In this session we will examine some of the 1,500 different fish species known from the floor of the deep sea and the waters immediately above it. They are mainly black, dark-grey, or brown--without the light-coloured or silvery undersides typical of the camouflaging of fish from the sunlit upper levels. They fall broadly into two groups, the 'swimmers' and the 'standers'.

Nigel Merrett

Tripod fish, Bathypterois grallator. Their common name comes from the enormously elongated fin-rays on which they stand on the ocean floor. The fish face into the current and spread their other fins out like radio antennae.

Swimmers

	Ready Reference
	Crustaceans One of the divisions of the arthropod phylum. Marine crustaceans include the crabs, lobsters and shrimps, as well as many smaller creatures including the copepods. All arthropods are encased in a more or less rigid external skeleton, which is periodically shed to enable the animal to grow into a new and larger skin. Trenches The deepest parts of the ocean at depths of greater than 6,000m.

The neutrally buoyant swimmers either have gas bladders to stop them sinking, or very light bones and flesh. This allows them to wander up and down in the water column in search of food without spending too much energy in this food-poor environment. They include members of many different fish groups, but most of them are eel-like in appearance--long and thin, often with very elongated tails. The most abundant and species-rich deep-sea group, the macrourids commonly called grenadiers or rat-tails, have large heads and taper more or less continuously to the tip of the tail so that they resemble large tadpoles. Distant relatives of the cod family of shallow waters, they are represented by about 200 species worldwide at depths from about 250 metres to the ocean trenches. Adults range in length from about 20centimetres to 1.5m, and although some are specialised for scooping up mud from the sea floor and extracting tiny animals from it, most of them will eat more or less anything they can catch--dead or alive. Slow swimmers, they probably catch their prey--crustaceans, molluscs, worms and other fish--by stealth rather than by speed, and are also attracted to dead carcasses lying on the bottom.

Although rat-tails are nowhere very common, they may be among the most numerous animals of their size range on Earth because of the vast size of the deep-sea environment. One species, Coryphaenoides armatus, seems to occur in all the major oceans of the world between about 3,000m and 6,000m depth, and is probably represented by 20 billion individuals, more than three times as many as there are humans on the planet.

Standers
standers tend to be heavier than water and sink if they stop swimming. They spend most of their time either resting on the bottom waiting for a suitable piece of food to arrive, or making short excursions across the sea floor in search of food. They include relatives of the bottom-living skates and rays of shallow waters, and also bottom-dwelling sharks, although these are rarely more than about 1 metre long.

Most of the bottom-dwelling bony fish are also fairly small, and many of them, like the rat-tails, have massive heads and rather small bodies. The Liparidae family, also known as 'sea snails', are typical--there are many different species, but few grow to more than about 35cm long. The deepest record for any fish goes to Abyssobrotula galatheae, a member of another deep-sea family, Ophidiidae, which was dredged from the bottom of the Puerto Rico Trench at a depth of 8,368m in 1970.

Tripod fish
Compared with many other deep-sea fishes, one group, the so-called tripod fish, are particularly elegant and interesting. These species have very tiny eyes and probably detect their prey by sensing vibrations. Other fish in the same group have quite short fins, but their eyes are enormous flat structures covering the whole of the upper surface of the head. Although light-sensitive, these structures are not believed to be efficient as eyes, and these tripods probably rely on vibration sensitivity like their stilted cousins.

All tripod fish exhibit both male and female sex organs. This phenomenon, hermaphroditism, is by no means uncommon, but in other hermaphrodite species the testes and ovaries mature at different times so that the eggs of one individual are fertilised by the sperm from another, thus ensuring a mixing of genetic material. In the tripod fish, both sets of organs mature at the same time. This is possibly because, like some of the stationary invertebrates mentioned above, the animals are so sparsely distributed that they may not find a potential mate when they need one. If all else fails, one tripod fish can reproduce on its own by fertilising its own eggs.