Photoregulatory behavior of bluegill,Lepomis macrochira,in a virtual light gradient

Synopsis Vertical movements of bluegill were monitored in gradients of light intensity to assess this fish's photoregulatory ability and mechanisms. A computerized monitoring and control system created virtual gradients of light intensity by adjusting an overhead lamp's output in response to fish movements, in a vertical tube, to produce a programmed intensity at the fish's depth position. This approach separated the process of gradient formation from normal clues for photoregulation and allowed formation of light gradients incompatible with natural taxic responses to intensity. Hourly shifts in gradient position minimized the possibility of confounding photoregulation with position regulation. Observed patterns of movement reduced the extremes of light intensity to which bluegill were exposed, compared to no movement or random movement. Seven fish were tested, producing 10 experiments. In 4 of 10 experiments, the fish effectively photoregulated in gradients in which light intensity decreased with depth, as in natural habitats. In 1 of 10 experiments, the fish photoregulated in an inverse gradient, with intensity increasing with depth. Evidence of regulation in an inverse gradient suggests that normal taxic responses are not essential for photoregulation in bluegill.     

Responses of Littoraria irrorata say (Mollusca: Gastropoda) to water-borne chemicals: a comparison of chemical sources and orientation mechanisms

The salt marsh periwinkle, Littoraria irrorata, exhibits orientation behaviors in response to a number of chemical stimuli. In this study, we quantified the response of L. irrorata to chemical stimuli derived from three sources: Spartina alterniflora (a food source), the blue crab Callinectes sapidus (a predator), and crushed conspecifics. Animals consistently moved toward sources of S. alterniflora extract and crushed conspecifics relative to controls, but did not approach or avoid extracts from C. sapidus relative to controls. Animals had lower response rates to stimuli from S. alterniflora than to controls, but not to putatively aversive chemical stimuli (C. sapidus or crushed conspecifics). However, periwinkles that moved in experimental groups and control groups did not differ in how far they crawled. In addition, we measured turning angles of pathways of periwinkles to determine the orientation mechanism used. Littoraria irrorata displayed a taxis in response to crushed conspecifics, but a kinesis in response to C. sapidus and S. alterniflora. Our results suggest that the ecological context of chemical cues influence the evolution of orientation mechanisms. Cues that require immediate response (crushed conspecifics) elicit directed movement using taxis, whereas those that represent less alarming stimuli (a blue crab in the vicinity) or that represent a ubiquitous resource (Spartina) elicit movement by kinesis.     

Advection–diffusion equations for generalized tactic searching behaviors

 Many organisms search for limiting resources by using repeated responses to local cues, which cumulatively cause movement towards more favorable parts of their environment. This paper presents a general asymptotic expression, derived under the assumption of shallow environmental gradients, for the population-level flux of organisms moving at a constant speed and reorienting at rates determined by the environmental conditions experienced since the last reorientation. The expression takes the form of an advection-diffusion equation, in which the diffusivity and advection velocity are determined by statistics of the turning algorithm that are directly comparable to empirical observations. This work provides a mechanism with which to systematically evaluate a wide variety of tactic and kinetic strategies for determining turning behaviors. The model is applied to searchers on spatially-variable, random distributions of discrete resource patches. Such algorithms are functions of the integrated resource density encountered between turns. It is shown that behaviors in which the turning time distribution is a function of integrated density cannot result in taxis. In contrast, behaviors in which the turning rate is a function of integrated density can result in taxis. These two classes of search algorithm differ in that the latter requires the searcher to “learn” about its local environment, whereas the former requires no such assessment. This suggests neural or physiological mechanisms for remembering previous encounters may be a biological requirement for searchers on discrete resource distributions. Received: 21 September 1995/Revised version: 18 July 1996