Sensory Biology and Ocean Ecology
Several of the OEUVRE white paper authors cited the critical link between stimulus production/transmission and behavior, or between individual behavior and resulting larger scale dynamics of abundance and spatial/temporal distributions. Drawing together these statements results in a paradigm for examining ecological interactions:
Interestingly, this paradigm seems to elide a critical transition by omitting from consideration the sensory capabilities of the animal itself. Investigations on sensory physiology, particularly in conjunction with behavioral studies, can fill a need in establishing the linkages between stimulus space, behavior, and demographic consequences of individual decisions.
In this vein one of the most important issues concerns how temporal and spatial properties of chemical and fluid mechanical signals interacts with the animals' sensory capabilities. Persistence of signals depends not only on their intensity, rates of generation and decay, but also on the threshold intensities perceivable by the organism. The ability of animals to navigate through gradients or fluctuating signals is strongly contingent on the temporal aspects of the signal, since patterns of stimulus history control the sensitivity and dynamic range of sensory neurons. Translating the properties of the chemical and fluid mechanical environment into patterns of population abundance and distributions may require us to examine the neural filter that transforms stimulus space into critical behaviors, and to model the impact of this process over many cycles. I offer two examples of the relevance of sensory biology to ocean ecology below.
1. Encounter models between predators and prey often depend on perceptual distance mediated by chemical or mechanosensory signals. Analysis of physiological thresholds provides a basis for determining this distance, given that the signal strength can be determined or estimated. Thus, the effects of differences in the degree of production or attenuation of signals, or interference by background noise can be examined. These models represent "virtual experiments", and are useful in establishing boundary conditions, or identifying critical levels of environmental variables that drastically affect perception, and hence, rates of predation. For zooplankton this approach is particularly appealing, since at this small scale (low Re), signal strength can be accurately modeled. This is of particular concern with respect to small scale turbulence and the performance of mechanoreceptive predators. Under conditions of constant perceptual volumes, turbulence increases the encounter rate between predators and prey. It is possible that background turbulence interferes with mechanoreceptive abilities, and diminishes perceptual volume, with corresponding decreases in predation rates.
2. The ability of crabs and lobster to track prey over long distances using odor plumes, or the ability of copepods to follow mating trails is critically dependent on spatial and temporal properties of odor signals, and sensory capabilities (adaptation-disadaptation, threshold sensitivity etc). Knowledge of both signal features and physiological capabilities can be used to predict the performance of animals in a given stimulus (i.e. hydrodynamic environment), or conversely, the suitability of given habitats for performing particular tasks. This is particularly useful to examine situations difficult to approach experimentally, e.g. long distance tracking of odor plumes by large macro-invertebrates. One critical element is that stimulus history appears to modulate neuronal sensitivity and thus perceptual abilities. Reproducing exact patterns of stimulus history, particularly for animals in turbulent plumes is challenging, but may possible using numerical simulations of plume structure combined with models of receptor temporal dynamics in relation to stimulus history. Again, this approach is very attractive with regard to small animals in simple flows and benign hydrodynamic regimes where diffusion dominates odorant transport. Copepods aggregate in areas of high Richardson's number or low energy dissipation rates. Uncertainty exists as to the exact reasons for this observation, but perhaps this is because these regions permit tracking of chemical trails that are otherwise easily disrupted by turbulence. Defining limits to perception, stimulus integration times etc. can help define the spatial and temporal scales over which these behaviors may occur, identify appropriate habitats, and establish critical nearest neighbor distances for mating or other activities. The perceptibility of slightly larger scale features also may be investigated in this manner. Thin layers may represent a critical scale because they produce a detectable gradient in odor intensity on time and space scales relevant to zooplankton. High bundance in such regions may reflect active tracking in addition to physical forcing. Obviously, the large mission of the Biological Oceanography program means that sensory work is unlikely to be the primary responsibility of this division. The most successful efforts will utilize physiological and behavioral methods, while relying heavily on tools used to visualize or quantitate signal structure. However, such studies on important species or model systems are likely to be hugely relevant to the scope and mission of BIO-OCE. I hope that the OEUVRE process, which seems to have a highly interdisciplinary nature, will focus our attention on the utility of these efforts, and that the biological oceanographic community will continue to encourage, support, and initiate such work.
Marc Weissburg
School of Biology
Georgia Institute of Technology
310 Ferst Ave.
Atlanta, Ga, 30332-0230
Tel: 404-894-8433
Fax: 404-894-0519
marc.weissburg@biology.gatech.edu
http://www.gatech.edu/biology/fac/weis.htm
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