<P> The swimbladders of deep sea fish are either absent or scarcely operational, and bathypelagic fish do not normally undertake vertical migrations . Filling bladders at such great pressures incurs huge energy costs . Some deep sea fishes have swimbladders which function while they are young and inhabit the upper epipelagic zone, but they wither or fill with fat when the fish move down to their adult habitat . </P> <P> The most important sensory systems are usually the inner ear, which responds to sound, and the lateral line, which responds to changes in water pressure . The olfactory system can also be important for males who find females by smell . Bathypelagic fish are black, or sometimes red, with few photophores . When photophores are used, it is usually to entice prey or attract a mate . Because food is so scarce, bathypelagic predators are not selective in their feeding habits, but grab whatever comes close enough . They accomplish this by having a large mouth with sharp teeth for grabbing large prey and overlapping gill rakers which prevent small prey that have been swallowed from escaping . </P> <P> It is not easy finding a mate in this zone . Some species depend on bioluminescence . Others are hermaphrodites, which doubles their chances of producing both eggs and sperm when an encounter occurs . The female anglerfish releases pheromones to attract tiny males . When a male finds her, he bites on to her and never lets go . When a male of the anglerfish species Haplophryne mollis bites into the skin of a female, he releases an enzyme that digests the skin of his mouth and her body, fusing the pair to the point where the two circulatory systems join up . The male then atrophies into nothing more than a pair of gonads . This extreme sexual dimorphism ensures that, when the female is ready to spawn, she has a mate immediately available . </P> <P> Many forms other than fish live in the bathypelagic zone, such as squid, large whales, octopuses, sponges, brachiopods, sea stars, and echinoids, but this zone is difficult for fish to live in . </P>

Name the type of fish which live in deep sea