ROCK POOL FISH
The seas surrounding the British Isles provide a home for a large number of different fish.
Although not so varied as tropical seas, at least 50 different species of fish can be discovered in pools at low tide. Many of these fish will be young of larger fish or others that get trapped by the ebbing tide.
This article will explore the fish that are often found in pools and under rocks on the shore.
Blenny (Lipophrys pholis)
This little green fish is found on rocky shores the length of the British coastline. It is a very common fish that hides under rocks and in crannies when the tide is out. It is called by other names as well, and is well known as the Shanny and the Sea-frog. The latter name may be because of its habit of basking on weeds out of the water and jumping back into the pool with a plop when it is disturbed. The Blenny has slime-covered skin. It also has sharp comb-like teeth which it uses to crunch barnacles off rocks and to attack dead crabs and pieces of carrion that it cannot swallow whole. The adults are up to 16 cm long and they come inshore to breed during early spring, where the adult male can be found guarding eggs in the large pools. However, it is the young fish that are much more common under rocks and even in holes in the rocks. Blennies swim with an undulating snake-like motion. They can be distinguished from the gobies by a continuous dorsal (top) fin that runs the length of the body.
There are two other blennies that can be found between the tides. The larger and fiercer Tompot Blenny Parablennius gattorugine is brownish-orange and has two distinct tentacles on its head. The Montagu's Blenny, Coryphoblennius galerita, is similar, but is covered in pale blue dots, and has a flap of skin on its head. Montagu's Blenny is only found in the south-west of Britain.
Rock Goby (Gobius paganellus)
The gobies are the other family of fishes that are common on the shore. There are several species, but the Rock Goby is the one that is often discovered under rocks when the tide goes out. The adults grow to 12 cm long and are almost black in colour. However, if they are living on sand they may be much lighter. Young fish are much more common than adults on the shore and may only be 45 mm in length. All the gobies have two dorsal fins and a pelvic (underneath) fin that is fused into a weak suction cup. They have small scales. The Black Goby, Gobius niger, looks very much like a Rock Goby. The Rock Goby is not found on the east coast. Rock Gobies eat tiny shrimps and worms and rarely tackle anything they cannot swallow in one go.
Common Goby (Pomatoschistus microps)
Drop a pebble into a shallow sandy pool and lots of very small fish will dart rapidly in all directions before coming to rest. They are difficult to see because they are coloured to blend in with the sandy bottoms on which they rest. The Common Goby will be found on rocky shores where there are sandy pools. It is a small fish that only attains a length of 64 mm. Like all gobies it has two dorsal fins. It feeds on small crustaceans.
The Common Goby only lives for one year and the male guards the eggs that will be deposited on the underside of a rock or seashell. This fish also lives in estuaries. There is an almost identical, but larger, fish called the Sand Goby, Pomatoschistus minutus, which is found in pools nearer the low tide mark. It comes inshore to breed in the late summer.
Butterfish (Pholis gunnellus)
Even longer and thinner, the Butterfish or Gunnel looks like a snake and swims, or rather wriggles along the bottom, in an undulating snake-like fashion. During spring and autumn small specimens of this orange-brown fish can be discovered in small water-filled hollows under rocks when the tide recedes. It can be easily recognised by about 13 large spots spaced out along the top of its body. These are false-eye spots, and may fool hungry fish into thinking it is a much bigger fish and not a tasty worm. The Butterfish gets its name because it is so slippery. It is almost impossible to pick up in your hand and it is best to use a net. Butterfish reach a length of 25 cm, but shore specimens are usually smaller and feed on small crustaceans. The adults eat worms. The male fish guards the eggs that are laid in shallow water.
5-Bearded Rockling (Ciliata mustela)
The bronze coloured Rockling is unusual in several ways. The first dorsal fin consists of a single small ray, followed by a fringe of tiny rays in a slot. These rays vibrate and may help the Rockling to find its food. This rockling has five barbels on its head that it uses to find worms buried in the sand. The 5-Bearded Rockling grows to 25 cm long, although fish found on the shore are usually smaller. It can be discovered both in pools and under rocks. Eggs and sperm are released into the sea where fertilisation takes place. The larvae are silver and live with the surface plankton. They are known as 'Mackerel-midges'. In mid- summer they move inshore and change colour from silver to brown. Several other species occur, of which the Shore Rockling, Gaidropsarus mediterraneus, has only three barbels.
Bullhead (Taurulus bubalis)
Seen from above, this fish looks like a rock, while it waits in ambush for a prawn or small fish to swallow in its expandible mouth. In the larger pools, this fish can be a very common predator. It is known by lots of different names like Rockfish, Clobberhead and Sea Scorpion in different places. It can be many different colours to match the background where it lives. However, the most usual is a patchwork brown and cream. Adults grow to 18 cm long. Fish of this size will only be found in the large pools. Smaller and younger fish are common during the summer months in the company of the prawns on which they feed. Like most of the shore fish, the Bullhead lacks the buoyancy organ called the swim bladder. It is heavier than water, so when it stops swimming it must rest on the bottom.
Great Pipefish (Syngnathus acus)
With their trumpet-like snouts, all pipefish suck in their food of fish larvae and small crustaceans. The Great Pipefish can be recognised quite easily by its brown and white banded body. During the late summer they follow the tide in. The Worm Pipefish, Nerophis lumbriciformis, can be mistaken for a thin strip of brown weed wedged under a rock. It needs a practised eye to spot it, but in the south-west it is a common fish at low tide. However, it likes warm water and is absent from the north and the east coasts of Britain. Like the sea-horses, which belong to the same family, it is the male pipefish that looks after the young in a special pouch along his belly. In the shallow seas where the bottom is sandy, the Lesser Pipefish, Syngnathus rostellatus, is common.
Corkwing Wrasse (Crenilabrus melops)
The wrasse are a large family of colourful fishes. Five species breed around Britain. Of these it is the Corkwing that lives in the shallow water offshore where it breeds, building a nest amongst weeds. Most rock pool fish are squat or elongate, and adapted for a life in amongst the rocks, but wrasse are ordinary-looking fish and are covered in scales. The colour is greenish-brown with black horizontal lines and a black spot just in front of the tail fin. However, when resting, or caught in a net, the lines and spot are obscured by black vertical bars. In late summer the young are very common in the lower shore pools feeding on tiny crustaceans. The adults are aggressive with sharp teeth to attack hard-shelled crabs and prawns. Corkwing Wrasse can grow to 25 cm in length. The Corkwing is found all around the coast of Britain apart from parts of the east coast. It could be mistaken for the much rarer Rock Cook, Centrolabrus exoletus, or confused with very young Ballan Wrasse, Labrus bergylta, which are about one in every hundred of the young wrasse caught inshore.
2-Spot Goby (Gobiusculus flavescens)
This small goby is unusual as it does not rest on the bottom but hangs motionless in mid- water on the fringes of the weeds. Small shoals can be found in intertidal pools, and if there are no large fish present they can be seen in the clear water. It is the most attractive of the common gobies of the shore. In some areas it is an orange colour. It is only 6 cm in length when fully grown. There is a clear black spot just in front of the caudal fin; the second spot on the side is less clear.
Sea Stickleback (Spinachia spinachia)
As thin and as long as a pencil, the Sea Stickleback is very fierce and may attack fish larger than itself. It reaches 15 cm long and has a pointed snout with sharp teeth. There are 15 spines in front of the single dorsal fin. The Sea Stickleback is brown on the upper side, silvery underneath, and has a prodigious appetite for small crustaceans and fish larvae. It can become trapped in the larger pools at low tide. The male builds a nest of weed in summer.
Lumpsucker (Cyclopterus lumpus)
By far the largest of the shore fishes that visit the intertidal zone regularly, the football-sized and shaped Lumpsucker moves inshore in February to breed on the shore. This is a northern fish and is common on Scottish coasts where it is eaten by otters, but the Lumpsucker also lives in the English Channel. During the early months of the year the shore zone can be very rough. The male fish guards the eggs on the shore, sticking himself to the rocks with a strong sucker on its belly. Adult Lumpsuckers spend most of their life in deeper water. Their diet includes comb- jellies. The young hatch from an orange egg mass and by March in the English Channel they appear as pea-sized blobs in brown and green living in the mid-tide pools. They are the first fish larvae of the year to be found on the shore.
Lesser Weever (Echiichthys vipera)
Beware! the black dorsal fin of this fish contains a poison that if touched or trodden on can cause a painful sting. Fortunately, it is not not very common on the shore and generally lives below low tide mark in the sandy shallows. It buries itself completely in the sand. Adult Weevers reach a length of 14 cm. They possess very sharp teeth and eat worms, shrimps and small fish.
Other Rock Pool Fish
The larvae of almost any fish can get trapped in pools as the tide goes out. Adult fish that live in the shallow water often venture into the lower pools, and adult specimens of the sandy coloured Dragonet, Callionymus lyra, the orange Montagu's Sea Snail, Liparis montagui, and the Cornish Sucker Lepadogaster lepadogaster are all part of the shore fauna. On the east coast the Eelpout, Zoarces viviparus hides under rocks at low tide. It gives birth to live young instead of laying eggs. A small flatfish called the Topknot, Zeugopterus punctatus, clings to the underside of rocks in pools in the south-west.
Field key to the Shore Fishes of the British Isles by Alwyne Wheeler. (Field Studies journal Vol. 8 No. 3 p 481-521).
Key to the Fishes of Northern Europe by Alwyne Wheeler [Warne 1978]
If you are interested in the marine life around the British Isles and wish to find out more, please write to to: British Marine Life Study Society, Glaucus House, 14 Corbyn Crescent, Shoreham-by-Sea, Sussex. BN43 6PQ. enclosing stamps to the value of 59 pence.