The word 'beach' conjures up an image of gently sloping sand between the sea and the land, where you can relax and enjoy fresh sea air and sunshine. Rocky and shingle beaches may not seem so inviting, but are worth exploring.
A shingle beach is a harsh environment; waves roll the stones about, scraping them together and crushing seaweeds and animals. Higher up the beach, beyond the high water mark, plants colonise areas where shingle was thrown up by waves. Although there is little soil and freshwater there, sea kale, (a wild cabbage), bright blue viper's bugloss, pink sea bindweed and many other plants thrive. In spring, terns and plovers lay their eggs on the shingle; the patterns on the eggs camouflage them from predators such as gulls, kestrels and foxes.
A rocky shore is a wild place to explore at low tide, but tread carefully, it is all too easy to slip or damage marine life. Sheltered rocky beaches support a rich diversity of life, with seaweeds and animals living near low water levels being different to those found around the high water levels. For simplicity, ecologists divide a rocky shore into four zones, each defined by relative time of submergence by tides.
The 'splash zone' is rarely flooded by tides and the maximum spread of sea spray can be gauged from the extent of black lichen covering the rock. Few shore species survive there, including rough and small periwinkles as well as the sea slater, which resembles a large woodlouse. These species are also found on the upper shore, below the splash zone.
The 'upper shore' floods only at mean to extreme high water spring tides, but channelled wrack (seaweed) is common, and its blackened dried-up strands rehydrate quickly when submerged. The water in rock pools on the upper shore becomes hot on sunny days and saltier as water evaporates. However, when it rains the water is diluted, decreasing salinity. Only seaweeds, sea lettuce, the filamentous Enteromorpha, and a few animals (such as beadlet sea anemones and small crustaceans) can survive such extremes.
The 'middle shore' contains rocks covered with limpets, which graze on algae; other rocks are covered by barnacles. Their shells open at high tide and their limbs emerge, making grabbing movements to collect plankton.
Groups of mussels tie themselves to rocks by special 'byssus' threads. Both barnacles and mussels have free-swimming larvae, which swim with plankton for a few weeks, until they settle on rock surfaces and become attached, where they are prey for fish and dog whelks. Brown seaweeds, spiral, bladder and knotted wracks may blanket the middle shore.
The slimy feel of seaweeds derives from the jelly-like mucilage that protects them from drying out at low tide. Look under fronds of seaweed to find brightly coloured flat periwinkles, painted top shells, and crabs sheltering there. Top shells and periwinkles are marine snails, which breathe by gills that extract oxygen from seawater; the gills are closed off at low tide.
Rock pools in the middle and lower shore are the most interesting as they have the greatest diversity of life. Delicate red seaweeds can be seen as well as sea lettuce. Greenish breadcrumb sponges encrust the surfaces of submerged rock; they filter out tiny food particles from the water. Sea lemons, which are bizarre-looking sea slugs, eat breadcrumb sponges. Hermit crabs, periwinkles, snakelocks anemones and prawns are common, and fish including blennies, gobies, suckers and wrasse may be seen. Cowries and colonial sea squirts may be found in lower shore rock pools, as well as starfish, crabs and sea urchins.
Seaweeds and animals in the lower shore zone are exposed for the shortest periods each day. Typical seaweed there is kelp, (Laminaria), which has a long leathery stalk known as a 'stipe,' anchored to rock by a strong holdfast. Marine worms live in the holdfasts, and other animals shelter there at low tide.
Sandy beaches look beautiful but quite devoid of life, apart from the seagulls flying around. Yet look below the sand at low tide and you will find animals hiding there, especially in sheltered beaches where there is both mud and sand. Wet or damp sand hides molluscs, crustaceans and worms from predators, and protects soft-bodied worms from drying out.
High up the beach on the strand line (the highest point reached by the tides) you may find bits of seaweed amongst the driftwood and litter. Digging around there may cause sandhoppers to jump into the air. They are harmless shrimp-like crustaceans, which eat rotting seaweed.
Clues on the surface help you to find the animals buried in wet sand close to the low tide mark. Large worm casts, each about 6-10 cm from a small circular depression in the sand are made by lugworms. The worm cast marks the rear end of a lugworm in its U-shaped burrow, and the depression marks where the head is eating sand. If you can dig up a lugworm, you will see that it resembles an earthworm, but has bristles on each segment. Other animals in wet sand include bivalve molluscs, such as cockles, tellin and razor shells.
At high tide cockles and tellin extend their feeding siphons into the water. Tellin hoover up particles from the surface of the sand, whereas cockles suck in water and filter out tiny planktonic organisms and food particles. On some beaches, you can find empty seashells that provide clues to the animals in the sand. Shallow pools close to the low tide mark contain sand gobies and shrimps, which blend so well with the sand that they are difficult to see.
You may also find netted dog whelks in wet sand. They are hungry scavengers; large numbers of netted dog whelks will gather round a dead fish, in a slow-motion feeding frenzy.
Animals buried in sand form a crucial part of the food web of a sandy shore. Look for clues that indicate which predators have eaten the inhabitants of empty seashells. Small round holes in tellin shells were drilled by necklace shells, predatory snails, to access and eat the animal inside.
Sandhoppers are popular prey for turnstones, rock pipits and sandpipers foraging on the strand line. Oystercatchers probe wet sand for cockles and tellin, using their long red beaks. Few waders have beaks long enough to get lugworms; they include curlews and godwits. Plovers and other waders with short beaks catch shrimps and other small crustaceans at shallow depths in the sand.
Our beaches provide varied habitats for communities of seaweeds and animals. Rocky shores give us glimpses of colourful marine life and simple observations can tell us a lot about the complex food chains linking different animal species. So next time you are on a beach go exploring and take a pocket guide to seashore life with you, especially if you are on a rocky shore!