Closer inspection shows cephalisation, however, with multiple branches in the head region. The lateral line system enables detection of both vibrations in the water and changes in the pressure field set up as the fish swims. Ambush predators of deep water often have the lateral line nerve endings exposed, rather than buried in canals. The lateral line sense is sufficiently directional to enable blinded fish to find and capture prey. A very low concentration, perhaps a single molecule in some cases, is necessary to excite nerve impulses from the olfactory epithelium.
Fish have elaborate nostrils and enlarged parts of the brain associated with smell.
The nostril does not lead into the pharynx as in mammals, but is usually designed with some sort of flow-through system. The simplest is a forward-directed flap that divides the nostril into incurrent and excurrent openings and deflects water into it when the fish is moving relative to the water. Some fish have cilia that create a continuous current through a U-tube nostril. Others have a relatively thin membrane separating the nostril and the buccal cavity so that pressure changes in the buccal cavity associated with breathing create an aspiratory effect to draw water in and force it back out. Male finds female in many deep-sea fishes.