Marine Invertebrates
MARINE INVERTEBRATES
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Invertebrates are those animals lacking a vertebral column or backbone. These animals have developed various forms of protection such as shells or bony exoskeletons. Others have adapted in other ways. More than 98% of earth’s species are invertebrates. The most common marine invertebrates include:
• Sponges
• Cnidarians
• Marine Worms
• Arthropods
• Molluscs
• Echinoderms
• Hemichordates
• Lophophorates
There are too many marine invertebrate species to describe in one chapter. Therefore we will cover some of the more common phyla, classes and species.
PHYLUM CNIDARIA
Cnidarians include hydroids, anemones, jellyfish and corals. Cnidarians are the simplest organisms that have attained a tissue level of organisation. Cnidarians are more complex animals than sponges for this reason. As such, these animals may have the ability to swim, engulf prey or do other things, which more simple animals without specialised tissues cannot do. Many cnidarians live in colonies – the best known examples are the reef-building corals in the tropics, however some are also solitary such as the Hydrozoans.
Cnidarians are true modular organisms that often form upright branching colonies, with the individual modules, the polyps, along the branches. The polyps have simple body plans, with a ring of tentacles surrounding a single mouth-anus opening.
Cnidarians consist of little more than an outer covering, the epidermis, and an inner gastrodermis with a non-cellular mesoglea sandwiched between the two. Mucus-secreting cells cover the animal with a protective slime. A single opening serves as the entrance to the gastrovascular cavity, within which prey is digested. Lacking an anus, any undigested material is ejected through the mouth. With most of the living cells in direct contact with the water, specialised excretory, circulatory, and respiratory are not needed. All members of this phylum possess stinging structures called nematocysts housed in specialised cells (cnidocytes).
Class Scyphozoa (True Jellyfish)
These are the true jellyfish and occur in marine environments. Jellyfish are the largest of the planktonic Cnidaria. They are typified by a reduced polyp stage and an extended free-swimming medusa stage. They look like the medusa stage of the hydrozoa but their digestive cavity is more complex, their jelly layer is thicker and their bodies are much larger and often covered with sensory structures. They do not have a sessile polyp stage.
Jellyfish are carnivores. They are relatively fast swimmers for planktonic organisms and propelled through the water by rhythmic contractions of muscle rings that force water out of a bell-shaped structure. After the contraction, the jellyfish starts to sink and the tentacles hanging from the bell can touch the prey and eject its nematocysts.
Class Hydrozoa (Portugese Man o’ War & Freshwater Jellyfish)
The hydrozoa are the simplest cnidarians. They are a diverse group that inhabit a variety of marine and fresh-water environments. Some hydrozoans are bush-shaped, others live as layers of polyps attached to seaweeds or rocks. The polyps are specialised into feeding polyps, reproductive polyps, spines, and protective polyps. The reproductive polyps bud off small individual shaped-like jellyfish, known as medusa. The medusa spawn gametes which then form lava called a planula, which swims for a time before forming the sessile colony.
Class Anthozoa (Sea Anemones, Hard & Soft Corals)
Corals
Corals are small animals noted for the formation of a heavy skeleton of lime (calcium carbonate). This skeleton persists long after the death of the animals and they help in the formation of reefs, atolls and islands. Coral reefs exist in tropical and subtropical waters across the world. Individual coral animals, most of which exists in colonies, are the primary building blocks of the reef. They lie just beneath the surface of the seawater along the tropical and subtropical shores or in shallow warm seas. The reefs are caused by a buildup of marine organisms, both animals and plant. Their skeletal deposits of calcium carbonate (limestone) form the reefs that have occasionally become inhabitable islands.
Corals consist of thousands of polyps interconnected by soft tissue. Stony corals secrete calcium carbonate and may have large, branching skeletons or mounds. Dead coral is a hard, stony substance made up of the skeletons of polyps. Individual coral polyps have soft bodies topped by a ring of stinging tentacles for catching food. These polyps occupy the cuplike cavities in the dead coral. The polyps are fixed in the cups and they are connected to each other by a living membrane that covers the coral rock. The skeleton is secreted by the outer surface of the polyps.
Sea Anemones
These are sessile, predatory animals in the Order Actiniaria. Many anemones are solitary, though some consist of hundreds of individuals that have been produced by one original animal. This is referred to as a polyp. The polyp is attached to substrate by an adhesive foot. The mouth is in the centre of an oral disc surrounded by tentacles. The tentacles surrounding the polyp have radial symmetry. They have stinging organelles called nematocysts. These contain toxins and a filament attached to a sensory hair. When touched, the hair will cause a trigger from a harpoon-shaped structure that injects poison into prey.
Some anemones share a symbiotic relationship with clown fish species. The sea anemone protects the fish from potential predators and provides food from residual prey materials. The clown fish in turn protects the anemone from predators and removes parasites.
A few sea anemone species can be pelagic. These animals have a gas chamber within their pedal disc that enables them to float inverted in the water.
PHYLUM ECHINODERMATA (ECHINODERMS)
Echinoderm means "spiny skin.” Echinoderms have a system of internal water-filled canals, which in many echinoderms form suckered "tube feet", with which the animal may move or grip objects.
Aside from a few species that inhabit brackish waters, all echinoderms are benthic animals living in marine environments. Echinoderms inhabit depths ranging from shallow waters at tide lines to the deep sea. They include sea stars, sea cucumbers and sea urchins.
Class Holothuroidea (Sea Cucumbers)
Sea cucumbers are tubular-shaped with a crown of tentacles, used for feeding on sediment and zooplankton. Their locomotion is by muscle contractions of the body wall and by the use of tubular feed that press against the bottom.
When attacked, sea cucumbers disembowel their internal organs, i.e. they expel their organs through the mouth or anal openings. The sea cucumber can then escape and regenerate. Tropical sea cucumbers have highly toxic substances on their skin which they also use as a defence mechanism.
At the anterior end of the body, the anus opens into a large cloaca, which typically gives rise to an extensively branched system of tubes called the respiratory trees. These penetrate forwards into the body cavity. As their name suggests, they appear to have a respiratory function, although gas exchange via the soft body wall must also be important.
Class Echinoidea (Sea Urchins)
This class includes sea urchins and sand dollars. Sea urchins differ from the other groups of echinoderms because they are globular, oval or sometimes disc-shaped echinoderms which lack arms.
Echinoids are usually covered in spines. The spines on sea urchins are long and thin, providing a defence against predators. Sand dollars and heart urchins have short spines that aid in burrowing.
Echinoids have a jaw-like structure called an Aristotle’s lantern, constructed from ossicles connected by ligaments. Five teeth on the jaw allow the animal to feed by tearing seaweed or scraping algae from rocks and other hard surfaces. Sea urchins feed on attached algae and some species also have long tube feet that catch drifting seaweed.
Class Asteroidea (Sea Stars)
This is probably the most recognised class of Echinoderms. Members of this class are characterised by a flexible, radially-symmetrical star shaped body which has a central disk with five or more radiating arms.
Sea stars are mobile carnivores. They are slow-moving and found on rocky, sandy or mud substrates They feed on crustaceans, snails, bivalves and polychaetes. They grasp food with their tube feet. Their stomach extends from the mouth to swallow and digest prey.
A fascinating characteristic of starfish is their ability to regenerate a complete set of new arms and disk after an attack, as long as the central disk is largely undamaged. A sea star cut into five pieces can potentially grow into five new individuals.
PHYLUM MOLLUSCA (MOLLUSCS)
This Phylum includes three classes found in the ocean – gastropods, bivalves and cephalopods. This phylum has a species total in excess of 100, 000. A group of this size exhibits amazing diversity. Most types of mollusc have a shell of calcium carbonate (such as snails) although some (the nudibranchs and cephalopods) have lost their shells, or they have been reduced. Another common feature is the radula, specialised horny teeth used for feeding. Molluscs also have well-developed sets of gills used for respiration.
Class Gastropoda (Gastropods)
These are the most common type of shell fish in coral reefs. Gastropods have a single solid shell that is often coiled in some mode. This shell offers protection from predators. Most of the body is hidden within the shell, usually the only part seen being the muscular foot at the shell opening. This foot provides a measured form of locomotion. The foot secretes mucus, much like land snails, also Gastropods, which decreases friction. The mantle on the inner surface of the shell secretes the calcium carbonate of the shell throughout its lifespan. Below are some representatives from this class.
Bivalves
Bivalves, as suggested by their name, are enclosed and protected by a pair of shell valves hinged together along the back by means of an elastic ligament. This ligament stretches when the animal clamps the valves together and springs them apart when the animal relaxes. Bivalves have very limited mobility and they must either cement one valve to the substratum or burrow into the sand. Only a few can swim.
Almost all the bivalves are filter feeders. They suck water in through an inhalant syphon, sieving it through enlarged sheet-like gills. The waste water is expelled through the exhalant syphon.
Most of the body consists of organs producing germ cells for sexual reproduction. Most species shed enormous numbers of sperm and eggs into the water, where they develop into planktonic larvae. Common marine bivalves include mussels and oysters.
• Mussels - The mussel's external shell is composed of two valves that protect it from predators and desiccation. Like most bivalves, mussels have a large organ referred to as a foot, which is tongue-like in shape with a groove on the ventral surface. They are filter-feeders, feeding on plankton through cilary-mucus feeding.
• Oysters - The name oyster is used for a number of different groups of molluscs which grow for the most part in marine or brackish water. Inside a usually highly calcified shell is a soft body. The gills filter plankton from the water. Strong adductor muscles are used to hold the shell closed. These are also bivalve molluscs. The "true oysters" are the members of the family Ostreidae, and this includes the edible oysters. They are sedentary filter feeders that depend on currents created by cilia in their gills to provide them with food from the surrounding water. Oysters attach their shells to a surface by secreting cement – literally gluing themselves to the sea bed.
Nudibranchs and Sea Slugs (Subclass Opisthobranchia)
In contrast to many other gastropods nudibranchs and sea slugs have either reduced or completely lost their shell. It many species the shell is present in the larval stage but disappears in the adult form. They are found in nearly all reef habitats from tide pools down to the greatest depths. They feed on algae, sponges, bryozoans, ascidians and cnidarians.
Most of the colourful Opisthobranchs belong to the Order Nudibranchia. These bright coloured and very slow moving creatures are extremely visible to predators. However, their flesh is frequently toxic or distasteful due to various chemicals. The bright colours also serve to warn potential predators that they taste bad. Once a fish taste this sour nudibranch it is unlikely to go back for more. Nudibranchs also utilise camouflage colours being found among seaweeds and sponges.
The tubercular nudibranchs (covered with numerous finger-like appendages) feed on various hydroids and other cnidarians (anemones, soft corals). They ingest the stinging cells of their prey without them firing (i.e. the nematocysts). The nematocysts are then integrated into their own tissues and used as a defence.
Both sexes are present in a single nudibranch but self-fertilisation does not occur. Special sperm sacs are exchanged during copulation. It may take weeks for the eggs to develop. They are then deposited in colourful strands in the favoured food (algae or sponges).
Abalone
These are commercially important edible sea snails. They can vary greatly in size from 20 mm to 200 mm. Abalone shells are rounded or oval with a large dome towards one end. The shell has a row of respiratory pores. The snail living in the shell has a muscular foot with strong suction power that permits it to clamp tightly to rocky surfaces.
They generally remain sedentary, only moving during storms. Adult abalone are herbivorous, feeding on macroalgae such as red or brown algae. They can be prey to whelks, crabs and octopus during their young crevice-living stage. As adults, abalone are prey to fish, octopus, lobster, starfish and stingray.
Class Cephalopoda (Cephalopods)
Cephalopods (head-foot) are a class of the Phylum Mollusca that have adapted swimming as a mode of life. They include squids, cuttlefish, octopuses and nautili. The first three groups have a variety of species, but there is only one surviving genus of the nautili, the Nautilus. There are about 700 species of Cephalopods. Many live in the open ocean or the depths of the ocean e.g. giant squid. Octopus and cuttlefish can also be found on rocky shores or shallow depths. Below are some representatives from this class.
Squid (Order Teuthida) – All squid are predators. They move using “jet propulsion”. Water is forced out of a funnel in the mantle which then propels the animal forward. Some move along the sea bed using their tentacles to move over coral and sand and slide into crevices.
Cuttlefish (Order Sepiida) – This species is similar in structure to the squid. It has the ability to change skin colour rapidly. Pigmented chromatophores overlay reflective pigment cells on a large membrane that is folded and retracted. This causes the distinctive colour changes in this order.
Octopus (Order Octopoda) – Similar body structure to squid. They also move by jet propulsion. The mantle is positioned behind advanced eyes with horizontal rectangular pupils. The mantle contains the vital organs of the organism. They possess eight arms or tentacles, with suckers that are used in locomotion and to capture prey. The mouth of the octopus is underneath the mantle. It has a strong beak that can be used to shred the flesh of prey.
Nautilus (Order Nautiloidea) – There are only six extant species of nautilus. These species are characterised by their smooth, curved and laterally compressed shell. They also use jet propelled locomotion, drawing water into and out of their living chamber. They have poor vision, relying on their olfactory senses for locating prey and identifying potential mates.
PHYLUM ARTHROPODA (ARTHROPODS)
Phylum Arthropoda includes insects and crustaceans. The term arthropod comes from the Greek “joint” and “foot”. This reflects the jointed limbs that characterise this group. Marine arthropods include well known representatives such as lobsters, shrimps and crabs but also an array of microscopic organisms that form a large portion of both the planktonic and benthic fauna.
Arthropods are segmented with limbs adapted to a variety of lifestyles. They each possess an exoskeleton which cannot increase with the size of the animal. As the animal outgrows its exoskeleton it is discarded. The animal excretes a new exoskeleton during molting.
Interestingly arthropods possess an image-forming eye. This is quite advanced for an invertebrate species. Only arthropods and cephalopods possess this ability to form an image of their external environment.
Subphylum Crustacea
Crustaceans are the marine representatives of this Phylum. They are incredibly diverse with regards to size, shapes, colours and lifestyle. Many of these species are of commercial importance such as crab, prawn (shrimp) and lobster.
Crustaceans have a rigid, calcium carbonate-based external skeleton. Growth is continuous throughout life therefore the animal will periodically moult the shell and replaces this with a new one. Before moulting the calcium component of the shell is partly absorbed and digested, while the new shell begins to form. When the new shell is still forming, having already disposed of the old shell, the animal is particularly vulnerable to predators and will stay in a burrow or hiding place. After moulting the animal often changes its morphology, e.g. the swimming appendages may change, or its food requirements may vary. Below is a selection of species from this subphylum.
Lobster (Order Decapoda) – this includes two large crustacean species. These can be differentiated by the presence or absence of paired claws. Like other crustaceans, they have a hard protective exoskeleton which they molt as they grow. They have ten (10) walking legs, the front three with claws. They are bilaterally symmetrical however they may have varying specialised claws for grasping prey. They are slow-moving along the sea floor. When threatened, they can retreat quickly by curling and uncurling their abdomen.
Crab (Order Decapoda) – These representatives are characterised by their reduced abdomen which is tucked under the thorax. The hermit crab is one exception. It possesses a large fleshy abdomen with the last pair of appendages modified to grasp the shell in which it lives. In most crabs the body is flattened and generally wider than it is long. This body shape allows them to walk sideways which is typical to the group.
Clam Shrimps (Order Ostracoda) – These are small pelagic and benthic crustaceans that can be found in both freshwater and marine environments. They are found throughout ocean environments around the world. They have two shells (bivalve) held together by a muscle. This shell is often clear. They are dorso-ventrally with compound eyes located on the forehead. They are hermaphroditic and only live between 2 and 3 weeks.
Barnacle (Order Cirripoda) – Most of the 900 species of this order live their lives permanently attached to some substrate. Others can be free-living. They can attach to other animals such as whales and turtles as well as logs and the underside of boats. However, most are benthic, adhering to rock substrates. It is a shrimp-like crustacean. It stands on its head, using bristled appendages on its thorax to sweep food particles from the water.
Krill (Order Copepoda) – These are planktonic herbivores, similar in shape to shrimp. They are found in all of the world’s oceans both in surface waters and in the deep sea. They are generally very small (between 1-5mm in length). The abdomen is reduced and bears no appendages. The thorax is fused to the head. The exoskeleton is transparent in most species. They have two antennae and several pairs of legs along the thorax. These are used for feeding and grooming.
PHYLUM ANNELIDA (ANNELIDS)
Annelids are worms with a body plan divided into segments. They have a tubular gut that connects the mouth to the anus. The reproductive organs, digestive glands and locomotory appendages are usually repeated in each segment. The body contains a coelom – a fluid-filled space which serves as a hydrostatic skeleton. The annelids have a nervous system –which consists of a double nerve cord that extends from a brain through the length of the body. The nerve cord coordinates the muscular contractions that allow the worms to swim. There are around 8,700 marine species that are mostly benthic. These worms burrow into sediment or crawl across or under various substrates.
Two of the more commonly recognised aquatic members of this phylum are:
Christmas Tree Worms (Class Polychaeta)
Christmas Tree Worms Spirobranchus giganteus are small sedentary marine worms that build tubes anchored to corals. These are calcerous tubes secreted by the worm. Christmas tree worms are found in tropical waters around the world. As the name suggests these worms have two distinctive tree-shaped crowns. They vary greatly in colour but are generally around 4cm across. The tentacles of the worm are used for feeding on food particles and plankton floating in the water. They then pass the food down cilary tracts and sort particles based on size and texture. Sand grains are stored in sacs for future tube building.
Christmas tree worms use sexual reproduction. Egg fertilisation occurs in the water. These eggs develop into larvae that then burrow into coral to form tubes. Adult worms are anchored to their tubes/burrows built on coral. They react quickly to disturbance, retracting rapidly when shadows pass.
Feather Duster Worm (Class Polychaeta)
Feather duster worms are also known as Sabellidae. They are also sedentary marine worms that build tubes for protection. These worms are common in intertidal zones worldwide.
As with their relations, they have a crown of feeding appendages in two fan-shaped clusters for feeding. They feed on plankton and organic detritus. The head of the worm is concealed within the tube. The branches of the worm create a comb for feeding.
Sizes can vary greatly. The largest species is the giant feather duster worm (Eudistylia polymorpha). This species can grow to about 25 cm long.
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