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A common question in the aquatic sciences is that of how zooplankter movement can be modeled. It is well-established in the literature that there exists a randomness to this movement, but the question is how to characterize this randomness. The most common methods for doing this involve the random walk and correlated random walk (CRW) models. Here, we present a time series model that allows a better description the randomness in Daphnia motion when the amount of time that elapses between observations of their position is small. Our approach is adaptable to description of tracks of a multitude of animal species through re-estimation of model parameters. The model we propose uses information about how the animal moved during the previous two time intervals to explain how it moves currently. We demonstrate that the proposed model provides better predictive accuracy and fit than do the CRW and random walk models.

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Models of marine ecosystem productivity rely on estimates of small-scale interactions, particularly those between copepods and their algal food sources. Rothschild and Osborn [Rothschild, B. J. & Osborn, T. R. (1988) J. Plankton Res. 10, 465-474], hypothesized that small-scale turbulence in aquatic systems increases the perceived abundance of prey to predators. We tested this hypothesis by exposing the planktonic copepod Centropages hamatus to turbulent and nonturbulent environments at different prey concentrations. Our results fell into two main categories. First, the response to turbulence was characterized by an initial period having a high number of escape reactions. This period was followed by one of increased foraging. C. hamatus responded to the higher encounter rates due to turbulence as if it were experiencing altered prey concentrations. Second, the termination of turbulence resulted in an increased foraging response, which was not directly related to the encounter rate. Functional response curves do not adequately explain this foraging response because the time course of the foraging response depends on prior encounter experience and foraging motivation.

RESUMEN

The creation of feeding currents by calanoid copepods increases encounter rates of copepods with their food and provides and advantage in dilute nutritional environments. Small-scale turbulence has also been hypothesized to increase the encounter rate between planktonic predators and their food. Centropages hamatus was exposed to turbulent and nonturbulent environments at two prey concentrations to quantify the influence of turbulence on feeding current efficacy. Turbulent energy dissipation rates used in the experiment were in the range of 0.05-0.15 cm2. sec-3. In the nonturbulent environments, feeding currents increased the encounter rates of C. hamatus 3-5 times that of control encounter areas. In turbulent environments, encounter rates were not increased by feeding currents, yet C. hamatus continued to create feeding currents. Energetic calculations indicate a tradeoff in the value of turbulence to a copepod feeding on phytoplankton. While turbulence is probably beneficial at low food concentrations, it may be deleterious at high food concentrations.

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The interaction between planktonic herbivorous calanoid copepods and their food, planktonic algae, is investigated to increase our understanding of the physiological adaptations these small marine animals have acquired in the course of evolution. Emphasis is given to the centimetre -second scale where calanoids encounter algae, select and capture them, or reject them either passively or actively. Most calanoid copepods create feeding currents which can be subdivided into three cores: motion, viscous, and sensory cores. Algae contained in the sensory core are perceived and then re-routed towards the capture area. The perimeter encompassing all the points of these re-routings can be defined as the reactive field of awareness surrounding the calanoid. An analysis of typical biological oceanographic feeding experiments reveals that direct observations are necessary to understand the feeding behaviours and strategies of calanoid copepods. To facilitate further studies, a new experimental set-up has been described and two hypotheses have been formulated. The method allows direct observations, in all three dimensions, of free-swimming herbivorous calanoids and their food in a 6-litre vessel. The two hypotheses are based on the fact that calanoids create feeding currents and orient their bodies within the water column. The first hypothesis states that calanoid copepods create species-specific, and maybe even age-specific, feeding currents. The second one proposes that ambient water motions may act as a mechanism for niche separation in herbivorous calanoid copepods. This latter hypothesis is based on the inference that ambient water motions may interfere with the flow field of the feeding current thereby making it more difficult for calanoids to successfully re-route algae contained in the sensory core of the feeding current.

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Feeding currents of free-swimming calanoid copepods, observed through an expanded krypton laser beam and a back-focus dark-field optical system, show that these planktonic animals generate a double shear field to help in detecting food. The interrelation between flow field, perception of food items, and body orientation explains why these animals are generally negatively buoyant.

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Ultrastructural studies of the mouthparts of the calanoid copepod Diaptomus pallidus have revealed the presence of numerous chemoreceptors, and the apparent absence of mechanoreceptors. The setae contain no muscles, and the setules are noncellular extensions of their chitin wall. This allows a new insight into the selective feeding of zooplankters.

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Ultrastructural studies of the setae of the first antennae of Cyclops scutifer (Sars) have revealed their sensory function. The setae are the extension of modified ciliary structures which function as mechanoreceptors. The setae apparently act to detect gravitational and inertial forces. This is of particular importance in sensing disturbances generated by prey or predators.