Turning kinematics of the scyphomedusaAurelia aurita.

Scyphomedusae are widespread in the oceans and their swimming has provided valuable insights into the hydrodynamics of animal propulsion. Most of this research has focused on symmetrical, linear swimming. However, in nature, medusae typically swim circuitous, nonlinear paths involving frequent turns. Here we describe swimming turns by the scyphomedusaAurelia auritaduring which asymmetric bell margin motions produce rotation around a linearly translating body center. These jellyfish 'skid' through turns and the degree of asynchrony between opposite bell margins is an approximate predictor of turn magnitude during a pulsation cycle. The underlying neuromechanical organization of bell contraction contributes substantially to asynchronous bell motions and inserts a stochastic rotational component into the directionality of scyphomedusan swimming. These mechanics are important for natural populations because asynchronous bell contraction patterns are commonin situand result in frequent turns by naturally swimming medusae.

Ink Release and Swimming Behavior in the Oceanic Ctenophore Eurhamphaea vexilligera.

Of the more than 150 ctenophore species, the oceanic ctenophore Eurhamphaea vexilligera is notable for its bright orange-yellow ink, secreted from numerous small vesicles that line its substomodeal comb rows. To date, in situ observations by scuba divers have proved the most fruitful method of observing these animals' natural behavior. We present the results of one such contemporary scuba-based observation of E. vexilligera, conducted in the Gulf Stream waters off the coast of Florida, using high-resolution photography and video. Utilizing underwater camera systems purpose built for filming gelatinous zooplankton, we observed E. vexilligera ink release and swimming behavior in situ. From these data, we describe the timeline and mechanics of E. vexilligera ink release in detail, as well as the animal's different swimming behaviors and resulting ink dispersal patterns. We also describe a rolling swimming behavior, accompanied and possibly facilitated by a characteristic change in overall body shape. These observations provide further insight into the behavioral ecology of this distinctive ctenophore and may serve as the foundation for future kinematic studies.

Fluid interactions that enable stealth predation by the upstream-foraging hydromedusa Craspedacusta sowerbyi.

Unlike most medusae that forage with tentacles trailing behind their bells, several species forage upstream of their bells using aborally located tentacles. It has been hypothesized that these medusae forage as stealth predators by placing their tentacles in more quiescent regions of flow around their bells. Consequently, they are able to capture highly mobile, sensitive prey. We used digital particle image velocimetry (DPIV) to quantitatively characterize the flow field around Craspedacusta sowerbyi, a freshwater upstream-foraging hydromedusa, to evaluate the mechanics of its stealth predation. We found that fluid velocities were minimal in front and along the sides of the bell where the tentacles are located. As a result, the deformation rates in the regions where the tentacles are located were low, below the threshold rates required to elicit an escape response in several species of copepods. Estimates of their encounter volume rates were examined on the basis of flow past the tentacles, and trade-offs associated with tentacle characteristics were evaluated.

A wake-based correlate of swimming performance and foraging behavior in seven co-occurring jellyfish species.

It is generally accepted that animal-fluid interactions have shaped the evolution of animals that swim and fly. However, the functional ecological advantages associated with those adaptations are currently difficult to predict on the basis of measurements of the animal-fluid interactions. We report the identification of a robust, fluid dynamic correlate of distinct ecological functions in seven jellyfish species that represent a broad range of morphologies and foraging modes. Since the comparative study is based on properties of the vortex wake--specifically, a fluid dynamical concept called optimal vortex formation--and not on details of animal morphology or phylogeny, we propose that higher organisms can also be understood in terms of these fluid dynamic organizing principles. This enables a quantitative, physically based understanding of how alterations in the fluid dynamics of aquatic and aerial animals throughout their evolution can result in distinct ecological functions.

Phenotypic plasticity in juvenile jellyfish medusae facilitates effective animal-fluid interaction.

Locomotion and feeding in marine animals are intimately linked to the flow dynamics created by specialized body parts. This interaction is of particular importance during ontogeny, when changes in behaviour and scale challenge the organism with shifts in fluid regimes and altered functionality. Previous studies have indicated that Scyphozoan jellyfish ontogeny accommodates the changes in fluid dynamics associated with increasing body dimensions and velocities during development. However, in addition to scale and behaviour that-to a certain degree-underlie the control of the animal, flow dynamics are also dependent on external factors such as temperature. Here, we show phenotypic plasticity in juvenile Aurelia aurita medusae, where morphogenesis is adapted to altered fluid regimes imposed by changes in ambient temperature. In particular, differential proportional growth was found to compensate for temperature-dependent changes in viscous effects, enabling the animal to use adhering water boundary layers as 'paddles'-and thus economize tissue-at low temperatures, while switching to tissue-dominated propulsion at higher temperatures where the boundary layer thickness is insufficient to serve for paddling. This effect was predicted by a model of animal-fluid interaction and confirmed empirically by flow-field visualization and assays of propulsion efficiency.

Functional morphology and fluid interactions during early development of the scyphomedusa Aurelia aurita.

Scyphomedusae undergo a predictable ontogenetic transition from a conserved, universal larval form to a diverse array of adult morphologies. This transition entails a change in bell morphology from a highly discontinuous ephyral form, with deep clefts separating eight discrete lappets, to a continuous solid umbrella-like adult form. We used a combination of kinematic, modeling, and flow visualization techniques to examine the function of the medusan bell throughout the developmental changes of the scyphomedusa Aurelia aurita. We found that flow around swimming ephyrae and their lappets was relatively viscous (1 < Re < 10) and, as a result, ephyral lappets were surrounded by thick, overlapping boundary layers that occluded flow through the gaps between lappets. As medusae grew, their fluid environment became increasingly influenced by inertial forces (10 < Re < 10,000) and, simultaneously, clefts between the lappets were replaced by organic tissue. Hence, although the bell undergoes a structural transition from discontinuous (lappets with gaps) to continuous (solid bell) surfaces during development, all developmental stages maintain functionally continuous paddling surfaces. This developmental pattern enables ephyrae to efficiently allocate tissue to bell diameter increase via lappet growth, while minimizing tissue allocation to inter-lappet spaces that maintain paddle function due to boundary layer overlap.

Transitions in morphology, nematocyst distribution, fluid motions, and prey capture during development of the scyphomedusa Cyanea capillata.

Like that of most scyphozoans, the ontogeny of Cyanea capillata medusae involves substantive alterations in feeding structures and mechanics. We used video and optical microscopy approaches to quantify these ontogenetic changes in morphology, flow, and feeding of C. capillata medusae. We found that alterations in gross morphology and nematocyst distributions coincided with a shift from prey capture on the manubrium or lappets of ephyrae (bell diameter 0.2-0.4 cm) to capture primarily on the tentacles in adult medusae (diameter >1.0 cm). These changes occurred within a hydrodynamic framework that itself changed due to medusan growth. Viscous forces were important in flows around small ephyrae (maximum Re <10(1)), whereas viscosity was less influential in the inertially dominated flows around adult medusae (Re > 10(2)). The relative timing of these events indicates that ontogenetic processes are closely synchronized with alterations in the hydrodynamic environment within which C. capillata medusae develop.

Grazing in a turbulent environment: behavioral response of a calanoid copepod, Centropages hamatus.

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.

Grazing in a turbulent environment: energy dissipation, encounter rates, and efficacy of feeding currents in Centropages hamatus.

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.