In this session we will learn about many of the creatures that populate the deep-sea floor and observe how evolution has adapted them to survive in the murky world at the bottom of the ocean.

Southampton Oceanography Centre

Sea cucumber, or holothurian Peniagone diaphana, from the deep-sea floor. Unlike most sea cucumbers, this one can swim using slow undulations of its muscular body.

Seen from afar, from a submersible for instance, the creamy-brown mud on the abyss floor appears to stretch away absolutely flat in all directions. However, closer inspection reveals many mounds and depressions up to 30-40 centimetres across and 5-10cm high, all over the sea floor. Some are smooth and rounded, others are conical with a hole in the top like mini-volcanoes, and sometimes with a circle of holes surrounding the base. In some places, and at certain times of the year, greenish masses of material accumulates where phytoplankton remains have reached the bottom and been swept by water currents to gather around the mounds and in the depressions. The surface of the mud is also marked by a variety of strange grooves and little pock-marks meandering across the bottom like tiny tyre tracks.

The bumps, hollows and grooves on the sediment surface are the work of the sea-floor animals. Many of the mounds and holes are made by different types of worms living in the mud and either stretching their bodies over the surface to feed, or throwing out the contents of their burrows to form the mounds. Other holes are produced by molluscs, crustaceans and even starfish and brittlestars, while irregular gashes in the mud may have been caused by fish or shrimps as they pounced on a tasty morsel.

Larger animals
The 'tyre tracks' are produced by larger, bottom-living animals, such as deep-sea crustaceans, snails, starfish and sea cucumbers as they move around in search of food. Sea cucumbers are one of the dominant sea-floor groups all over the world. They have more-or-less sausage-shaped bodies with defined front and back ends. They come in all shapes and sizes, ranging from tiny ones no more than a centimetre long and living in burrows in the mud, through barrel-shaped versions with lots of stiff little legs, to massive football-sized giants, sometimes with long tails sticking up into the water. A few can even take off from the bottom and swim in the water column with remarkable undulations of their bodies. Almost all of them 'hoover' the surface of the mud for edible scraps as they move across it and periodically dump the unwanted contents of their guts as faecal casts. Because sea cucumbers are so universal, their trails and faecal casts are among the most common features on photographs of the deep-sea floor.

Patience

Southampton Oceanography Centre

Most starfish, whether in deep or shallow water, obtain their food from the sea bed itself. But some, like this one, Freyella elegans, photographed at a depth of 4,000m in the north Atlantic, spread their arms into the water above the bottom and catch food particles carried in the water currents.

Many deep-sea animals including sponges, sea anemones, tube worms and barnacles, adopt a 'sit and wait' strategy-- staying for long periods, sometimes permanently, in one place and depending on food particles that either fall on to them or are carried to them in the currents. Barnacles project only a few centimetres above the sediment surface. Other sedentary animals raise their feeding structures well above the bottom on long, thin stalks.

This sedentary life style is so successful that it has evolved independently several different times Although it solves the problems of finding food, it creates another problem--how to find a mate and reproduce--because different individuals of these stalked species often live tens or hundreds of metres apart.

In most of the species studied by scientists so far, the males apparently release their sperm into the water which then fertilise the eggs within the female. Just how the male cells survive long enough to cross the large distances involved, and how they locate a female with eggs, are two of the many deep-sea mysteries that still have not been solved.

Vultures of the abyss

Southampton Oceanography Centre

Eurythenes gryllus, a deep-sea scavenging amphipod. Most amphipods attracted to dead carrion on the deep-sea floor are no more than a centimetre or two long. But Eurythenes is a giant, growing to about 15cm long.

With rare exceptions, immediately after a large animal such as a squid, shark, dolphin or whale dies, its carcass will start to sink. Once it reaches the bottom, it is likely to be feasted upon by hundreds, or even thousands of amphipods--shrimp-like creatures that are found at all depths in the sea. They are usually no more than a couple of centimetres long, but on the deep-sea floor some species reach a length of 10-15cm and have sharp, powerful jaws that slice flesh like a surgeon's scalpel. They can detect the odour of a carcass carried on the water currents, possibly from hundreds of metres away, and rapidly home in on it.

The amphipods will soon be joined by a number of other active swimmers, including scavenging fish and shrimps, and the sediment around the carcass will be disturbed rapidly by their feeding frenzy. When no suitable food is left, the tiny pieces remaining will attract slow-moving animals, such as molluscs, starfish, brittlestars and sea cucumbers. Finally, bacteria will finish the demolition job, even breaking down bones, so that within weeks of the arrival of the carcass there would be hardly any evidence that it had ever existed.

Natural History Museum

Foraminiferan remains from the White Cliffs of Dover, UK. The cliffs are made up of unimaginable numbers of chalky shells of long-dead marine animals. Foraminifera are very simple organisms that consist mainly of protoplasm. They feed by engulfing bacteria or other tiny pieces of organic matter. By secreting a chalky shell, or by sticking together mud particles or pieces of other animals' shells into balls, tubes, or multi-chambered little houses in which they live, these tiny animals produce a bewildering variety of strange shapes.

Smaller beasts
Apart from when they are brought together by these food bonanzas, animals big enough to be seen with the naked eye are distributed fairly thinly on the deep-sea floor with perhaps one animal every 10 or 20m. But for every one of these big animals there are many thousands of tiny ones, no more than a few millimetres long and living hidden away in the abyssal mud. Many different animal groups are represented here, but just four or five groups dominate in terms of numbers.

Natural History Museum

Acanthometra, a radiolarian with skeletal spicules of strontium sulphate, and foraminiferans from the Indian Ocean.

First, at the lower end of the size range are the single-celled animals, the foraminiferans and their relatives. These are very simple organisms which feed by engulfing bacteria or other tiny pieces of organic matter into their protoplasm, and are extremely numerous. By secreting a chalky shell, or by sticking together mud particles or pieces of other animals' shells into balls, tubes or multi-chambered little houses in which they live, these tiny animals produce a bewildering variety of strange shapes.

Another important group of single-celled animals are the Radiolaria. They have skeletons made of glass-like silica or strontium sulfate, often in radiating spicules--hence their name. Most of the 4,000 or so species are planktonic, but a few live on the bottom.

Natural History Museum

Marine nematode worm. Nematodes or thread worms are one of the dominant groups in deep-sea muds. There are certainly thousands, possibly tens of thousands, of species still to be discovered.

The second most abundant group, and among the smallest of the many-celled animals, are the nematodes or thread worms. Nematodes are found in all environments--marine, fresh-water and terrestrial--and may outnumber all other many-celled animals on Earth. In the deep sea they range in length from a few tenths of a millimetre to a centimetre or more, and include bacterial grazers (specialist predators that can puncture the prey's cell membranes and suck out the internal juices), and active and deadly hunters.

The crustaceans are the next in importance. They include amphipod shrimps, which are related to, but much smaller than, the big scavenging amphipods, and several other groups specialised in producing tunnels or in pushing their way through the sediment by simply moving aside mud particles or other animals. Deep-sea isopods appear in a variety of forms--some resemble their terrestrial relatives, woodlice; others have a flattened form; some have long, thin, matchstick-like bodies ideal for ploughing through the sediment; others have long spindly legs well-suited for walking across the soft surface of the mud.

The most abundant of the mud-dwelling crustaceans, however, are the harpacticoid copepods, related to the dominant zooplankton group in the mid-water realm, but specialised for living in the sediment and feeding on bacteria or organic detritus.

Natural History Museum

Newly discovered polychaete worm, Sigambra sp., from the floor of the abyssal Atlantic. Though some polychaetes reach 10-20cm, most deep-sea species, like this one, are a few millimetres long.

Next come the polychaete worms which are found at all depths and include very mobile species, which crawl actively across the bottom, or even swim; and sedentary species, which live in tubes or burrows, and either forage over the sediment surface for pieces of food or catch floating particles with feather-like tentacles.

Only one other deep-sea, mud-dwelling group, the bivalve molluscs (relatives of the shallow-water clams and mussels with two shells, or valves, hinged together), approaches the polychaetes in abundance. Most deep-sea bivalves live buried in the sediment and send out feeding palps to gather pieces of food from the surface of the mud, while a few have become accomplished carnivorous hunters.

But most surprising of all, there are specialist deep-sea, wood-boring bivalves. Their environment is just about as far from a source of any wood as is possible. These bivalves will attack the hulls of sunken wooden ships, but they were around long before humans first sailed over the deep ocean. Unlikely as it seems, trees from coastal forests must fall into the sea and eventually sink into the abyss sufficiently often to make this strange lifestyle worthwhile.

The very smallest

	Ready Reference
	Abyss Greek word meaning 'bottomless'. The deep part of the oceans, between about 3,000m and 6,000m deep. Amphipods Group of shrimp-like crustaceans ranging in length from a few millimetres to more than 10 cm. Bacteria Tiny living organisms, neither clearly plants nor animals, which have no clear nucleus like the cells of most other creatures. Copepods One of the most important groups of planktonic crustaceans in the oceans. Most are small, just a few millimetres long, and many feed on phytoplankton cells. Hydrothermal vents Springs of very hot, chemically-laden water gushing through the sea floor along the mid-ocean ridge system. Species The basic unit into which scientists divide the living world. Members of a species can interbreed with one another, but not with members of another species.

Tiny bacteria cells occur everywhere on Earth where life is remotely possible and their presence in the deep ocean has been known for decades. Until the 1970s, however, just a few rather similar oceanic types were recognised, but new techniques have revealed a bewildering variety in two quite distinct kingdoms, the true Bacteria and a newly recognised one, the Archaea.

Recognition of the importance of these organisms in oceanic processes has also grown enormously in recent years. They occur on and within all deep-sea animals, where they perform a variety of crucial roles, ranging from producing bioluminescence to helping with digestion or even providing a source of food themselves. They also live independently of other organisms, being involved in a multitude of chemical processes in the water column and sea-bed sediments, in and around hydrothermal vents, and are associated with natural oil and gas seeps.

Bacteria have also been cultured from mud cores collected from hundreds of metres beneath the sea bed in sediments deposited millions of years ago. How they survive in this unbelievably hostile environment is still something of a puzzle, but the discovery of this deep biosphere is potentially enormously important. It suggests that tiny microbes may inhabit vast regions of the Earth's interior, way beyond the reach of any other life-forms. If so, despite their diminutive individual size the Bacteria and Archaea may in combination outweigh all the rest of the planet's living organisms put together. Unravelling the mysteries of this new-found, but ancient, world will be one of the great challenges to ocean scientists in the twenty-first century.