli from crayfish and other taxa (e.g., fish). Our conclusions are contrasted with the two previous reports on chemical communication in crayfish in which experimental animals were tested in static systems. We suggest that a temporal separation of molting and copulation and a long reproductive receptivity period for females (which would allow abundant intersexual encounters) could account for a lack of selective pressure to evolve long-distance sex pheromones.

Predator-prey relationships among larval dragonflies, salamanders, and frogs.

Tadpoles of the barking tree frog, Hyla gratiosa, are abundant in spring and summer in some ponds and Carolina bays on the Savannah River Plant near Aiken, South Carolina. To determine how these tadpoles survive in the presence of predaceous salamander larvae, Ambystoma talpoideum, and larvae of an aeshnid dragonfly, Anax junius, we determined fields densities and sizes of the predators and the prey and conducted predation experiments in the laboratory. Tadpoles rapidly grow to a size not captured by Ambystoma, although Anax larvae can capture slightly larger tadpoles. Differing habitat preferences among the tadpoles and the two predator species probably aid in reducing predation pressure. Preliminary work indicates that the tadpoles may have an immobility response to an attack by a predator. In addition, the smallest, most vulnerable tadpoles have a distinctive color pattern which may function to disrupt the body outline and make them indiscernable to predators.

Effects of chronic cadmium exposure on crayfish survival, growth, and tolerance to elevated temperatures.

Crayfish Cambarus latimanus were subjected over a 5-month period to 0.02, 5, and 10 microgram Cd/L in a continuous flow system at ambient winter temperatures. Uptake of cadmium, growth, and mortality were measured. One-half of the surviving crayfish were analyzed for Cd after completion of the 5-month experiment, while the remaining half were removed from the contaminated water and subjected in a static laboratory experiment to stepwise, daily increases in temperature. The results indicated that (1) the rate of Cd uptake in crayfish was positively related to external concentration, (2) mortality increased significantly with higher Cd concentrations, and (3) neither growth nor thermal tolerance was significantly related to the Cd concentration in the crayfish tissues. The importance of chronic, low-level studies is discussed.