In Rock Pools

 

Portrush is spectacular with its two beaches – features which have always made it a very popular holiday destination. Surrounded by a strong ecological area, the rock pools of Portrush are unsurprisingly teeming with diversity.

At first glance, the rock pools seem still and rather uninteresting, but each one is filled with different types of creatures and plants unique to the shallow, sheltered areas of water which rock pools provide. These little spaces are created when the tide brings wildlife and water with it during high tide, when the water draws further up the shore, then withdraws during low tide and leaves fresh seawater and life in each little ‘pocket’ of rock. Some creatures will only stay in the rock pools until the next high tide arrives and they can escape back to the sea, while others are permanent residents of the much warmer, more shallow pools.

The seaweed provides perfect hiding places and shade for the creatures that live in rock pools. If you wait patiently, you may be able to see air bubbles or vibrations disturbing the surface of the pools and indicate the presence of wildlife. Fish that may be be seen in the rock pools of Portrush include:

BLENNY (LIPOPHRYS PHOLIS)

Blenny Lipophrys Pholis
Blenny Lipophrys Pholis

This little green fish is found on rocky shores all along the coastline. It is a very common fish that hides under rocks and in crannies when the tide is out. It is also 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. TheBlenny 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.

BLENNY (LIPOPHRYS PHOLIS GOBIUS PAGANELLUS)

Rock Goby
Rock Goby

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)

Common Goby

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)

Butterfish

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 -pots 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)

5 Bearded Rockling

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)

Bullhead

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.

CORKWING WRASSE (CRENILABRUS MELOPS)

Corkwing Wrasse

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.

LESSER WEEVER (ECHIICHTHYS VIPERA)

Lesser Weever

Beware! the black dorsal fin of this fish contains a venom that if touched or trodden on can cause a painful sting. Fortunately, it is 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.

2-SPOT GOBY (GOBIUSCULUS FLAVESCENS)

2 Spot Goby

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)

Sea Stickleback

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.  (BRITISH ROCK POOL FISH by Andy Horton & Jane Lilley)

HERMIT CRAB (PAGURUS BERNHARDUS)

Hermit Crab

This soft-bodied crab lives in an empty spiral shell, moving up sizes as it grows. When inside, its right pincer seals the entrance. Common in rock pools.

COMMON (SHORE) CRAB (CARCINUS MAENAS)

Common Shore Crab

Our most abundant crab, found on all coasts. The shell has a jagged front edge with tooth-like projections. Colour varies, but often dark green.

BROAD-CLAWED PORCELAIN CRAB (PORCELLANA PLATYCHELES)

Broad Clawed Porcelain Crab

Hard to find as it’s tiny (up to 1.5cm) and clings under rocks. Downy haired with wide flattened claws, it lives low down the shore, so search at low tide.

VELVET SWIMMING CRAB (NECORA PUBER)

Velvet Swimming Crab

Bright red eyes give this its other name of devil crab. Hiding under rocks low on sheltered coasts, it’s feisty, snapping with claws if threatened.

EDIBLE CRAB (CANCER PAGURUS)

Edible Crab

A seafood shack staple, this red-brown crab can grow 25cm wide (the largest ones live in deep water). Its crimped edge shell is like a Cornish Pasty.

COMMON SPIDER CRAB (MAJA SQUINADO)

Common Spider Crab

A big, spiny species with spindly legs and impressive pincers up to 45cm long. Commonest offshore but in summer moves into shallower water.

Crabs only walk sideways because their legs are placed on the sides of their bodies facing outwards, and their ‘knees’ only bend that way. Our legs work in the same way- we can only bend our knees forward so we can only walk forwards or backwards[1](without using our hip joints which enable us to walk sideways).

The easiest and most common creatures to find at rock pools are limpets and barnacles. Invisible at first glance, these soft creatures protect themselves by attaching themselves to the rocks and forming shells over them. They are extremely strong and difficult to detach from the rocks.

COMMON LIMPET (PATELLA VULGATA)

Common Limpet

Limpets are dome-shaped molluscs with no spiralling of the shell. Patella is the genus name for the most abundant group. They can live up to 16 years and may achieve a maximum diameter of 7 centimetres. The Black Lichen, Verrucaria mucosa, can often be seen growing on the shell.

These are very important seashore herbivores, feeding on microscopic algae which covers the rocks like a fine carpet. Because they graze so efficiently they remove young seaweeds and may, therefore prevent the establishment of larger seaweeds, like Bladderwrack. They feed by scraping the radula (“tongue”) across the rock surface. The radula has teeth (toughened with iron) on it and acts like a file. Each sweep of the radula removes fine algae and leaves a grazing mark on the rock. Re-colonisation by the algae soon takes place. Often the only algae to be found is that growing on the shell where they cannot reach.

The attachment to rock is legendary. This adhesion to the surface is by the muscular foot and the secretion of a chemical. They can hold on to rock with a force of 75 lbs/sq.in. Clamping down at low tide will prevent drying out. As Limpets settle down they rotate the shell and grind it into rock which produces a good fit but also, on death, leaves a scar on the rock surface. To breathe they remove oxygen from the water. This is drawn in to the gills via a hole above the head. When the tide goes out they have a problem with the lack of water. They clamp down and reduce their metabolism which in turn reduces the need for water. The shell has a high degree of waterproofing to conserve water.

The shell shape varies with wave action, time spent emersed and possibly food supply (Ballantine, 2005, pers.comm.). A limpet whose muscles are tensed as shell is secreted produces a domed result, relaxed limpets produce flatter shells. Well-fed limpets, whose tissue pushes out to the shell margin, produce flatter shells than hungry limpets. Increased wave action and/or time spent emersed should produce taller limpets but the pattern (especially on exposed shores) is complicated by local topography. The sexes are separate but as in many molluscs they often begin life as males changing to a female as they get older. For the first year of P. vulgata‘s life they are all neuter, then change into males. After 4-7 years approximately 34% change into females. Gametes fertilise externally in the seawater. The release of gametes from the adults is triggered by a lowering of the temperature in autumn, with 11 degrees C being the critical value. This, coupled with rough weather, stimulates the realise. The eggs hatch into planktonic larvae which live in the plankton for barely 2 weeks before settling out on to the shore. All this sex stuff means that they are protandrous hermaphrodites.

[1] Our Marine Species, Crabs

BARNACLES

Barnacles

We often overlook Barnacles as they are so small and look like part of the rocks themselves rather than individual animals.

They are, in fact, tiny prawn-like animals which build a shell fixed permanently to the rock. They live inside their shells and never leave it.  When the tide is in, they open a trap door at the top of their shell and stick out their legs to catch plankton.

DARK ACORN BARNACLE AKA VOLCANO BARNACLE (BALANUS PERFORATUS)

Dark Acorn Barnacle

These are often our most easily recognisable barnacles are they tend to be larger being at least a centimetre in height in our rockpools in comparison with just a few millimetres that our other barnacles reach. Further out to sea, they can be even larger. They are purplish or grey in colour and tapered in a volcano shape. Their sides are often heavily ridged as can be seen in the photo on the left, and they are frequently found in overlapping groups. The photos below show the legs of the barnacle coming out to feed on plankton in the water.

SEA ANEMONES

Sea Anemone

Sometimes called the ‘flowers of the sea’, Sea Anemones are actually beautiful animals, closely related to jellyfish and corals. Like jellyfish and corals, anemones belong to the group Cnidarians. The name Cnidaria comes from the Latin cnidae which means ‘nettle’. All of the animals within this group have stinging cells which they use for the capture of prey and to protect themselves against predators. Sea Anemones are simple animals, often attached to hard surfaces such as rocks and boulders. However there are also burrowing anemones that bury themselves in sand, mud or gravel on the sea floor.

Most Sea Anemones live attached, catching passing food with their tentacles. Sea Anemones can move slowly by gliding on their base. Many are also capable of moving rapidly to avoid predation or competition by detaching, catching a current and re-attaching elsewhere. The diet of most anemones consists of small animals such as plankton, crabs and fish, however a number of bigger sea anemones will eat much larger prey. For example, dahlia anemones can be greedy feeders that will prey on starfish and jellyfish. Anemones have rings of tentacles surrounding their central mouth. Tentacles have specialised stinging cells called nematocysts. They use these to immobilise their prey so that the tentacles are then able to move the food into the mouth. The extending tentacles can also be used to catch passing food as it drifts past.

Sea Anemones vary in size, with some tropical species reaching more than a metre in diameter. One of the largest in British waters is the Horesman Anemone (Urticina eques), reaching sizes of 35cm across. One of the smallest in Britain is the Rare Anemone (Gonactinia prolifera), which rarely grows more than 5mm tall. Some sea anemones are very long lived and have been known to reach 60-80 years. Because anemones are able to clone themselves they do not age and therefore have the potential to live indefinitely in the absence of predators or disease.

From and on the beaches in Portrush, you will see many types of wildlife.

JELLYFISH

Jellyfish

Along the shore you may come across jellyfish along the beach, especially in the summer. These clear blobs of an elastic-like substance called mesoglea are dangerous to move as they can sting if they feel endangered. Jellyfish don’t have a heart, brain or blood and yet still manage to go about their daily lives!