Shane P. Windsor

Swimming kinematics and hydrodynamic imaging in the blind Mexican cave fish (Astyanax fasciatus)

SUMMARY Blind Mexican cave fish (Astyanax fasciatus) lack a functioning visual system, and are known to use self-generated water motion to sense their surroundings; an ability termed hydrodynamic imaging. Nearby objects distort the flow field created by the motion of the fish. These flow distortions are sensed by the mechanosensory lateral line. Here we used image processing to measure detailed kinematics, along with a new behavioural technique, to investigate the effectiveness of hydrodynamic imaging. In a head-on approach to a wall, fish reacted to avoid collision with the wall at an average distance of only 4.0±0.2 mm. Contrary to previous expectation, there was no significant correlation between the swimming velocity of the fish and the distance at which they reacted to the wall. Hydrodynamic imaging appeared to be most effective when the fish were gliding with their bodies held straight, with the proportion of approaches to the wall that resulted in collision increasing from 11% to 73% if the fish were beating their tails rather than gliding as they neared the wall. The swimming kinematics of the fish were significantly different when swimming beside a wall compared with when swimming away from any walls. Blind cave fish frequently touched walls when swimming alongside them, indicating that they use both tactile and hydrodynamic information in this situation. We conclude that although hydrodynamic imaging can provide effective collision avoidance, it is a short-range sense that may often be used synergistically with direct touch.

Rhythmic actomyosin-driven contractions induced by sperm entry predict mammalian embryo viability

Fertilization-induced cytoplasmic flows are a conserved feature of eggs in many species. However, until now the importance of cytoplasmic flows for the development of mammalian embryos has been unknown. Here, by combining a rapid imaging of the freshly fertilized mouse egg with advanced image analysis based on particle image velocimetry, we show that fertilization induces rhythmical cytoplasmic movements that coincide with pulsations of the protrusion forming above the sperm head. We find that these movements are caused by contractions of the actomyosin cytoskeleton triggered by Ca2+ oscillations induced by fertilization. Most importantly, the relationship between the movements and the events of egg activation makes it possible to use the movements alone to predict developmental potential of the zygote. In conclusion, this method offers, thus far, the earliest and fastest, non-invasive way to predict the viability of eggs fertilized in vitro and therefore can potentially improve greatly the prospects for IVF treatment.

Active wall following by Mexican blind cavefish (Astyanax mexicanus).

When introduced into a novel environment that limits or prevents vision, a variety of species including Mexican blind cavefish (Astyanax mexicanus) exhibit wall-following behaviors. It is often assumed that wall following serves an exploratory function, but this assertion remains untested against alternative artifactual explanations. Here, we test whether wall following by cavefish is a purposeful behavior in which fish actively maintain a close relationship with the wall, or an artifactual consequence of being enclosed in a small concave arena, in which fish turn slightly to avoid the wall whenever it impedes forward movement. Wall-following abilities of fish were tested in a large, goggle-shaped arena, where forward motion along the convex wall was unimpeded. In this circumstance, cavefish continued to follow the wall at frequencies significantly above chance levels. Lateral line inactivation significantly reduced the ability of fish to follow convex, but not concave or straight, walls. Wall-following abilities of normal fish decreased with decreasing radius of wall convex curvature. Our results demonstrate that cavefish actively follow walls of varying contours. Radius-of-curvature effects coupled with the difficulties posed by convex walls to lateral line-deprived fish suggest a partially complementary use of tactile and lateral line information to regulate distance from the wall.

The flow fields involved in hydrodynamic imaging by blind Mexican cave fish (Astyanax fasciatus). Part II: gliding parallel to a wall.

SUMMARY Blind Mexican cave fish (Astyanax fasciatus) are able to sense detailed information about objects by gliding alongside them and sensing changes in the flow field around their body using their lateral line sensory system. Hence the fish are able to build hydrodynamic images of their surroundings. This study measured the flow fields around blind cave fish using particle image velocimetry (PIV) as they swam parallel to a wall. Computational fluid dynamics models were also used to calculate the flow fields and the stimuli to the lateral line sensory system. Our results showed that characteristic changes in the form of the flow field occurred when the fish were within approximately 0.20 body lengths (BL) of a wall. The magnitude of these changes increased steadily as the distance between the fish and the wall was reduced. When the fish were within 0.02 BL of the wall there was a change in the form of the flow field owing to the merging of the boundary layers on the body of the fish and the wall. The stimuli to the lateral line appears to be sufficient for fish to detect walls when they are 0.10 BL away (the mean distance at which they normally swim from a wall), but insufficient for the fish to detect a wall when 0.25 BL away. This suggests that the nature of the flow fields surrounding the fish are such that hydrodynamic imaging can only be used by fish to detect surfaces at short range.

Phospholipase C-ζ-induced Ca2+ oscillations cause coincident cytoplasmic movements in human oocytes that failed to fertilize after intracytoplasmic sperm injection

Objective To evaluate the imaging of cytoplasmic movements in human oocytes as a potential method to monitor the pattern of Ca2+ oscillations during activation. Design Test of a laboratory technique. Setting University medical school research laboratory. Patient(s) Donated unfertilized human oocytes from intracytoplasmic sperm injection (ICSI) cycles. Intervention(s) Microinjection of oocytes with phospholipase C (PLC) zeta (ζ) cRNA and a Ca2+-sensitive fluorescent dye. Main Outcome Measure(s) Simultaneous detection of oocyte cytoplasmic movements using particle image velocimetry (PIV) and of Ca2+ oscillations using a Ca2+-sensitive fluorescent dye. Result(s) Microinjection of PLCζ cRNA into human oocytes that had failed to fertilize after ICSI resulted in the appearance of prolonged Ca2+ oscillations. Each transient Ca2+ concentration change was accompanied by a small coordinated movement of the cytoplasm that could be detected using PIV analysis. Conclusion(s) The occurrence and frequency of cytoplasmic Ca2+ oscillations, a critical parameter in activating human zygotes, can be monitored by PIV analysis of cytoplasmic movements. This simple method provides a novel, noninvasive approach to determine in real time the occurrence and frequency of Ca2+ oscillations in human zygotes.

The influence of viscous hydrodynamics on the fish lateral-line system

Fish exhibit many behaviors that involve sensing water flows with their lateral-line system. In many situations, viscosity affects how the flow interacts with the body of the fish and the neuromasts of the lateral line. Here we discuss how viscosity influences the stimulus to the fish lateral-line system. The movement of a fishs body creates flows that can interfere with the detection of external signals, but these flows can also serve as a source of information about nearby obstacles and the fishs own hydrodynamic performance. The viscous boundary layer on the surface of the skin alters external signals by attenuating the low-frequency components of stimuli. The stimulus to each neuromast depends on the interaction of the fluid surrounding the neuromast and the structural properties of that neuromast, including the number of mechanosensory hair cells it contains. A consideration of the influences of viscosity on flow, at both the whole-body and receptor levels, offers the promise of a more comprehensive understanding of the signals involved in behaviors mediated by the lateral-line system.

Hydrodynamic Imaging by Blind Mexican Cavefish

Blind Mexican cavefish (Astyanax mexicanus) live in complete darkness in underground streams and pools. These eyeless fish use hydrodynamic imaging to sense their surroundings. Hydrodynamic imaging involves fish using their mechanosensory lateral line system to sense changes in the water flows around their body caused by the presence of nearby objects. This allows them to sense detailed information about their surroundings as they move through complex environments. The fluid dynamics associated with this remarkable ability have been revealed using experimental flow measurements and computational modelling. Measurements of the fish’s behavior and of the flow fields around the fish show that hydrodynamic imaging has a short range, of the order of 10 % of the fish’s body length, and that fish need fast reactions in order to use it for collision avoidance. Due to the fluid dynamics of the flow fields involved, this sensory range is not increased when fish swim faster, contrary to previous expectations. This chapter summarises the behaviors and fluid dynamics involved with hydrodynamic imaging as used by blind cavefish.

Fine-scale flight strategies of gulls in urban airflows indicate risk and reward in city living

Birds modulate their flight paths in relation to regional and global airflows in order to reduce their travel costs. Birds should also respond to fine-scale airflows, although the incidence and value of this remains largely unknown. We resolved the three-dimensional trajectories of gulls flying along a built-up coastline, and used computational fluid dynamic models to examine how gulls reacted to airflows around buildings. Birds systematically altered their flight trajectories with wind conditions to exploit updraughts over features as small as a row of low-rise buildings. This provides the first evidence that human activities can change patterns of space-use in flying birds by altering the profitability of the airscape. At finer scales still, gulls varied their position to select a narrow range of updraught values, rather than exploiting the strongest updraughts available, and their precise positions were consistent with a strategy to increase their velocity control in gusty conditions. Ultimately, strategies such as these could help unmanned aerial vehicles negotiate complex airflows. Overall, airflows around fine-scale features have profound implications for flight control and energy use, and consideration of this could lead to a paradigm-shift in the way ecologists view the urban environment. This article is part of the themed issue ‘Moving in a moving medium: new perspectives on flight’.

Vision-based flight control in the hawkmoth Hyles lineata.

Vision is a key sensory modality for flying insects, playing an important role in guidance, navigation and control. Here, we use a virtual-reality flight simulator to measure the optomotor responses of the hawkmoth Hyles lineata, and use a published linear-time invariant model of the flight dynamics to interpret the function of the measured responses in flight stabilization and control. We recorded the forces and moments produced during oscillation of the visual field in roll, pitch and yaw, varying the temporal frequency, amplitude or spatial frequency of the stimulus. The moths’ responses were strongly dependent upon contrast frequency, as expected if the optomotor system uses correlation-type motion detectors to sense self-motion. The flight dynamics model predicts that roll angle feedback is needed to stabilize the lateral dynamics, and that a combination of pitch angle and pitch rate feedback is most effective in stabilizing the longitudinal dynamics. The moths’ responses to roll and pitch stimuli coincided qualitatively with these functional predictions. The moths produced coupled roll and yaw moments in response to yaw stimuli, which could help to reduce the energetic cost of correcting heading. Our results emphasize the close relationship between physics and physiology in the stabilization of insect flight.

No role for direct touch using the pectoral fins, as an information gathering strategy in a blind fish

Blind Mexican cave fish (Astyanax fasciatus) lack a functional visual system and have been shown to sense their environment using a technique called hydrodynamic imaging, whereby nearby objects are detected by sensing distortions in the flow field of water around the body using the mechanosensory lateral line. This species has also been noted to touch obstacles, mainly with the pectoral fins, apparently using this tactile information alongside hydrodynamic imaging to sense their surroundings. This study aimed to determine the relative contributions of hydrodynamic and tactile information during wall following behaviour in blind Mexican cave fish. A wall was custom built with a ‘netted’ region in its centre, which provided very similar tactile information to a solid tank wall, but was undetectable using hydrodynamic imaging. The fish swam significantly closer to and collided more frequently with the netted region of this wall than the solid regions, indicating that the fish did not perceive the netted region as a solid obstacle despite being able to feel it as such with their pectoral fins. We conclude that the touching of objects with the pectoral fins may be an artefact of the intrinsic link between pectoral fin extensions and tail beating whilst swimming, and does not function to gather information. During wall following, hydrodynamic information appears to be used strongly in preference to tactile information in this non-visual system.