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: Fauna of the sea floor, Ecomare
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Fauna of the sea floor

At first, one does not see much on the bottom of the North Sea. However, appearances are deceptive: most of the animals which live here are hiding. Worms, crustaceans and shellfish live in holes and tunnels under the surface of the sea floor. Looking carefully, one can identify life on the bottom: shrimp, crabs and starfish crawl around and other animals are attached to stones or shipwrecks. Every different area in the North Sea (such as various bottoms) has its own characteristic species of animals.

At home in sand in mud
By far, the largest section of the bottom of the North Sea consists of sand and silt. A lot of animals live there burrowed in, such as worms and shellfish. Some worms, such as rag-worms, move through the bottom while digging. Others live permanently in tubes sticking out just above the floor surface, for example, the Pygospio elegans. Its tube is built up of sand grains stuck together with slime. If the tube is covered over by sand, the animal just extends it.
Most species of shellfish live in the bottom. They maintain contact with the surface via their siphons. These tubes provide the intake and return of the seawater. The animals use the oxygen in the water and filter out the nutrients. Well known examples of shellfish living in the bottom are the Baltic tellin, the cockle and the gaper.
Regions with lots of silt contain more animals, in other compositions of species as well, than regions with lots of sand. Silty bottoms contain many more remnants of dead plants and animals, which means more food is available for the benthic animals.
Hard bottoms
Rocky bottoms are not very common in the North Sea. They are found mostly by the Straits of Dover and in the vicinity of Scotland and Norway. Fields of stones are spread throughout the North Sea. There are a few places in the North Sea region where gravel beds are found on the surface of the sea floor. These are the remains from river beds in the time when the North Sea was dry. Barnacles, mussels, sea anemones and sponges spend their lives attached to rocks and stones. Crabs and lobsters like to hide between stones. These 'rock' inhabitants are also found in 'artificial rocky areas', for example in marine harbours, on or in the vicinity of shipwrecks, and at the foot of breakwaters.
There's another group of bottom inhabitants crawling over the bottom. This group consists mostly of scavengers and carrion eaters such as crabs, starfish and sea urchins. Flatfish and rays also belong to this category. These fish, such as plaice and sole, live on the sea floor and feed on benthic animals and small fish. They often have a color pattern similar to the seabed where they reside in order to camouflage themselves better. Turbot are even capable of adapting their color to their environment! Benthic fish dig themselves in.
Predators and scavengers
Starfish consuming a stickleback, Ecomare
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In the bottom of the tidal area, there are animals crawling around which eat other benthic animals with their strong jaws, dead or alive, such as the rag-worm and the nephthys hombergi. Most predators and scavengers live on the bottom, for instance, shrimp, crabs and fish. However, few species are true carrion eaters, as are certain gammarids and Baltic isopods. Bacteria and fungi convert the organic material into nutrients. The starfish uses its suction feet not only to move around but also to catch mussels. It can break open a mussel by placing two arms on the left valve and the other arms on the right valve. The mussel will close its valves with the strongest constrictor muscle, however the starfish waits patiently. When the mussel tires, the starfish begins to pull, and after holding out for a few hours, the shell halves will separate. The starfish everts its stomach into the shell and digests the mussel.
Common dogwhelks and necklace shells eat mostly other molluscs and barnacles. Using a combination of a grating tongue and a dissolving acid, the snail drills a hole into the victim's shell. The snail then devours the soft animal in the shell. On the beach, one regularly finds shells with a nice round hole. You now know how its inhabitant's life was ended.
Dead algae and marine animals also end up on the sea floor, together with excretions from the still living animals. This forms food for the masses of bacteria, which break down the nutrients into silicic acid, nitrate and phosphate. These are used by the plants in the sea (the phytoplankton) in order to grow. Beside the bacteria, crabs, shrimp and brittle-stars also feast upon the dead animals.