Sean P. Colin

Flow patterns generated by oblate medusan jellyfish: field measurements and laboratory analyses

SUMMARY Flow patterns generated by medusan swimmers such as jellyfish are known to differ according the morphology of the various animal species. Oblate medusae have been previously observed to generate vortex ring structures during the propulsive cycle. Owing to the inherent physical coupling between locomotor and feeding structures in these animals, the dynamics of vortex ring formation must be robustly tuned to facilitate effective functioning of both systems. To understand how this is achieved, we employed dye visualization techniques on scyphomedusae (Aurelia aurita) observed swimming in their natural marine habitat. The flow created during each propulsive cycle consists of a toroidal starting vortex formed during the power swimming stroke, followed by a stopping vortex of opposite rotational sense generated during the recovery stroke. These two vortices merge in a laterally oriented vortex superstructure that induces flow both toward the subumbrellar feeding surfaces and downstream. The lateral vortex motif discovered here appears to be critical to the dual function of the medusa bell as a flow source for feeding and propulsion. Furthermore, vortices in the animal wake have a greater volume and closer spacing than predicted by prevailing models of medusan swimming. These effects are shown to be advantageous for feeding and swimming performance, and are an important consequence of vortex interactions that have been previously neglected.

Morphology, fluid motion and predation by the scyphomedusa Aurelia aurita

Although medusan predators play demonstrably important roles in a variety of marine ecosystems, the mechanics of prey capture and, hence, prey selection, have remained poorly defined. A review of the literature describing the commonly studied medusa Aurelia aurita (Linnaeus 1758) reveals no distinct patterns of prey selectivity and suggests that A. aurita is a generalist and feeds unselectively upon available zooplankton. We examined the mechanics of prey capture by A. aurita using video methods to record body and fluid motions. Medusae were collected between February and June in 1990 and 1991 from Woods Hole, Massachusetts and Narragansett Bay, Rhode Island, USA. Tentaculate A. aurita create fluid motions during swimming which entrain prey and bring them into contact with tentacles. We suggest that this mechanism dominates prey selection by A. aurita. In this case, we predict that medusae of a specific diameter will positively select prey with escape speeds slower than the flow velocities at their bell margins. Negatively selected prey escape faster than the medusan flow velocity draws them to capture surfaces. Faster prey will be captured by larger medusac because flow field velocity is a function of bell diameter. On the basis of prey escape velocities and flow field velocities of A. aurita with diameters of 0.8 to 7.1 cm, we predict that A. aurita will select zooplankton such as barnacle nauplii and some slow swimming hydromedusae, while faster copepods will be negatively selected.

Latitudinal Differentiation in the Effects of the Toxic Dinoflagellate Alexandrium spp. on the Feeding and Reproduction of Populations of the Copepod Acartia Hudsonica

Blooms of the dinoflagellate Alexandriumspp. increase in their frequency, toxicity and historical presence with increasing latitude from New Jersey (USA) to the Gaspe peninsula (Canada). Biogeographic variation in these blooms results in differential exposure of geographically separate copepod populations to toxic Alexandrium. We hypothesize that the ability of copepods to feed and reproduce on toxic Alexandriumshould be higher in copepods from regions that are frequently exposed to toxic Alexandriumblooms. We tested this hypothesis with factorial common environment experiments in which female adults of the copepod Acartia hudsonicafrom five separate populations ranging from New Jersey to New Brunswick were fed toxic and non-toxic strains of Alexandrium, and the non-toxic flagellate Tetraselmis sp. Consistent with the hypothesis, when fed toxic Alexandriumwe observed significantly higher ingestion and egg production rates in the copepods historically exposed to toxic Alexandriumblooms relative to copepods from regions in which Alexandriumis rare or absent. Such differences among copepod populations were not observed when copepods were fed non-toxic Alexandrium or Tetraselmis sp. These results were also supported by assays in which copepods from populations both historically exposed and na¨ ove to toxic Alexandrium blooms were fed mixtures of toxic Alexandrium and Tetraselmis sp. Two-week long experiments demonstrated that when

Flow and Feeding by Swimming Scyphomedusae

The mechanical basis of prey capture by scyphomedusae has been largely ignored, despite the importance of these predators in a variety of planktonic ecosystems. Interactions between swimming, fluid motions, and prey capture were examined during 1991–1992 for a species from the three scyphozoan orders having planktonic medusae: Rhizostomeae, Stomolophus meleagris Agassiz, 1862; Coronatae, Linuche unguiculata (Schwartz, 1788); and Semaeostomeae, Cyanea capillata (Linnaeus, 1758). All three species used flow created during bell pulsation to capture prey, but the type of flow used for prey capture and the capture surface morphology were different for each species. The mechanics of capture by these species of diverse morphology and taxonomic affinity suggests that the use of bell pulsation-induced flow for prey entrainment and capture is widespread among the scyphomedusae.

Testing for resistance of pelagic marine copepods to a toxic dinoflagellate

With few exceptions, the evolutionary consequences of harmful algae to grazers in aquatic systems remain unexplored. To examine both the ecological and evolutionary consequences of harmful algae on marine zooplankton, we used a two-fold approach. In the first approach, we examined the life history responses of two geographically separate Acartia hudsonica (Copepoda Calanoida) populations reared on diets containing the toxic dinoflagellate Alexandrium fundyense . One copepod population was from a region, Casco Bay, Maine, USA, that has experienced recurrent blooms of highly toxic Alexandrium spp. for decades; whereas the other population from Great Bay, New Jersey, USA, has never been exposed to toxic Alexandrium blooms. The life history experiment demonstrated that when the copepod population from New Jersey was reared on a diet containing toxic A. fundyense it exhibited lower somatic growth, size at maturity, egg production and survival than the same population reared on a diet without toxic A. fundyense . In contrast, toxic A. fundyense did not affect the life-history traits of the Maine population. Fitness, finite population growth rate (λ), was significantly reduced in the New Jersey population, but not in the Maine population. These results are consistent with the hypothesis of local adaptation (resistance) of the historically exposed copepod population to the toxic dinoflagellate. In the second approach, we further tested the resistance hypothesis with a laboratory genetic selection experiment with the naïve New Jersey copepod population exposed to a diet containing toxic A. fundyense. This experiment demonstrated that the ingestion and egg production of adult females of naïve copepods fed A. fundyense improved after three generations of being reared on a diet containing the toxic dinoflagellate. The results of the present study have important implications for understanding how grazer populations may respond to the introduction of toxic algae to their environment, and suggest that grazer resistance may be a feedback mechanism that may lead to bloom control.

Faking Giants: The Evolution of High Prey Clearance Rates in Jellyfishes

Jellyfish process prey at the same rates as fish, suggesting that a shift to jellyfish-dominated systems is possible. Jellyfishes have functionally replaced several overexploited commercial stocks of planktivorous fishes. This is paradoxical, because they use a primitive prey capture mechanism requiring direct contact with the prey, whereas fishes use more efficient visual detection. We have compiled published data to show that, in spite of their primitive life-style, jellyfishes exhibit similar instantaneous prey clearance and respiration rates as their fish competitors and similar potential for growth and reproduction. To achieve this production, they have evolved large, water-laden bodies that increase prey contact rates. Although larger bodies are less efficient for swimming, optimization analysis reveals that large collectors are advantageous if they move through the water sufficiently slowly.

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

SUMMARY 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.

The Numerical Comparison of Flow Patterns and Propulsive Performances for the Hydromedusae Sarsia Tubulosa and Aequorea Victoria

SUMMARY The thrust-generating mechanism of a prolate hydromedusa Sarsia tubulosa and an oblate hydromedusa Aequorea victoria was investigated by solving the incompressible Navier–Stokes equations in the swirl-free cylindrical coordinates. The calculations clearly show the vortex dynamics related to the thrust-generating mechanism, which is very important for understanding the underlying propulsion mechanism. The calculations for the prolate jetting hydromedusa S. tubulosa indicate the formation of a single starting vortex ring for each pulse cycle with a relatively high vortex formation number. However, the calculations for the oblate jet-paddling hydromedusa A. victoria indicate shedding of the opposite-signed vortex rings very close to each other and the formation of large induced velocities along the line of interaction as the vortices move away from the hydromedusa in the wake. In addition to this jet propulsion mechanism, the hydromedusas bell margin acts like a paddle and the highly flexible bell margin deforms in such a way that the low pressure leeward side of the bell margin has a projected area in the direction of motion. This thrust is particularly important during refilling of the subumbrella cavity where the stopping vortex causes significant pressure drag. The swimming performances based on our numerical simulations, such as swimming velocity, thrust, power requirement and efficiency, were computed and support the idea that jet propulsion is very effective for rapid body movement but is energetically costly and less efficient compared with the jet-paddling propulsion mechanism.

Stealth predation and the predatory success of the invasive ctenophore Mnemiopsis leidyi

In contrast to higher metazoans such as copepods and fish, ctenophores are a basal metazoan lineage possessing a relatively narrow set of sensory-motor capabilities. Yet lobate ctenophores can capture prey at rates comparable to sophisticated predatory copepods and fish, and they are capable of altering the composition of coastal planktonic communities. Here, we demonstrate that the predatory success of the lobate ctenophore Mnemiopsis leidyi lies in its use of cilia to generate a feeding current that continuously entrains large volumes of fluid, yet is virtually undetectable to its prey. This form of stealth predation enables M. leidyi to feed as a generalist predator capturing prey, including microplankton (approximately 50 μm), copepods (approximately 1 mm), and fish larvae (>3 mm). The efficacy and versatility of this stealth feeding mechanism has enabled M. leidyi to be notoriously destructive as a predator and successful as an invasive species.

Passive energy recapture in jellyfish contributes to propulsive advantage over other metazoans

Significance Jellyfish have the ability to bloom and take over perturbed ecosystems, but this is counterintuitive because jellyfish are described as inefficient swimmers and rely on direct contact with prey to feed. To understand how jellyfish can outcompete effective visual hunters, such as fish, we investigate the energetics of propulsion. We find that jellyfish exhibit a unique mechanism of passive energy recapture, which can reduce metabolic energy demand by swimming muscles. Contrary to prevailing views, this contributes to jellyfish being one of the most energetically efficient propulsors on the planet. These results demonstrate a physical basis for the ecological success of medusan swimmers despite their simple body plan and have implications for bioinspired design, where low-energy propulsion is required. Gelatinous zooplankton populations are well known for their ability to take over perturbed ecosystems. The ability of these animals to outcompete and functionally replace fish that exhibit an effective visual predatory mode is counterintuitive because jellyfish are described as inefficient swimmers that must rely on direct contact with prey to feed. We show that jellyfish exhibit a unique mechanism of passive energy recapture, which is exploited to allow them to travel 30% further each swimming cycle, thereby reducing metabolic energy demand by swimming muscles. By accounting for large interspecific differences in net metabolic rates, we demonstrate, contrary to prevailing views, that the jellyfish (Aurelia aurita) is one of the most energetically efficient propulsors on the planet, exhibiting a cost of transport (joules per kilogram per meter) lower than other metazoans. We estimate that reduced metabolic demand by passive energy recapture improves the cost of transport by 48%, allowing jellyfish to achieve the large sizes required for sufficient prey encounters. Pressure calculations, using both computational fluid dynamics and a newly developed method from empirical velocity field measurements, demonstrate that this extra thrust results from positive pressure created by a vortex ring underneath the bell during the refilling phase of swimming. These results demonstrate a physical basis for the ecological success of medusan swimmers despite their simple body plan. Results from this study also have implications for bioinspired design, where low-energy propulsion is required.