Test Design Lighthouse

Down under – On the ocean floor

If we could freely observe the ocean floor, there would be more to see than mere abysses. In fact, the sea hides an imposing marine landscape as varied as that above ground, with mountains and valleys, high plateaus and deep-sea plains, and extensive mountain ranges. It is covered by an average of 3,650 metres of water and even at a depth of 500 metres, there is total darkness. Moreover, at a fairly constant 1 - 3°C as far as anyone can judge, the temperature is hardly warm and the hydrostatic pressure of the water increases by approximately one atmosphere for every 10 metres. At a depth of eleven kilometres, that equates to 1,100 atmospheres!


Around 80 per cent of the ocean floor lies below a depth of 1,000 metres and thus for its inhabitants, the standard hydrostatic pressure is over a hundred times the atmospheric pressure we are comfortable with. This variation of water pressure with depth is a factor that organisms living in the oceans cannot escape, and must consequently adapt to. However, it is clear that these conditions are not an obstacle to the development of a diverse animal population, since living organisms are found even at the bottom of the deepest abysses.


For a long time it was thought that the deep ocean was a uniform, sparsely-populated habitat, largely due to lack of nutrients, but the picture has changed in recent decades. Increasingly, the discovery and exploration of hitherto unknown biotic communities at hydrothermal vents, or the amazing diversity around underwater mountains, has confirmed the impression that this most inaccessible ecosystem is subject to considerable spatial-temporal variability.


The transitions between the individual zones are fluid (right). The eulittoral and sublittoral are roughly demarcated by the tides and the position of the shelf edge; the bathyal encompasses the continental slope, the abyssal the continental rise, the deep-sea plains and the mid-ocean ridge. Deep-sea trenches belong to the hadal.



The cylindrical anemones Cerianthus borealis live on the soft floor of the north Atlantic continental slope. (Photo: R. Cooper, NURP)

Ocean floor community – Life on the bottom

Particular zones of the sea floor, from the coast down to the deep ocean, are inhabited by characteristic biotic communities, whose members – the individual species – each require similar environmental conditions, e.g. water pressure and hence depth, illumination, and water quality. These factors also have a critical influence on the distribution of bottom-dwellers called benthic organisms, as they tie certain species to particular locations.


Unlike land or freshwater, the sea is home to a diverse and ecologically important group of static (sessile) animals, many of which look like plants (as is suggested by such names as 'sea anemone', 'sea pansy' etc). The zonation of these animals on the ocean floor is often just as striking as the zonation of trees on a mountainside, and provides a similar basis for the classification of marine communities to that used for the large terrestrial plants.


Soft corals, feather star and sea pen look for a hard substrate to fasten onto. (Photo: NURP)

The composition of the bottom or substrate largely determines the presence or absence of particular benthic organisms. On rocky ground the main forms that occur colonise the surface directly. Known as epifauna, in many cases they attach themselves firmly to the substrate. In general, the animal population consists of cnidarians (stinging creatures) such as sea anemones and corals, sponges, barnacles, tubeworms, bivalves such as blue mussels and oysters, and sea squirts. In amongst them, free-moving starfish, sea urchins, molluscs and crustaceans are found. Where there is sufficient light and secure anchorage, large algae grow, providing additional protection and food for further species.


The majority of the ocean floor, however, is covered with loose sediments – a habitat above all for burrowing forms known as infauna, which invade the substrate or bore tubes and burrows. The relationships created between the species in this process can be either direct, as with predator and prey, or indirect, if, for instance, tubes previously inhabited by one species and then abandoned are occupied by other species.


Food supply from above

For nourishment, benthic organisms depend almost entirely on particles that flutter down like snow. Except in the illuminated coastal zones, there is virtually no primary production by plants on the ocean floor, because the light necessary for photosynthesis cannot penetrate to such great depths.


Delicate feather stars orientate themselves towards the current in order to filter particles out of the water. (Photo: NURP)
With tentacles at the front end, the tubeworm scoops particles out of the water, hiding its body in a self-made tube. (Photo: NURP)

What reaches the bottom, in what form and, importantly, what quantity, depends in turn on a variety of factors. Sometimes, it consists of sizeable amounts of terrestrial plant matter such as wood and leaf remains, which can be found even at great depths. It is, however, the remains of pelagic organisms, those that swim or drift in the upper water levels of the open sea, that provide the real food for bottom fauna.


Seasonal changes on the surface, such as the summer blooming of phytoplankton and the associated population fluctuations in zooplankton, are ultimately reflected on the menu of the bottom fauna. Thus in temperate regions, the number of silica algae (diatoms) sinking to the bottom in summer can reach levels a hundred times the winter level, thus triggering seasonal fluctuations in the weight of bottom fauna.


Yet the deeper the water, the more meagre the fare. In shallow waters, the majority of food required by benthic organisms finds its way to the bottom. But at greater depths, so many particles are consumed by other members of the food web whilst still in the water column that only a fraction of the surface production reaches the depths. Deep-ocean bottom fauna has to make do with what is left over – or try something different.


For in the end, the cell walls, shells and skeletons, that reach the deep ocean are the very least digestible components of organic material. They are finally decomposed by bacteria which occur in large numbers in surface sediment, and which are an important connecting link in the marine food web. Organic compounds, which animals cannot use directly, are 'cracked' by bacteria and channelled back into the nutrient cycle by bacteria-filtering organisms, in the form of bacterial biomass. Nature works on a principle of almost 100% recycling.


On lucky days, bottom fauna can feast on the cadavers of large animals from the pelagic zone – sharks or whales, for instance. Unlike the fine particles of 'sea snow', these sink to the bottom at the rate of several thousand metres per day. In depths of up to 3,000 metres, bull sharks are grateful consumers; below that, sand fleas up to 20cm long, grenadier fish and squid take their share of the cadavers. Clearly, then, some of the animals living in total darkness on the ocean floors can move often enough, and far enough, to be able to devour the meat rations that come their way, albeit occasionally and unpredictably. In fact, in the Philippine Trench, scavenging crabs have been found at depths up to 9,000 metres.


Typical specimens

The diversity of species on the ocean floor is incredible. On a test site measuring only 50 square metres on the continental rise in the north-west Atlantic, almost 1,600 invertebrate species were recorded. As depth increases, so too does the proportion of small life forms, or meiofauna, within the overall fauna. Typical deep-sea organisms include such bizarre specimens as the xenophyophores, giant protozoan up to 25 centimetres in diameter with shells cemented together from foreign matter. Or their distant relations, the much smaller komokiacea, whose fine, irregular meshes are thought to contribute to the formation of ferromanganese nodules.


Galatheid crabs such as Munida iris are common burrow-dwellers on the continental slope. (Photo: S. Ross, NURP)
Octopuses live in all oceans, including on the muddy bottom of the deep-sea. (Photo: L. Levin, NURP)

Glass sponges use long spicules to anchor themselves in sediment, or directly onto the hard substrate. In faster-moving water, horny and stone corals may also be found on hard sea beds. Sea pens which, like corals, are cnidarians, bear a few large polyps arranged in star formation on long, whip-like branches. Burrowing sea anemones are also common on soft sediments.


Primitive species, long known only in fossil form, have been rediscovered: the primitive limpet Neopilina is only known at a depth of around 4,000 metres in the abyssal zone. Generally, bivalves and molluscs are represented in large numbers. The primitive bivalves of the sub-class Protobranchia, which use their elongated mouth lobes to collect edible particles from the substrate, are found only in the deep ocean, whereas the more highly developed bivalves, which feed by filtering their respiratory water, become fewer in number with increasing depth.


Most large species of epifauna belong to the same family groups as are found in the shelf seas. These include sea cucumbers, brittle stars, prawns and siliceous sponges. However, the greater the depth, the lower their occurrence per square metre of substrate. A whole array of mostly worm-like creatures of the endofauna belong to animal groups which are barely represented, if at all, outside of the oceans. Their strange bodies provoke as many questions as their mode of life. All that is known of some, for instance the green spoonworm (Echiurida), is from body parts recovered from soil samples taken at great depths.


The deep ocean is one of the habitats where closer inspection can still lead to the discovery of hitherto unknown species, and even new animal phyla. Estimates put the number of species as high as 10 million, and if this is accurate, only half of them have been described. Moreover, one 'hotspot' of marine biodiversity has only been known for about 30 years: the biotic communities around hydrothermal springs.