In animal physiology, hydrodynamic reception refers to the ability of some animals to sense water movements generated by biotic (conspecifics, predators, or prey) or abiotic sources. This form of mechanoreception is useful for orientation, hunting, predator avoidance, and schooling. Frequent encounters with conditions of low visibility can prevent vision from being a reliable information source for navigation and sensing objects or organisms in the environment. Sensing water movements is one resolution to this problem. This sense is common in aquatic animals, the most cited example being the lateral line system, the array of hydrodynamic receptors found in fish and aquatic amphibians. Arthropods (including crayfish and lobsters) and some mammals (including pinnipeds and manatees) can use sensory hairs to detect water movements. Systems that detect hydrodynamic stimuli are also used for sensing other stimuli. For example, sensory hairs are also used for the tactile sense, detecting objects and organisms up close rather than via water disturbances from afar. Relative to other sensory systems, our knowledge of hydrodynamic sensing is rather limited. This could be because humans do not have hydrodynamic receptors, which makes it difficult for us to understand the importance of such a system. Generating and measuring a complex hydrodynamic stimulus can also be difficult.
Overview of hydrodynamic stimuli
Definition “Hydrodynamic” refers to the motion of water against an object that causes a force to be exerted upon it. A hydrodynamic stimulus is therefore a detectable disturbance caused by objects moving in a fluid. The geometry of the disturbance depends on properties of the object (shape, size, velocity) and also on properties of the fluid, such as viscosity and velocity. These water movements are not only relevant to animals that can detect them, but constitute a branch of physics, fluid dynamics, that has importance in areas such as meteorology, engineering, and astronomy. A frequent hydrodynamic stimulus is a wake, consisting of eddies and vortices that an organism leaves behind as it swims, affected by the animal's size, swimming pattern, and speed. Although the strength of a wake decreases over time as it moves away from its source, vortex structure of a goldfish's wake can remain for about thirty seconds, and increased water velocity can be detected several minutes after production.
Uses of hydrodynamic stimuli
Since movement of an object through water inevitably creates movement of the water itself, and this resulting water motion persists and travels, the detection of hydrodynamic stimuli is useful for sensing conspecifics, predators, and prey. Many studies are based upon the question of how an aquatic organism can capture prey despite darkness or apparent lack of visual or other sensory systems and find that the sensing of hydrodynamic stimuli left by prey is probably responsible. As for detection of conspecifics, harbor seal pups will enter the water with their mother, but eventually ascend to obtain oxygen, and then dive again to rejoin the mother. Observations suggest that the tracking of water movements produced by the mother and other pups allows this rejoining to occur. Through these trips and the following of conspecifics, pups might learn routes to avoid predators and good places to find food, showing the possible significance of hydrodynamic detection to these seals. Hydrodynamic stimuli also function in exploration of the environment. For example, blind cave fish create disturbances in the water and use distortions of this self-generated field to complete spatial tasks, such as avoiding surrounding obstacles.
Visualizing hydrodynamic stimuli Since water movements are difficult for humans to observe, researchers can visualize the hydrodynamic stimuli that animals detect via particle image velocimetry (PIV). This technique tracks fluid motions by particles put into the water that can be more easily imaged compared to the water itself. The direction and speed of water movement can be defined quantitatively. This technique assumes that the particles will follow the flow of the water.
Invertebrates To detect water movement, many invertebrates have sensory cells with cilia that project from the body surface and make direct contact with surrounding water. Typically, the cilia include one kinocilium surrounded by a group of shorter stereocilia. Deflection of stereocilia toward the kinocilium by movement of water around the animal stimulates some sensory cells and inhibits others. Water velocity is thus related to the amount of deflection of certain stereocilia, and sensory cells send information about this deflection to the brain via firing rates of afferent nerves. Cephalopods, including the squid Loligo vulgaris and cuttlefish Sepia officinalis, have ciliated sensory cells arranged in lines at different locations on the body. Although these cephalopods have only kinocilia and no stereocilia, the sensory cells and their arrangement are analogous to the hair cells and lateral line in vertebrates, indicating convergent evolution. Arthropods are different from other invertebrates as they use surface receptors in the form of mechanosensory setae to function in both touch and hydrodynamic sensing. These receptors can also be deflected by solid objects or water flow. They are located on different body regions depending on the animal, such as on the tail for crayfish and lobsters. Neural excitation occurs when setae are moved in one direction, while inhibition occurs with movement in the opposite direction.
Fish
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