As we have seen, the seaweeds of a rocky shore grow in different areas – zones – depending on their ability out of the water. The animals show a similar zonation, but because they can move around it is not so obvious. Like the plants, the animals are greatly influenced by the tide. Some species live almost as land animals around the high water-mark of spring tides, and others are only briefly exposed to the open air at the low water-mark of spring tides.
All the animals are adapted to feed and breed during the few hours when they are covered by the tide. When the tide is out, birds such as gulls and oystercatchers search the shore, probably amongst the seaweeds for animals.
Here are some of the more common animals you can expect to find on the rocky shore.
Brittle Star
Brittle stars are not fish at all but are echinoderms, distantly related to sea stars. Some can swim but most crawl along the sea bed using their flexible arms. They usually have 5 arms covered with spines. Their name refers to how easily their arms will fall off when touched. It can also cast off an arm if it is injured. This isn't a problem for them as it can grow another arm within months and if the broken arm has a piece of the central disc attached, it can turn into a whole new brittle star!
They feed on plankton and detritus - decaying matter which they put in to their mouths, a hole which they also use to extrete from. In shallow water they are more active at night to avoid predators and will hide in dark crevices in the day. They are often eaten by their relative the common star fish.
Barnacles
The upper shore is usually dominated by barnacles, particularly the acorn barnacle. These crustaceans start life as tiny larvae floating in the sea with the plankton, and most come to rest in the intertidal area of a rocky shore, cementing themselves firmly to the rock. When exposed to the air, the barnacle closes its opening with a hinged trapdoor-like operculum – this is made of two limy plates which join up with other plates to protect the body. When covered by the sea, the operculum opens and six pairs of feathery legs (the equivalent of other crustaceans’ legs) appear and filter out particles of food.
Acorn Barnacle
The barnacle thrives in exposed conditions high up the shore, but even here it is not safe from predators. It is the favourite prey of the dog whelk which wanders up from the middle shore to attack the barnacles. The whelk also attacks other molluscs, particularly the limpet, by either producing a shell-dissolving acid which makes a hole in the shell or by boring a hole through the shell using its rough, belt-like tongue. The flesh is then sucked out. If you find an empty limpet shell with a small, neat hole in it, this is the work of the dog whelk. The dog whelk itself may be attacked by crabs and herring gulls.
Limpet Lifestyle
The limpets are perhaps the most well-adapted of all the marine snails for a life on the exposed rock surfaces. Each limpet has its own ‘home’ – an exact spot on the rock where it stays when the tide is out. On soft rock, the limpet grinds it with its shell to make an exact fit; on hard rock, the shell is ground down to fit the rock’s shape. This tight fit allows the limpet to trap a spoonful of water inside to stop it drying up. A strong foot muscle gets a firm grip on the rock, making it difficult for birds to prise off the limpet!
When the tide covers their rock, or in wet, cool weather, the limpets leave their base and wander about, grazing on young seaweeds which have started growing on the rock surface. Before the tide goes out, each limpet returns to its own ‘home’.
More about Molluscs
Apart from the dog whelks and limpets, several other molluscs live in the intertidal zone. Molluscs of the rocky shore are mostly uni-vavled (one shell) snails, whilst those of the sandy shore are mostly bivalves (two shells, such as a cockle). The bivalves can burrow down into the sand for safety, but the univalves, being unable to burrow into rock, have strong shells with an operculum (trapdoor attached to the foot muscle).
Small periwinkle: Up in the splash zone, the small periwinkle is found. It has a smooth, almost black, 4-6mm shell, and it is almost a land animal, eating lichens and being able to breathe air. However, the females have to release their eggs into the sea at high tides so the larvae can live amongst the plankton.
Rough periwinkle: The larger rough periwinkle, with its deeply grooved shell, is well-adapted for surviving on the upper shore, although it cannot breathe atmospheric air as well as the small periwinkle. It is often found thriving in the middle shore too. Unlike the other sea snails, the rough periwinkle produces live miniature snails, so it is almost set for a life totally on the land!
Flat periwinkle: The flat periwinkle, slightly larger than the rough, has a smooth shell ranging from white to black through shades of yellow, red and brown. It lives amongst the wracks of the middle shore, looking like the bladders (air-filled ‘bubbles’ on the fronds). The female sticks jelly-like egg masses underneath the fronds and one month later the baby snails chew their way out of the jelly.
Edible periwinkle: The edible, or common, periwinkle is the largest of the winkles, about 3.5cm with a more pointed shell. It lives all over the upper and middle shores, on bare rocks as well as beneath rocks and seaweeds.
Although the winkles are the most common molluscs on the middle shore, topshells may also be found here – and on the lower shore too. The most common is the purple topshell, up to 2cm across with red-purple ‘pyjama’ stripes.
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