Other Moray Firth Creatures
ANEMONES, CORALS AND JELLYFISH
Anemones, jellyfish, corals and their relatives belong to the group called cnidarians. They are simple animals but the variety of their colours and shapes contribute greatly to the beauty of the oceans. All cnidarians are radially symmetrical, where similar parts of the body are repeated around a central axis. They occur as one of two basic forms - a sessile polyp (a sac-like form) or a drifting medusa (jellyfish form). Some cnidarians may take on both body forms during their life cycle, while others spend their entire lives as one of the two.
Cnidarians are characterised by a centrally located mouth surrounded by numerous tentacles which are used to capture and handle food. The tentacles are armed with a battery of stinging cells, called nematocysts, which fire a harpoon-like structure when touched. Cnidarians are therefore carnivores, catching prey that come within range of their tentacles.
Common Moray Firth cnidarians include sea anemones, such as the beadlet anemone and the flower-like dahlia anemone, dead men's fingers (a soft coral common in the kelp reef habitat), the sea pen (which inhabits sandy areas) and the moon jellyfish.
The beadlet anemone is often found in mid and lower shore pools, and also out of pools in crevices and other damp places, looking like a blob of dark red, or green, jelly. Underwater, the anemone expands its tentacles to catch small creatures, such as prawns, and small fish like young blennies. Although flower-like in appearance, sea anemones are carnivorous animals, with poison in their tentacles that paralyses their prey.
The soft-bodied molluscs are a very successful group with around 110,000 species, including fresh water and terrestrial forms. They make up the largest group of marine animals and include snails, mussels, limpets, sea slugs, octopuses and the like.
Molluscs are characterised by a soft body protected by a calcium carbonate (chalky) shell. The shell is produced by the mantle, a thin layer of tissue covering the body. The body has a muscular foot, usually used for locomotion, and most species have a radula - a tongue with small teeth used for rasping food from surfaces. They possess eyes, which are particularly well developed in the octopus. Molluscs gain oxygen from the water through gills.
There are three main kinds of molluscs.
Gastropods (literally meaning 'stomach-footed') have a creeping foot and a single, often coiled, shell. Snails, limpets, whelks and cone shells are all in this group. Snails and limpets are vegetarians, using their radula to graze algae from rocks, while cone shells and whelks are carnivores. Whelks, common on the sea shore, have modified their radula for use as a drill with which to penetrate the hard shells of prey such as barnacles and mussels.
Bivalves (mussels, clams, scallops and razor shells) don't have a radula but they are recognised by having two parts to the shell, known as valves. They tend to be filter feeders and either burrow, attach to rocks or live freely on the sea bed. For example, clams burrow using their foot, sessile mussels produce strong byssus threads that attach them firmly to rocks, while scallops clap the two valves of their shell together to propel themselves away from danger.
Cephalopods are the third group of molluscs, it includes octopus, squid and cuttlefish and they have become specialised for active hunting. The typical molluscan shell is either internal, in squid and cuttlefish, or is lost altogether as in the octopus.
Sponges are very primitive animals, little more than a mass of cells, without any true tissues or organs. In fact, if a living sponge is forced through a sieve, not only would the cells survive, but they would also regroup to form a new sponge body.
A sponge body is made up of two layers of cells held together by a gel-like substance. Minute needles of silica (glass) within the gel, called spicules, give the sponge body support and offer some protection against predators.
Adult sponges are sessile, or anchored in place, and filter the water for food particles, drawing water in through tiny pores which perforate the sponge body. Sponges are widespread, living on rocks, kelp holdfasts and even spider crabs. Common sponges on the rocky shores of the Moray Firth include the elephant's ear sponge and the breadcrumb sponge.
'Echinoderm' means 'spiny skinned' which is an appropriate name for the members of this group - starfish, sea urchins, brittle stars, and sea cucumbers. Beneath the surface of these animals is an internal skeleton made up from many individual plates which often bear spines and other projections.
All echinoderms show a five-sided symmetry radiating out from a central disc - most noticeable in the starfish and brittle stars. They have no head or brain, though they do have a nervous system and distinct organ systems. Echinoderms also have a unique way of getting about - on hundreds of little sucker-tipped tube feet. These are hydraulically powered by a water vascular system - a network of canals containing pressurised body fluids. The tube feet extend as fluid flows into them. Tube feet also allow for the uptake of oxygen across the thin membranes and, in starfish, some, at the ends of the arms, are modified into simple eyespots.
Echinoderms are generally bottom dwellers. Starfish are common predators on the rocky shore, preying on molluscs by prising open the shells, extruding their stomach between the shells and secreting enzymes to digest the prey.
Sea urchins tend to be vegetarian, feeding particularly on kelp plants. They have a complex chewing apparatus, known as the Aristotle's lantern, which allows the urchin to graze on tough materials. A sea urchin's spines are effective deterrents to predators but also aid in locomotion.
As their name suggests, brittle stars' arms break very easily, but, like starfish, they are able to regenerate them. Brittle stars are often found in great numbers on the sea bed and may be filter feeders or scavengers.
Sea cucumbers have an elongated body, though this may still be divided into five identical parts. They creep over the sea floor and either eat the sediment, extracting organic material, or they filter feed using modified tube feet. If threatened, sea cucumbers can actually eject some of their guts to deter potential predators, and quickly regenerate their organs.
Also known as the 'insects of the sea', this diverse group includes animals with widely differing lifestyles, from parasites to filter feeders, plankton dwellers to beach hoppers.
Crustaceans are specialised for life in water and have gills for oxygen uptake. All possess an external skeleton made from tough chitin. This is like a suit of armour and provides protection and a place for muscles to attach to. It allows for great flexibility, combined with numerous paired limbs which are specialised for feeding, crawling and swimming. However, the exoskeleton restricts growth and all crustaceans must moult, or shed their exoskeleton, in order to grow.
All crustaceans also share a complicated life cycle, often involving several different larval stages. The number of these larval stages differ among species but most include a planktonic stage which look nothing like the adult.
Some crustaceans stay in the plankton throughout their lives - tiny shrimp-like krill are an important food source to the largest of animals, the blue whale. Other species' larvae develop into more familiar forms. Barnacle larvae settle out of the plankton and cement their heads to rock, develop hard plates to protect their bodies then wave their 'legs' in the water and kick food into their mouths!
Other crustaceans include crabs. They have 10 walking legs, a hard carapace (covering the main body) and pincers which are used for picking up food or for self defence. If attacked, crabs and other crustaceans can escape danger by deliberately shedding an injured or trapped limb - they can grow a new one when they next molt. Lobsters may grow to over 1m long, weigh over 6kg and live for over 60 years! The Norway lobster is now the most important commercial fishing in the Moray Firth, in terms of value at first sale.
There are many different kinds of worms in the sea, but probably the most familiar are the segmented worms. Almost all marine segmented worms are known as polychaetes. The most distinguishing feature of these animals is the presence of flattened extensions to each body section, called parapodia. These are sometimes covered in bristles and also possess gills. Parapodia are often used in locomotion.
Unlike the sponges and cnidarians, segmented worms are bilaterally symmetrical (the body has two identical halves) with a head at the front. The head has a brain, or at least, a collection of nerve cells and sensory organs such as eyes.
Polychaetes have very diverse lifestyles. Some, such as the ragworm, are carnivores with strong jaws for catching small invertebrates, which they actively hunt on the sea floor. Others live in tunnels and feed on organic particles in the sediment - the lugworm, for example, leaves a pile of discarded sand at the entrance to its burrow. Another family of polychaetes build and live in tubes, which may be made from mucus, sand or tiny fragments of shells, and extend many tentacles out into the water to catch floating food particles.