Tidal rapids
Tidal rapids are the most species-rich parts of sea lochs and often contain several different habitats and communities in a small area. The seabed looks like a mass of plant growth, but often these are animals in disguise with plant-like names such as sea anemones, sea firs and sea mosses (otherwise known as sea mats). Just as plants spread out branches and leaves to collect the maximum amount of light from the sun, these animals spread out arms and tentacles to capture small particles carried on the water currents. A range of mobile animals such as fish, sea urchins, starfish, squat lobsters and hermit crabs graze on the attached animals and plants or search for other mobile prey.
In the shallow tidal rapids, dense sea firs and sea mats grow beneath tall kelp plants and a large brown seaweed, the sea oak (Halidrys siliquosa). Magnificent sponges, particularly the green and yellow breadcrumb sponge, often wrap around kelp holdfasts. Myriads of tiny ghost shrimps hang all around. Small multicoloured anemones nestle in crevices and any rock not colonised by animals has a smooth hard living paint of coralline seaweed. In deeper water, dense orange or white soft corals, dead man’s fingers, are preyed on by the mushroom-coloured, giant triton seaslug, up to 20 centimetres long, and the red cushion star.
The mixed seabeds found in many tidal rapids probably represent the remains of glacial moraines from which all fine material has been winnowed out by the water currents. Various sizes of stones have been left among coarse, mobile gravel and support a wide variety of animals living on, between and under stones. Sea firs, brittlestars, butterfish, squat lobsters and queen scallops all abound while spectacular red and white striped dahlia anemones and sunstars add splashes of startling colour. A variety of exotically coloured seaslugs eat the sea firs and sea mats, and hermit crabs scuttle here, there and everywhere. Other animals need searching for – the elusive Yarrell’s blenny for instance with its topknot of tentacles lives hidden in crevices or even in old bottles.