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Environmental stressors induce rapid physiological and behavioral shifts in vertebrate animals. However, the neurobiological mechanisms responsible for stress-induced changes in behavior are complex and not well understood. Similar to mammalian vertebrates, zebrafish adults display a preference for dark environments that is associated with predator avoidance, enhanced by stressors, and broadly used in assays for anxiety-like behavior. Although the larvae of zebrafish are a prominent model organism for understanding neural circuits, fewer studies have examined the effects of stressors on their behavior. This study examines the effects of noxious chemical and electric shock stressors on locomotion and light preference in zebrafish larvae. We found that both stressors elicited similar changes in behavior. Acute exposure induced increased swimming activity, while prolonged exposure depressed activity. Neither stressor produced a consistent shift in light/dark preference, but prolonged exposure to these stressors resulted in a pronounced decrease in exploration of different visual environments. We also examined the effects of exposure to a noxious chemical cue using whole-brain calcium imaging, and identified neural correlates in the area postrema, an area of the hindbrain containing noradrenergic and dopaminergic neurons. Pharmaceutical blockade experiments showed that ɑ-adrenergic receptors contribute to the behavioral response to an acute stressor but are not necessary for the response to a prolonged stressor. These results indicate that zebrafish larvae have complex behavioral responses to stressors comparable to those of adult animals, and also suggest that these responses are mediated by similar neural pathways.

SUMMARYThe gills of teleost fishes are often discussed as an archetypal counter-current exchange system, capable of supporting the relatively high metabolic rates of some fishes despite the low oxygen solubility of water. Despite an appreciation for the physiology of exchange at the gills, many questions remain regarding the hydrodynamical basis of ventilation in teleost fishes. In this study, the hydrodynamic resistance and flow fields around the isolated gills of a tilapia, Oreochromis mossambicus, were measured as a function of the applied pressure head. At ventilatory pressures typical of a fish at rest, the hydrodynamic resistance of the gills was nearly constant, the flow was laminar, shunting of water around the gills was essentially absent, and the distribution of water flow was relatively uniform. However, at the higher pressures typical of an active or stressed fish, some of these qualities were lost. In particular, at elevated pressures there was a decrease in the hydrodynamic resistance of the gills and substantial shunting of water around the gills. These effects suggest mechanical limits to maximum aerobic performance during activity or under adverse environmental conditions.

A great diversity of aquatic animals detects water flow with ciliated mechanoreceptors on the body's surface. In order to understand how these receptors mechanically filter signals, we developed a theoretical model of the superficial neuromast in the fish lateral line system. The cupula of the neuromast was modeled as a cylindrical beam that deflects in response to an oscillating flow field. Its accuracy was verified by comparison with prior measurements of cupular deflection in larval zebrafish (Danio rerio). The model predicts that the boundary layer of flow over the body attenuates low-frequency stimuli. The fluid-structure interaction between this flow and the cupula attenuates high-frequency stimuli. The number and height of hair cell kinocilia and the dimensions of the cupular matrix determine the range of intermediate frequencies to which a neuromast is sensitive. By articulating the individual mechanical contributions of the boundary layer and the components of cupular morphology, this model provides the theoretical framework for understanding how a hydrodynamic receptor filters flow signals.

Larvae of many benthic marine animals set- tle and metamorphose in response to waterborne chem- ical cues. Can the behavioral responses of microscopic larvae in the water column to dissolved chemical cues affect their transport to the substratum in the turbulent, wave-driven flow characteristic of many shallow coastal habitats? We addressed this question using an individ- ual-based model of larvae of the sea slug Phestilla sibo- gae, transported in the oscillatory flow above coral reefs. Larvae of P. sibogae stop swimming and sink in response to a dissolved inducer released by their prey, the coral Porites compressa, and resume swimming when exposed to inducer-free water. The instantaneous fine-scale spa- tial distribution of inducer in the flow above a reef is fila- mentous; hence microscopic larvae swimming or sinking through the water encounter inducer above threshold concentration in on/off temporal patterns. Model results show that using a time-averaged inducer concentration gradient to calculate larval transport rates to the reef overestimates the rates by

SUMMARYUnderstanding how the shape and motion of an aquatic animal affects the performance of swimming requires knowledge of the fluid forces that generate thrust and drag. These forces are poorly understood for the large diversity of animals that swim at Reynolds numbers (Re) between 100 and 102. We experimentally tested quasi-steady and unsteady blade-element models of the hydrodynamics of undulatory swimming in the larvae of the ascidian Botrylloides sp. by comparing the forces predicted by these models with measured forces generated by tethered larvae and by comparing the swimming speeds predicted with measurements of the speed of freely swimming larvae. Although both models predicted mean forces that were statistically indistinguishable from measurements, the quasi-steady model predicted the timing of force production and mean swimming speed more accurately than the unsteady model. This suggests that unsteady force (i.e. the acceleration reaction) does not play a role in the dynamics of steady undulatory swimming at Re≈102. We explored the relative contribution of viscous and inertial force to the generation of thrust and drag at 100102) and low (

Although phototaxis has an important influence on the vertical distribution and settlement of marine invertebrate larvae, few studies have explored the mechanisms of taxis in larvae at the organismal level. We examined how phototaxis changes over ontogeny in larvae of the ascidian Aplidium constellatum and experimentally tested hypotheses about the kinematics of oriented swimming. By video recording their swimming movements at regular intervals over their ontogeny, we found that larvae switched from positive to negative phototaxis. We tested hypotheses about the kinematics of phototaxis by recording the three-dimensional movement of larvae in response to a change in the direction of illumination and by tracking the tail motion of tethered larvae in response to sinusoidal changes in light intensity. Larvae swimming with negative phototaxis changed their rate of rotation about their antero-posterior and dorso-ventral axes in response to a change in the direction of illumination. These changes in the rates of rotation caused the axis of the helical trajectory to orient away from the light source. Tethered larvae oscillated their tails at a constant tail beat frequency and with a slow periodicity that was correlated with the stimulus frequency. These findings suggest that ascidian larvae orient by changing tail motion in proportion to perceived changes in light intensity. This method of orientation predicts that larvae achieve the switch from positive to negative phototaxis by changing the delay of their kinematic response to changes in perceived light intensity.

SUMMARY We created physical models based on the morphology of ram suspension-feeding fishes to better understand the roles morphology and swimming speed play in particle retention, size selectivity and filtration efficiency during feeding events. We varied the buccal length, flow speed and architecture of the gills slits, including the number, size, orientation and pore size/permeability, in our models. Models were placed in a recirculating flow tank with slightly negatively buoyant plankton-like particles (∼20–2000 μm) collected at the simulated esophagus and gill rakers to locate the highest density of particle accumulation. Particles were captured through sieve filtration, direct interception and inertial impaction. Changing the number of gill slits resulted in a change in the filtration mechanism of particles from a bimodal filter, with very small (≤50 μm) and very large (>1000 μm) particles collected, to a filter that captured medium-sized particles (101–1000 μm). The number of particles collected on the gill rakers increased with flow speed and skewed the size distribution towards smaller particles (51–500 μm). Small pore sizes (105 and 200 μm mesh size) had the highest filtration efficiencies, presumably because sieve filtration played a significant role. We used our model to make predictions about the filtering capacity and efficiency of neonatal whale sharks. These results suggest that the filtration mechanics of suspension feeding are closely linked to an animal's swimming speed and the structural design of the buccal cavity and gill slits.

SUMMARYSand lances, fishes in the genus Ammodytes, exhibit a peculiar burrowing behavior in which they appear to swim rapidly into the substrate. They use posteriorly propagated undulations of the body to move in both water, a Newtonian fluid, and in sand, a non-Newtonian, granular substrate. In typical aquatic limbless locomotion, undulations of the body push against water, which flows because it is incapable of supporting the static stresses exerted by the animal, thus the undulations move in world space (slipping wave locomotion). In typical terrestrial limbless locomotion, these undulations push against substrate irregularities and move relatively little in world space (non-slipping wave locomotion). We used standard and X-ray video to determine the roles of slipping wave and non-slipping wave locomotion during burrowing in sand lances. We find that sand lances in water use slipping wave locomotion, similar to most aquatic undulators, but switch to non-slipping waves once they burrow. We identify a progression of three stages in the burrowing process: first, aquatic undulations similar to typical anguilliform locomotion (but without head yaw) push the head into the sand; second, more pronounced undulations of the aquatic portion of the body push most of the animal below ground; third, the remaining above-ground portion of the body ceases undulation and the subterranean portion takes over, transitioning to non-slipping wave locomotion. We find no evidence that sand lances use their body motions to fluidize the sand. Instead, as soon as enough of the body is underground, they undergo a kinematic shift and locomote like terrestrial limbless vertebrates.