Invertebrates

There are far too many different species of invertebrates in the seas around the Argyll Islands to even begin to list them. However some of these invertebrates are quite well known and easily recognisable perhaps because they are often seen along the seashore or maybe because they are a good food source or because they produce beautiful shells. The molluscs are probably the most familiar group of marine invertebrates, partly due to the fact that this is the largest and most diverse group of animals.

Molluscs are characterised by their calcareous shells which are secreted by the mantle. The mantle is a thick covering over the soft body, which has a head and a muscular foot. The mantle also covers the main organs such as the sensory organs, the heart, the gills and a mucus-secreting gland into which the anus, kidneys and reproductive organs all lead. The body also contains open spaces through which fluids can circulate to distribute oxygen to all the cells and to collect waste materials. All living cells require oxygen and most marine organisms acquire it from the surrounding water where it has dissolved. The more advanced animals have specialised gills to speed up the exchange of gases and most have methods to circulate the water over the gills. These gills are often used to obtain food too by filtering miniscule particles out of the water using threads of mucus at the gill system. The molluscs' shell grows as the animal grows, increasing in diameter and thickness as new shell material is added to the older parts. Each group of molluscs has its own particular type of shell, some being ornate and spiral, some made up of two parts (bivalves) and some whose shell cannot be seen.

The octopus is a mollusc but it actually does not have a shell at all. It is in the same class as the squids and cuttlefish but these two animals do have vestigial shells hidden within their soft bodies. The squids have a small cylindrical “pen” and the cuttlefish have a large, flat “cuttlebone”. These cuttlebones are often found washed up on the shore and are a good source of calcium for caged birds such as budgerigars.

The Common cuttlefish, S epia officinalis, is found on all shallow, sandy or muddyshores around the Argyll Islands . Its oval, flattened body grows to about 40cm long and has a soft flap, the lateral fins, running all around the edge. Like all the other octopods it has eight arms but also has a pair of retractile tentacles for catching its prey. Squid, such as Loligo forbesii, also possess these tentacles and both species feed on crustaceans such as crabs and bony fish. They locate their prey using their well developed eyes and squid can accelerate towards their prey using water jet propulsion! The cuttlefish on the other hand tend to hide in the sand and lash out with their elongated tentacles to engulf the prey. Both animals can rapidly change colour to become camouflaged but also show colour changes when stressed. The excitement of capturing its prey can cause a cuttlefish to completely lose its colourful camouflage! These soft bodied creatures are able to feed on the hard crustaceans and bony fish because in amongst their eight legs is a mouth with large, tough beak-like jaws. They also possess a radula, a horny strip, which is able to act like a tongue, pulling torn flesh into the mouth.

Squid are all edible and commercially important in many areas. The squid found around the Argyll waters generally grow to about half a metre at most but the giant squid, Architeuthis dux, which grows up to nearly 20 metres is occasionally washed up on our shores. One of these squid found washed up in New Zealand in 1933 had eyes which were 40cm across!

Octopus are also edible but are not particularly popular locally. There are two species of octopus commonly found in the shallow waters here. Both have the typical large fused head and mantle of all octopus and the eight, long arms covered with suckers. They have large, dark eyes which are extremely well developed for an invertebrate, being able to focus the lens and to alter the amount of light entering the eye by opening and closing the slit-like iris. An octopus is able to discriminate objects as small as 5 millimetres at a distance of 1 metre.

Octopus live in caves or amongst large rocks and either seize their prey as they pass by or venture out to stalk them. Crabs and fish are common prey for them and they are able to immobilise their prey with toxins. All octopus inject their prey with toxins but the most toxic of all octopus is the Blue-ringed octopus found in Australia . The toxin produced by this octopus is known to be able to kill a man in only a few minutes. Once immobilised the prey is flushed with enzymes which will start to break it down before it enters the mouth.

Octopus, like the squid and cuttlefish are able to change their colour very quickly. They are also able to produce ink. This ink was in fact the original source of ink used in pens and it contains a high concentration of melanin, the same chemical which creates the brown “tan” in humans. It is produced in a special ink sac which empties into the rectum, behind the anus. It is stored there until needed to repel a predator or to create a diversion for the prey. Either way, it is squirted out with force, creating a dense cloud in the water. The vent through which the octopus empties the anus is found at the side of the head since this is the only opening left through the fused mantle.

Another well known group of molluscs are the gastropods, often known as marine snails. These are the source of the wonderful variety of colourful shells found on our shores. They range from the tiny periwinkles, some of which reach only ½ cm in height, to the sturdy Red whelk growing up to 20 cm.

Gastropods have a muscular foot and special cells which secret mucus to allow them to move more easily. Their foot extends out beyond the protective shell as can the head but both can be withdrawn back into the shell and a tough, horny disc on their foot, the operculum, can seal the opening.

All the winkles have spiral shells with a marked groove beside the aperture. They are all grazing herbivores and can be found feeding on brown seaweeds or on rocks covered with slimy algae. Top shells are globular or pyramidal in shape with few whorls and are often very colourful. These gastropods graze on rock surfaces scraping off the algae growing there. Whelks on the other hand are carnivorous, often feeding on dead and dying prey.

The Dog whelk, Nucella lapillus, feeds on mussels and barnacles which surprisingly often use its shell as a good site to grow. Many of the marine invertebrates have an extended proboscis. This is a tube-like organ which contains the radula, mouth and oesophagus (food pipe to the stomach). It can be used to attack prey, prise open bivalve shells and even to inject poisons into the prey. The tropical cone shells inject poisons into their prey to stun them and they can be deadly to humans.

Limpets are often thought of as being completely stationary since once exposed by the tide they clamp themselves firmly into place and do not move until the water covers them again. Then they move across the rocks scraping off their food with their rough “radula” Their conical shells are grown to fit their chosen site on the rocks and they return to the exact same place every time. This ensures that the seal to the rock surface is watertight so that they don't dry out. Some limpets even carve out a piece of rock to fit them better.

Bivalve molluscs have bodies enclosed within a pair of hinged shells (valves). There are many different species of bivalve but it is the Common mussel, Mytilus edulis, with its sharp purplish shell which is most familiar and commercially important in the Argyll Islands . Mussels attach themselves to rocks or wooden posts, forming dense beds. They are held in position by threads called Byssus threads and when exposed they close their shells tightly. When under water however the shells open slightly and water is siphoned in to the gills. Around the gills the mucus threads trap food such as plankton and water and waste are siphoned out of the exhalent siphon.

Unlike mussels, clams, which are also bivalves, live buried in the sand with their siphons poking up towards the surface. Their shells are irregular and often multi-coloured and they have a whole range of local names such as Sand Gaper, Mya arenaria, and White Piddock, Barnea candida.

Scallops have very easily identifiable shells with radiating ribs. They are bivalves which do not remain attached to the sea bed although they do live at the sea bottom. They can swim very fast using water propulsion though usually only to escape predators. Under water the two shells open exposing the mantle with its frilled edge and beautiful blue eyes! These tiny eyes even have a cornea, retina and lens but can only detect changes in light such as a hand passing above them. Scallops are a popular food but because of over fishing for them using dredgers and trawlers they are now an expensive luxury. The edible part is the large, single muscle which is used to close the bivalves. This is a very powerful muscle and once closed it is very difficult to prise open.

Another popular edible bivalve is the oyster, Ostra edulis. Having been over-fished for many years this is now almost extinct in the wild although they are still fattened or “greened” commercially in tidal pools. These oysters also produce pearls which are calcareous growths around particles of sand or other foreign body which they are trying to isolate. The pearls from our local oysters are not particularly pretty since they lack the shiny, nacreous lustre of the valuable Pearl oysters, Pintada sp.

Echinoderms

Echinoderms are all invertebrates since they do not have a back bone but many of them have calcareous skeleton inside their bodies. Their soft body is covered with a tough skin which is either prickly, warty or spiny. The name Echinoderm is derived from the Greek echinos and derma meaning hedgehog skin. As well as this spiny skin they also have a special internal water system which allows them to alter the pressure within different parts of their bodies. For example they can extend or contract their tube-like feet which are used to pull them along but are also used for food collection and gas exchange.

Starfish are of course not fish but one of the Echinoderms and they have hundreds of tiny tube feet with suckers on the ends. These they use to move around but also to open the shells of their prey such as oysters and mussels. They are able to prise open the shells with these tube feet and then push their stomach out of their body and into the shell where it digests the mollusc before absorbing it.

The most familiar starfish around the Argyll islands is the Common Starfish, Asterias rubens, which is occasionally found in enormous groups up to a kilometre across. These aggregations move slowly along the coast eating crustaceans and bivalves as they go. The Common starfish is quite a large starfish, growing to 50 cm and they are found on all types of shore below the tide line. Most starfish have 5 arms but some are found with 6 or more.

The Common Sun Star, Crossater papposus, is similar to the starfish but has 8 – 12 narrow arms around a disc-like body. It is carnivorous and in fact its favourite food is the common starfish!

Brittlestars also have a central disc-like body but this is tiny compared to their five long, spiny arms. Their internal skeleton is embedded beneath their skin and arranged in plates down the arms giving a jointed appearance. When attacked the brittlestars drop the ends of their arms off and they can regenerate them later. Most of them are scavengers and use their tube feet to collect debris from the sea bed.

Another Echinoderm nis the beautiful sea urchin. Their brightly coloured spherical shells are often found washed up along the high tide line. They feed on algae especially kelp and have a mouth surrounde by 5 hard plates which is called the Aristotle's Lantern. The spines of the Common Sea Urchin, Echinus esculentus, are often red with violet tips and when broken off they reveal a deeper red or purple shell or “test” covered with white spine bosses. The spines grow from these bosses and work on a ball and socket arrangement allowing plenty of movement so that the spines can be used to move the urchin along and also to remove debris and for defense.

Crustaceans

The largest Phylum of invertebrate animals is the Arthropoda containing the crustaceans with over 50,000 marine species. These range from the microscopic copepod plankton to the common lobster which can grow up to one metre in length. Crustaceans have bodies which are divided into three main parts; the head, the thorax and the abdomen. The skeleton of a crustacean, like all arthropods is on the outside. This external shell, in addition to being protective also gives support for attachment of muscles. It is made up of many plates, connected by thin membranes and allowing joint movement. The word Arthropod comes from the Greek words arthron and pod meaning jointed leg. The exoskeleton does not grow as the animal inside it grows but must be periodically shed and a new skeleton produced. Before the old skeleton or shell is shed, calcium is reabsorbed from it and used to form a new skeleton underneath the old.

At first glance the barnacles do not look like crustaceans and they are the only sessile crustaceans, often being mistakenly thought of as molluscs. Within the barnacles shell however lives a tiny shrimp-like animal which is attached to the rock by a tough glue. It is attached head down with its six feathery legs upwards and encased by overlapping calcareaous plates or shells. These plates are kept tightly shut when the tide recedes but when under water the muscles open the shell plates and the barnacle feeds by waving its legs around to catch plankton. The legs are also used for gas exchange. Just like the other crustaceans, the exoskeleton of this shrimp-like animal must be periodically moulted and a new one grown. The shell-like plates however grow as the animal inside grows since new layers are being continually added.

There are two particularly common barnacles around the Argyll islands (pictured in the diagram above). Semibalanus balanoides is a symmetrical barnacle found around the upper tidal areas on rocky shores. When displaced from the rock it does not leave a white “scar”. If a white scar is found then it was probably Chthamalus montagui. Both these barnacles are known as “acorn barnacles” as opposed to the “goose barnacles” which have long flexible stalks or peduncles attaching them to rocks, wood, boats and even whales!

Unlike the barnacles, the members of the Decapoda are a very popular food. The decapoda includes all the crustaceans with ten legs such as the shrimps, crabs and lobsters. All these animals have five pairs of legs but the front pair are often enlarged and have claws.

As well as these five pairs of legs shrimps also have five pairs of “pleopods” on their abdomen which they use for swimming or crawling on the sand. Like all the decapods their head and thorax are fused at the back to form a carapace. In shrimps this is extended forwards to form a prominent rostrum from which protrudes a pair of stalked eyes and long antennae. Prawns differ from shrimps in having more than one pair of clawed legs but the common names are often misplaced and the Pink Shrimp, Pandalus montagui, is actually a prawn! Some prawns and shrimps are carnivorous whilst others eat plankton and detritus. All have gills located under the carapace where the legs join the body.

The exoskeleton of shrimps and prawns is quite thin whereas lobsters have a very thick exoskeleton and very well developed pincers! Their walking legs are also very well developed but they can also swim backwards very fast by flexing their abdomen. Most lobsters are scavengers, feeding on other dead animals and have sharp mouthparts. There are two species of lobster found around the Argyll Islands . One, the Common Lobster, Homanus gammarus, is large and aggressive but beautifully coloured. Unfortunately, because of its commercial value it is becoming more and more scarce. They breed only once a year and the female carries the eggs attached to her abdomen for six months. These eggs are considered a delicacy by some and therefore, rather than been returned to the water so that the eggs can hatch and develop into a new generation of lobsters, they are often killed. The other lobster found here, the delicate, pink Norway Lobster, Nephrops norvegicus, has also become quite scarce as has the Squat lobster, Munida rugosa. The squat lobster, just as its name implies, has a broad, flattened body without a “tail” and long thin pincers.

Crabs have been likened to folded-up lobsters since their abdomen is permanently folded beneath the carapace. Female crabs hold their eggs beneath the abdominal flap and so it is broad and triangular whereas the male crabs have a thin, narrow flap. They are also decapods though and have two pairs of clawed legs and four pairs of walking legs which they use to propel themselves sideways.

The largest crab around the Argyll Islands is the Edible Crab, Cancer pagurus. This grows to a width of more than 25 cm and its pinky, oval carapace is ridged around the edge like a good, old-fashioned pie crust. Once again they are being over fished and becoming far less common. Very few ever reach their maximum size.

The Green Shore Crab, carcinus maenus, is much smaller than the edible crab, growing up to only 5 cm, and is not so popular. It is therefore found in much greater numbers. This crab can live in a range of salinities so it is found in estuaries and salt marshes as well as along every coast. The green colour is usually mottled and ranges from almost black to pale yellow.

Very similar in size is the velvet Swimming Crab which has flattened paddle-shaped sections to its legs. This crab is fast-moving and can be quite nasty. When a group of them are left together in an enclosed place they soon end up quite literally “legless” as they chew the legs of each other! So watch your fingers when handling these crabs but do take the chance to feel the velvety carapace and admire its bright red eyes.

Krill

The term “krill” is used to cover over 90 species of shrimp-like crustaceans but should really only be used for a single species, Euphausia superba. The name “krill” comes from the Norwegian words for whale food and are small, shrimp-like animals that grow up to about 6 cm in length and live for up to 5 years. These are some of the largest members of the plankton, in fact because they swim so well some people think that they are more like little fish than drifting plankton. The krill usually live in dense swarms that may have more than 10,000 krill in each cubic metre of water. Occasionally krill swarms go on for miles and miles and contain thousands of tonnes of krill. They are a vital part of nearly all marine food webs, being eaten by many animals from seabirds to baleen whales. Krill themselves feed on plankton, both animal and plant as well as detritus. Their bodies contain up to 50% protein and this is one reason for their importance as a food source for larger animals. It is also why krill are harvested commercially to produce protein supplements for human and animals foodstuffs. Unfortunately, because of their swarming instinct, krill are very easy to catch and a trawler can net 10 tonnes of krill every hour! Their populations can easily be decimated if this is not controlled and there is a large risk to their predators from these trawl nets.

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