Changes in Larval Oyster Swimming Behavior with Salinity and Larval Age

AbstractEastern oysters (Crassostrea virginica) are sessile, relying on a larval phase to disperse in estuaries. Oyster larval swimming behavior can alter dispersal trajectories and patterns of population connectivity. Experiments were conducted to test how both (1) acclimation time to new environmental conditions and (2) larval swimming behavior change with salinity and larval age. Acclimation time to changes in salinity was longest in lower salinity (6 ppt) and decreased with age. To test changes in behavior with salinity, larvae were placed into four salinities (6, 10, 16, and 22 ppt) where swimming was recorded. To test changes in behavior with age, larvae aged 6, 12, and 15 days were recorded. In both experiments, swimming paths were mapped in two dimensions, behavior of each path was categorized, and speed, direction, and acceleration were calculated. The frequency of upward, neutral, and downward swimming behaviors did not differ across salinity treatments but did vary with age, whereas the frequency of behavior types varied with both salinity and ontogeny. As an example, diving was observed more frequently in low salinity, and more downward helices were observed in moderate salinity, while younger larvae swam upward with more frequency than older larvae. Surprisingly, diving was observed in 10%-15% of all larvae across all ages. Given the consequence of larval behavior to marine invertebrate dispersal, changes in swimming over larval age and in response to environmental changes have important implications to marine population stability and structure.

Surfing on Turbulence: A Strategy for Planktonic Navigation

In marine plankton, many swimming species can perceive their environment with flow sensors. Can they use this flow information to travel faster in turbulence? To address this question, we consider plankters swimming at constant speed, whose goal is to move upward. We propose a robust analytical behavior that allows plankters to choose a swimming direction according to the local flow gradients. We show numerically that such plankters can "surf" on turbulence and reach net vertical speeds up to twice their swimming speed. This new physics-based model suggests that planktonic organisms can exploit turbulence features for navigation.

Responding to the signal and the noise: behavior of planktonic gastropod larvae in turbulence

Swimming organisms may actively adjust their behavior in response to the flow around them. Ocean flows are typically turbulent, and characterized by chaotic velocity fluctuations. While some studies have observed planktonic larvae altering their behavior in response to turbulence, it is not always clear whether a plankter is responding to an individual turbulent fluctuation or to the time-averaged flow. To distinguish between these two paradigms, we conducted laboratory experiments with larvae in turbulence. We observed veliger larvae of the gastropod Crepidula fornicata in a jet-stirred turbulence tank while simultaneously measuring two-components of the fluid and larval velocity. Larvae were studied at two different stages of development, early-stage and late-stage, and their behavior was analyzed in response to different characteristics of turbulence: acceleration, dissipation, and vorticity. Our analysis considered both the effects of the time-averaged flow and the instantaneous flow around the larvae. Overall, we found that both stages of larvae increased their upward swimming speeds in response to increasing turbulence. However, we found that the early-stage larvae tended to respond to the time-averaged flow whereas the late-stage larvae tended to respond to the instantaneous flow around them. These observations indicate that larvae can integrate flow information over time and that their behavioral responses to turbulence can depend on both their present and past flow environments.

Brownian systems perturbed by mild shear: comparing response relations

We present a comprehensive study of the linear response of interacting underdamped Brownian particles to simple shear flow. We collect six different routes for computing the response, two of which are based on the symmetry of the considered system and observable with respect to the shear axes. We include the extension of the Green-Kubo relation to underdamped cases, which shows two unexpected additional terms. These six computational methods are applied to investigate the relaxation of the response towards the steady state for different observables, where interesting effects due to interactions and a finite particle mass are observed. Moreover, we compare the different response relations in terms of their statistical efficiency, identifying their relative demand on experimental measurement time or computational resources in computer simulations. Finally, several measures of breakdown of linear response theory for larger shear rates are discussed.

Sea Lice Are Sensitive to Low Frequency Sounds

The salmon louse Lepeophtheirus salmonis is a major disease problem in salmonids farming and there are indications that it also plays a role in the decline of wild salmon stocks. This study shows the first ultrastructural images of pathological changes in the sensory setae of the first antenna and in inner tissues in different stages of L. salmonis development after sound exposure in laboratory and sea conditions. Given the current ineffectiveness of traditional methods to eradicate this plague, and the strong impact on the environment these treatments often provoke, the described response to sounds and the associated injuries in the lice sensory organs could represent an interesting basis for developing a bioacoustics method to prevent lice infection and to treat affected salmons.

Engineering a 3D collective cancer invasion model with control over collagen fiber alignment

Prior to cancer cell invasion, the structure of the extracellular matrix (ECM) surrounding the tumor is remodeled, such that circumferentially oriented matrix fibers become radially aligned. This predisposed radially aligned matrix structure serves as a critical regulator of cancer invasion. However, a biomimetic 3D model recapitulating a tumor's behavioral response to these ECM structures is not yet available. In this study, we have developed a phase-specific, force-guided method to establish a 3D dual topographical tumor model in which each tumor spheroid/organoid is surrounded by radially aligned collagen I fibers on one side and circumferentially oriented fibers on the opposite side. A coaxial rotating cylinder system was employed to construct the dual fiber topography and to pre-seed tumor spheroids/organoids within a single device. This system enables the application of different force mechanisms in the nucleation and elongation phases of collagen fiber polymerization to guide fiber alignment. In the nucleation phase, fiber alignment is enhanced by a horizontal laminar Couette flow driven by the inner cylinder rotation. In the elongation phase, fiber growth is guided by a vertical gravitational force to form a large aligned collagen matrix gel (35 × 25 × 0.5 mm) embedded with >1000 tumor spheroids. The fibers above each tumor spheroid are radially aligned along the direction of gravitational force in contrast to the circumferentially oriented fibers beneath each tumor spheroid/organoid, where the presence of the tumor interferes with the gravity-induced fiber alignment. After tumor invasion, there are more disseminated multicellular clusters on the radially aligned side, compared to the side of the tumor spheroid/organoid facing circumferentially oriented fibers. These results indicate that our 3D dual topographical model recapitulates the preference of tumors to invade and disseminate along radially aligned fibers. We anticipate that this 3D dual topographical model will have broad utility to those studying collective tumor invasion and that it has the potential to identify cancer invasion-targeted therapeutic agents.

Departures from isotropy: the kinematics of a larval snail in response to food

The swimming behavior of invertebrate larvae can affect their dispersal, survival and settlement in the ocean. Modeling this behavior accurately poses unique challenges as behavior is controlled by both physiology and environmental cues. Some larvae use cilia to both swim and create feeding currents, resulting in potential trade-offs between the two functions. Food availability is naturally patchy and often occurs in shallow horizontal layers in the ocean. Also, larval swimming motions generally differ in the horizontal and vertical directions. In order to investigate behavioral response to food by ciliated larvae, we measured their behavioral anisotropy by quantifying deviations from a model based on isotropic diffusion. We hypothesized that larvae would increase horizontal swimming and decrease vertical swimming after encountering food, which could lead to aggregation at food layers. We considered Crepidula fornicata larvae, which are specifically of interest as they exhibit unsteady and variable swimming behaviors that are difficult to categorize. We tracked the larvae in still water with and without food, with a portion of the larvae starved beforehand. On average, larvae in the presence of food were observed higher in the water column, with higher swimming speeds and higher horizontal swimming velocities when compared with larvae without food. Starved larvae also exhibited higher vertical velocities in food, suggesting no aggregation behavior. Although most treatments showed strong anisotropy in larval behavior, we found that starved larvae without food exhibited approximately isotropic kinematics, indicating that behavioral anisotropy can vary with environmental history and conditions to enhance foraging success or mitigate food-poor environments.

Vertical distribution of brittle star larvae in two contrasting coastal embayments: implications for larval transport

The ability of marine invertebrate larvae to control their vertical position shapes their dispersal pattern. In species characterized by large variations in population density, like many echinoderm species, larval dispersal may contribute to outbreak and die-off phenomena. A proliferation of the ophiuroid Ophiocomina nigra was observed for several years in western Brittany (France), inducing drastic changes on the benthic communities. We here studied the larval vertical distribution in this species and two co-occurring ophiuroid species, Ophiothrix fragilis and Amphiura filiformis, in two contrasting hydrodynamic environments: stratified in the bay of Douarnenez and well-mixed in the bay of Brest. Larvae were collected at 3 depths during 25 h within each bay. In the bay of Brest, all larvae were evenly distributed in the water column due to the intense vertical mixing. Conversely, in the bay of Douarnenez, a diel vertical migration was observed for O. nigra, with a night ascent of young larvae, and ontogenetic differences. These different patterns in the two bays mediate the effects of tidal currents on larval fluxes. O. fragilis larvae were mainly distributed above the thermocline which may favour larval retention within the bay, while A. filiformis larvae, mostly concentrated near the bottom, were preferentially exported. This study highlighted the complex interactions between coastal hydrodynamics and specific larval traits, e.g. larval morphology, in the control of larval vertical distribution and larval dispersal.

Diversity of cilia-based mechanosensory systems and their functions in marine animal behaviour

Sensory cells that detect mechanical forces usually have one or more specialized cilia. These mechanosensory cells underlie hearing, proprioception or gravity sensation. To date, it is unclear how cilia contribute to detecting mechanical forces and what is the relationship between mechanosensory ciliated cells in different animal groups and sensory systems. Here, we review examples of ciliated sensory cells with a focus on marine invertebrate animals. We discuss how various ciliated cells mediate mechanosensory responses during feeding, tactic responses or predator-prey interactions. We also highlight some of these systems as interesting and accessible models for future in-depth behavioural, functional and molecular studies. We envisage that embracing a broader diversity of organisms could lead to a more complete view of cilia-based mechanosensation. This article is part of the Theo Murphy meeting issue 'Unity and diversity of cilia in locomotion and transport'.

Choosing the right home: settlement responses by larvae of six sea urchin species align with hydrodynamic traits of their contrasting adult habitats

Ocean organisms as diverse as seaweeds and sea cucumbers exhibit life cycles in which dispersal occurs primarily via microscopic larvae or spores, with adults exhibiting limited or even no dispersal. In benthic animals, the larval stage concludes with irreversible settlement into the benthos. The decision of where and when to settle is thus one of substantial import. Prior work has shown that settlement in two shoreline echinoids (a sea urchin and a sand dollar) is unexpectedly sensitive to an environmental feature (intense fluid turbulence) that can be considered as a signal to larvae of their arrival in the neighbourhood of the hydrodynamically energetic habitats in which these taxa live as adults. Here, we used a comparative approach to explore the evolution of turbulence responsiveness in late-stage echinoid larvae. We examined three pairs of closely related sea urchins that differ in the energetic exposure of their adult habitats and found that larval responsiveness to turbulence was more pronounced in urchins that settle in more hydrodynamically exposed locations. These results raise the possibility that evolutionary differences in larval responsiveness to environmental indicators of appropriate adult habitat might reinforce or even provide a mechanism for vicariance in the ocean.

Arms of larval seastars of Pisaster ochraceus provide versatility in muscular and ciliary swimming

Larval swimming with cilia, unaided by muscles, is the presumed ancestral condition for echinoderms, but use of muscles in swimming has evolved several times. Ciliation and musculature of the arms of brachiolaria-stage larvae in the family Asteriidae provide unusual versatility in the use of muscles in swimming. The muscles affect swimming in two different ways. (1) Contraction of muscles moves the arms, propelling the larva. (2) Contraction of muscles changes orientation of the arms, thereby changing direction of ciliary currents and direction of swimming. New observations of the brachiolaria of the asteriid seastar Pisaster ochraceus demonstrate more versatility in both of these uses of muscles than had been previously described: the posterolateral arms stroke in more ways to propel the larva forward and to change the direction of swimming, and more pairs of the arms point ciliary currents in more directions for changes in direction of swimming. Morphology of brachiolariae suggests that these uses of muscles in swimming evolved before divergence of the families Stichasteridae and Asteriidae within forcipulate asteroids. This versatile use of muscles for swimming, both alone and in combination with ciliary currents, further distinguishes the swimming of these brachiolariae from swimming by larvae of other echinoderms and larvae of acorn worms in the sister phylum Hemichordata.

Brief exposure to intense turbulence induces a sustained life-history shift in echinoids

In coastal ecosystems, attributes of fluid motion can prompt animal larvae to rise or sink in the water column and to select microhabitats within which they attach and commit to a benthic existence. In echinoid (sea urchin and sand dollar) larvae living along wave-exposed shorelines, intense turbulence characteristic of surf zones can cause individuals to undergo an abrupt life-history shift characterized by precocious entry into competence - the stage at which larvae will settle and complete metamorphosis in response to local cues. However, the mechanistic details of this turbulence-triggered onset of competence remain poorly defined. Here, we evaluate in a series of laboratory experiments the time course of this turbulence effect, both the rapidity with which it initiates and whether it perdures. We found that larvae become competent with turbulence exposures as brief as 30 s, with longer exposures inducing a greater proportion of larvae to become competent. Intriguingly, larvae can remember such exposures for a protracted period (at least 24 h), a pattern reminiscent of long-term potentiation. Turbulence also induces short-term behavioral responses that last less than 30 min, including cessation of swimming, that facilitate sinking and thus contact of echinoid larvae with the substratum. Together, these results yield a novel perspective on how larvae find their way to suitable adult habitat at the critical settlement transition, and also open new experimental opportunities to elucidate the mechanisms by which planktonic animals respond to fluid motion.

Sand Dollar Larvae Show Within-Population Variation in Their Settlement Induction by Turbulence

Settlement-the generally irreversible transition from a planktonic phase to a benthic phase-is a critical stage in the life history of many shoreline organisms. It is reasonable to expect that larvae are under intense selection pressure to identify appropriate settlement habitat. Several decades of studies have focused mainly on local indicators that larvae use to identify suitable habitat, such as olfactory cues that indicate the presence of conspecifics or a favored food source. Our recent work has shown that the larvae of seashore-dwelling echinoids (sea urchins, sand dollars, and kin) can be primed to settle following a brief exposure to a broader-scale indicator of their approach to shore: an increase in fluid turbulence. Here we demonstrate that this priming shows within-population variation: the offspring of certain Pacific sand dollar (Dendraster excentricus) parents-both specific fathers and specific mothers, regardless of the other parent-are more responsive to turbulence than others. In particular, the observation of the effect correlating, in some cases, with specific fathers leads us to conclude that these behavioral differences are likely genetic and thus heritable. We also report that turbulence exposure causes larvae to temporarily sink to the bottom of a container of seawater and that larvae that respond in this way are also more likely to subsequently settle. We hypothesize a two-step scenario for the evolution of turbulence responsiveness at settlement and suggest that the evolutionary origin of these behaviors could be a driving force for population differentiation and speciation.

Waves cue distinct behaviors and differentiate transport of congeneric snail larvae from sheltered versus wavy habitats

Marine population dynamics often depend on dispersal of larvae with infinitesimal odds of survival, creating selective pressure for larval behaviors that enhance transport to suitable habitats. One intriguing possibility is that larvae navigate using physical signals dominating their natal environments. We tested whether flow-induced larval behaviors vary with adults' physical environments, using congeneric snail larvae from the wavy continental shelf (Tritia trivittata) and from turbulent inlets (Tritia obsoleta). Turbulence and flow rotation (vorticity) induced both species to swim more energetically and descend more frequently. Accelerations, the strongest signal from waves, induced a dramatic response in T. trivittata but almost no response in competent T. obsoleta Early stage T. obsoleta did react to accelerations, ruling out differences in sensory capacities. Larvae likely distinguished turbulent vortices from wave oscillations using statocysts. Statocysts' ability to sense acceleration would also enable detection of low-frequency sound from wind and waves. T. trivittata potentially hear and react to waves that provide a clear signal over the continental shelf, whereas T. obsoleta effectively "go deaf" to wave motions that are weak in inlets. Their contrasting responses to waves would cause these larvae to move in opposite directions in the water columns of their respective adult habitats. Simulations showed that the congeners' transport patterns would diverge over the shelf, potentially reinforcing the separate biogeographic ranges of these otherwise similar species. Responses to turbulence could enhance settlement but are unlikely to aid large-scale navigation, whereas shelf species' responses to waves may aid retention over the shelf via Stokes drift.

Turbulence induces metabolically costly behaviors and inhibits food capture in oyster larvae, causing net energy loss

Planktotrophic invertebrate larvae require energy to develop, disperse and settle successfully, and it is unknown how their energetics are impacted by turbulence. Ciliated larvae gain metabolic energy from their phytoplankton food to offset the energetic costs of growth, development and ciliary activity for swimming and feeding. Turbulence may affect the energetic balance by inducing behaviors that alter the metabolic costs and efficiency of swimming, by raising the encounter rate with food particles and by inhibiting food capture. We used experiments and an empirical model to quantify the net rate of energy gain, swimming efficiency and food capture efficiency for eyed oyster larvae (Crassostrea virginica) in turbulence. At dissipation rates representative of coastal waters, larvae lost energy even when food concentrations were very high. Both feeding activity and turbulence-induced behaviors incurred high metabolic costs. Swimming efficiency was concave up versus dissipation rate, suggesting that ciliary activity for food handling became more costly while swimming became more efficient with turbulence intensity. Though counter-intuitive, swimming may have become more efficient in turbulence because vorticity-induced rotation caused larvae to swim more horizontally, which requires less effort than swimming vertically against the pull of gravity. Overall, however, larvae failed to offset high activity costs with food energy gains because turbulence reduced food capture efficiency more than it enhanced food encounter rates. Younger, smaller larvae may have some energetic advantages, but competent larvae would lose energy at turbulence intensities they experience frequently, suggesting that turbulence-induced starvation may account for much of oysters' high larval mortality.