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