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