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Scavengers of the deep, these long-tailed fish use their acute sense of smell to sniff out food in the darkness of the deep ocean.
They live typically for around 20-30 years.
They are about 30-60cm (1-2ft).
Rat-tails have large heads and long, tapering tails which makes them look like large tadpoles. They have a blunt snout and chin barbels that help them to feel around on the sea bed for food. The mouth is on the underside of the head.
They are found in deep waters, from the Arctic to Antarctic.
They are benthic or bottom-dwelling fish found to depths of over 3,000m.
Rat-tails have a varied diet, which includes fish, crustaceans and cephalopods.
Some feed on animals swimming above the seabed, others pick their food off the sea floor. Some have hard, pointed snouts which allow them to plough through the sediment in search of worms and other small invertebrates.
Special projections on the inner gill arches act as sieves to extract the food as sediment passes through.
Rat tails are the most abundant of the sea-floor species. They are the fish most commonly seen from submersibles. They move with their heads angled down into the current and their tails elevated.
They have swim bladders which makes them buoyant and allows them to move up and down the water column, but they tend to be slow-moving to conserve energy in this demanding habitat.
Free-floating eggs are released into the water.
They are not on the IUCN Red List
Some rat-tail species have a complex light-producing organ near their anus, which contains a lens and mirrors to direct the beam. The function of this is a mystery, but it may be used as a searchlight to look for food.
Some species can produce sounds by means of paired muscles attached to the swim bladder. There are around 200 species of rat-tails, and they are also known as grenadiers.
Some rat-tails have large and extremely sensitive eyes, with a larger number of rods than any other animal.