Reference: Biol. Bull. 198: 213-224. (April 2000) Life in Transition: Balancing Inertial and Viscous Forces by Planktonic Copepods JEANNETTE YEN Marine Sciences Research Center, State University of New York at Stony Brook, Stonv Brook, New York 11794-5000 Abstract. Copepods (1-10 mm aquatic crustaceans mov-ing at 1-1000 mm s~') live at Reynolds numbers that vary over 5 orders of magnitude, from 10~ 2 to 10 3 . Hence, they live at the interface between laminar and turbulent regimes and are subject to the physical constraints imposed by both viscous and inertial realms. At large scales, the inertially driven system enforces the dominance of physically derived fluid motion; plankton, advected by currents, adjust their life histories to the changing oceanic environment. At Kol-mogorov scales, a careful interplay of evenly matched forces of biology and physics occurs. Copepods conform or deform the local physical environment for their survival, using morphological and behavioral adaptations to shift the balance in their favor. Examples of these balances and transitions are observed when copepods engage in their various survival tasks of feeding, predator avoidance, mat-ing, and signaling. Quantitative analyses of their behavior give measures of such physical properties of their fluid medium as energy dissipation rates, molecular diffusion rates, eddy size, and eddy packaging. Understanding the micromechanics of small-scale biological-physical-chemi-cal interactions gives insight into factors influencing large-scale dynamics of copepod distribution, patchiness, and encounter probabilities in the sea. Introduction Flow patterns created by solid objects moving through a fluid are influenced by viscous and inertial forces. To eval-uate the relative importance of viscous and inertial momen-Received 7 July 1999; accepted 8 February 2000. E-mail:
[email protected] This paper was originally presented at a symposium titled Chemical Communication and Ecology. The symposium, which was held in San Diego, California, on 30 December 1998, was invited by the Western Society of Naturalists as part of its annual meetings. turn fluxes in this solid-fluid interaction, the Reynolds num-ber can be estimated: Re = UL/D (Table l,Eq. 1) where U is the relative flow speed between object and fluid, L is the diameter of the object moving against the flow, and v is the kinematic viscosity (Table 1, Eq. 4). For pelagic copepods, L varies from 10 /xm for the width of a food-particle-capturing seta to 1-3 cm for the span of the antennules. These copepods exhibit movement speeds encompassing a range from less than 1 mm s~ ' for flow past setae to as much as 1 m s~' for escape speeds. Hence, the range of Re realms experienced by copepods spans 5 orders of magnitude, from Re = [10^ 2 ] to [10 3 ]. This is considered a transition zone in flow, where routine swimming and feeding take place at low Re regimes and laminar fields, whereas escapes and captures occur at high Re realms in quasi-turbulent fields. Naganuma (1996) suggests that the "flourishing dominance of the calanoids is based on the advantage of living on the border of different worlds," the viscous and inertial. The author gives examples of activities executed by the copepod in both regimes, but does not discuss mechanisms by which the copepods derive an ad-vantage from these physical forces. In this article, I consider how planktonic copepods make this transition to enhance the detection of chemical and fluid-mechanical signals. Signaling in a Laminar and Quasi-Turbulent Regime Every time a copepod moves through water, it causes a fluid deformation. If the flow is above the threshold for a mechanosensory response or contains a chemical whose concentration is above the threshold for a chemosensory response, the feature is a signal, conveying information about the propagator. Such information as size, shape, scent, speed, distance, and direction of movement are important 213
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