Deep-sea scientists received a great shock in the late 1970s when springs of hot water were discovered gushing up through the sea floor into the icy waters of the deep Pacific. Since then many more have been discovered in all the main oceans. They are almost always associated with the crest of the mid-ocean ridge system at depths of between 2,000 and 3,000 metres. The existence of such leaks of hot water through the ridges had been predicted by geophysicists for many years, but in this session we will discover why the form they took, and particularly the remarkable animal communities associated with them, took everyone totally by surprise.
The water coming out of the vents was once ordinary sea water that percolated downwards many hundreds of metres through cracks and fissures left in the new sea floor as it solidified and spread away from the ridge. At a depth of a kilometre or more into the seabed the water met hot rocks of the oceanic crust and was both heated and chemically changed. The hot water was now very buoyant and rose, again through cracks in the rocks, to emerge into the sea through vents near the ridge crest. The temperature of the water coming out of the vents ranges from a modest 10-30° Celsius to a staggering 350-400°C. Because of the very high pressure, this superheated water does not boil and turn to steam as it would in air. Instead, it simply gushes out of the vent and mixes with the surrounding water, usually cooling more or less to the background temperature (2-3°C) within a few metres. When the hot-vent water comes into contact with the cold sea water, some of the chemicals it picked up from the deep rocks may be precipitated out as tiny particles, colouring the plume of water. Depending on the temperature, the rising plume of water may be black or white, and the corresponding types of vents are called black smokers or white smokers. They often produce their own chimneys, tubes of precipitated chemicals standing up to 30m above the surrounding sea floor until they become unstable and crash on to the bottom to start the process all over again.
The water gushing out of deep-sea vents is laden with a cocktail of chemicals that would be deadly to most forms of life, so it is the specialised organisms associated with them that make the vents really remarkable. These organisms not only can withstand the toxic chemicals, but actually thrive in them. Many vents are surrounded by fantastic communities in which life may be hundreds of times more abundant than on the adjacent sea floor. This is because the vent animals have their own rich source of food, which is totally independent of the input from the photosynthesizing plants in the overlying surface waters. The food is produced by special bacteria which can make the complex chemicals the animals need. But instead of using the energy in sunlight, like plants, they get their energy from some of the chemicals in the vent water in a process called chemosynthesis. Some of these bacteria are free-living and are eaten by other vent-dwellers; they can be so abundant that they form thick mats around the vents and even over the animals living there. Others live in close association with the animals, sometimes even inside their bodies, in a strange partnership from which both partners benefit.
Moving on
Many scientists believe that their strange combination of chemistry, temperature and pressure make the vents prime candidates for the place of origin of the very first life on Earth. Individual vents are not long-lived, probably lasting no more than a few tens of years before they stop gushing forth their hot chemical soup as suddenly as they started. When this happens, the community of animals living around them is doomed so they must be able to dispatch their young ones to find and colonise new vents. This is probably one of the reasons why the communities around vents that are reasonably close to one another on the same section of the ridge system tend to be very similar, but quite different from those on other sections, and particularly in different oceans.
Giant worms and monster clams
There are many hundreds of bivalve molluscs, such as mussels, oysters and cockles, in the deep sea but almost all of them are tiny, no more than a few millimetres long. So in the late 1970s scientists were amazed to see photographs from the deep eastern Pacific which showed the sea floor strewn with empty bivalve shells up to 25 centimetres long. These turned out to be the remains of vent communities that had died when the vent stopped working. Later, scientists in submersibles examining animal communities in the vicinity of living vents observed hundreds of bivalves crammed into crevices, and often several deep. The bivalves were of two distinct types, one related to the shallow-water mussels, the other to clams. The mussel, later named Bathymodiolus thermophilus, sometimes occurs in enormous numbers, up to 300 per square metre.
![[image]](giantmussel.jpg)
Natural History Museum |
Giant vent mussel Bathymodiolus elongatus, from a hydrothermal vent in the central Pacific at a depth of 2,800 metres. These animals obtain most of their nutrition from chemosymbiotic bacteria contained in their gills. These bacteria oxidise hydrogen sulphide emanating from the vents. |
They grow to a length of about 20cm and, like their shallow-living relatives, have well-developed gills and a functional mouth and gut. Also like shallow-living mussels they spend their lives anchored to the seabed with a beard of tiny threads, the byssus, although from time to time they can detach themselves, move to a new spot, and become re-attached. So, like mussels the world over, they probably filter water through the gills and pass the collected food particles, in this case vent bacteria, to the mouth. But the gut in Bathymodiolus is rather small, and it seems that they have another source of food as indicated by the very high numbers of bacteria that actually grow inside the mussels' gills and are attached to their surfaces. The bacteria seem to thrive in this situation even though many of them are destined to be eaten by their host.
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| Giant worms | |
| Yet another amazing feature of the giant worms is the rate at which they grow. What little is known of the growth of animals in the food-poor deep sea away from the vents suggests very slow growth rates, with many of them, including the small ones, taking years, sometimes, decades, to reach full size. In stark contrast, Riftia appears to be able to do this in only a couple of years, rivalling almost any animal in tropical shallow waters. But in 2000, scientists working with a related species from hydrocarbon seeps in the Gulf of Mexico that also grows to a length of 2m, found that this species takes around 200 years to reach full size, making it the longest-lived (and probably slowest-growing) marine invertebrate, apart from colonial animals like corals. | |
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The giant clam, Calyptogena magnifica, which grows even bigger than the mussel, also has a rich bacterial population associated with its gills. Unlike the mussel, it moves about quite actively, using a large, fleshy foot, which is pushed out from between its shells to either anchor the clam into a crevice or pull it along to a new location. Whereas when Calyptogena's shallow-living relatives move in this way when searching for food, the vent clam is probably searching for the best conditions to grow its gill bacteria. The clam has lost all trace of a gut and seems to be totally dependent upon its bacterial garden for its food.
The scientists were further amazed by one of the other animals at the Pacific vents--tube worms almost 2 m long and as thick as a man's wrist, growing in clusters like strange extra-terrestrial flowers. Named Riftia pachyptila, meaning the thick-feathered vent worm and referring to the brilliant red plume of gills sticking out of the top of the worm's white tube, these animals were placed by scientists in a totally new phylum, the Vestimentifera, which now has several other members found in other vent fields and around cold hydrocarbon seeps. Like the vent clam, Riftia has no mouth or gut. Up to half of its weight is made up of special chemosynthetic bacteria living in enormous numbers in the tissues of its body. The worm's blood delivers all the chemicals the bacteria need, having collected them from the vent water across the surface of the gill plume. The plume is richly supplied with blood vessels, which give its red colour.
Along with these giants, lots of smaller, and previously unknown, vent animals are found, including other molluscs and worms, shrimps, sea anemones and even fish. The Pompeii worm, Alvinella pompejana, is found in extraordinarily hot environments--particularly at vents in the Pacific--around white smokers where vent water emerges at around 150°C and black smokers with water at 350°C.
The tubes of hundreds of worms living together form a snowball-like mass on the side of the chimney close to the opening so that hot fluid flows through the tubes and over the worms' bodies. It is an incredibly dangerous place to live, within a few centimetres of water that is so hot that it would cook the worms within seconds, and with the ever present possibility that the chimney will suddenly collapse and destroy the whole worm colony. Even more incredibly, the animals seem happy to sit with their tails at 70°C or more and with the head at around 20°C, a gradient of no less than 50°C along the length of the body. No other creature on Earth could stand such a gradient for more than a few minutes, so Alvinella pompejana amply deserves its name.
Heat-seeking shrimps
In the Atlantic ocean, shrimp dominate the vent faunas. Thousands upon thousands of white shrimps, about 5 cm long, mass around the vents like bees swarming round a hive. There are a number of different species, found only at the vents, each with its own special life style and living in more or less distinct zones around the outflowing hot water. They feed on the vent bacteria living on the surfaces bathed by the outflowing water, but the shrimps are also covered in these bacteria, including the surfaces of the mouthparts or jaws and the lining of the gill chambers on either side of the animal's body above the bases of the legs.
It seems that the shrimp can clean these bacteria away with particularly flexible limbs and pass them to its mouth to be eaten. In fact it may be that this is the most important source of food and that the shrimps jostle one another to stay in water at the best temperature for their bacteria to grow. In other words, they seem to be farming their food on their own bodies.
Summary
Fifty years ago, biologists thought they understood deep ocean life rather well. They realised that many undescribed species remained to be discovered, but they were fairly confident that the supposedly monotonous deep-sea environment would not throw up any major surprises. Since then, the realisation that the deep-sea floor is extremely variable and species rich, and particularly the discovery of the amazing hydrothermal vent communities, has changed our view of the oceans completely. More clearly than ever before, we now know that by their influence on the earth's climate and the potential effects of global warming, the oceans are crucial to our well-being as well as that of the animals living in them. As we move into the new century, with rapidly improving technology, who knows what previously unimagined secrets of the oceans will be revealed.
