Marine invertebrates

Marine invertebrates

Barnacles feedingBarnacles feeding Invertebrates are animals that do not have backbones. Many are only found in the sea. Marine invertebrates are a extremely large and diverse group that includes corals, sea anemones, sea urchins, starfish and many crustaceans, molluscs and worms. Crustaceans and molluscs are sometimes grouped together and called “shellfish”. Among the marine invertebrates are some of the largest animals on Earth (the colossal squid may measure up to 14 metres / 46 feet long) as well as some of the deadliest (the box jellyfish).

Krill, a kind of shrimpKrill, a kind of shrimp


Crustaceans are a group of arthropods—animals that have jointed legs and an external skeleton. As they grow, they moult their external skeleton (exoskeleton) and grow a new one. Crustaceans include crabs, lobsters, shrimp and barnacles. They all have two pairs of sense receptors called antennae on their heads.

Shrimp are stalk-eyed crustaceans with long, narrow abdomens, long antennae and slender, fragile legs. They swim using legs called swimmerets attached to their abdomens.

As adults, barnacles attach themselves permanently to a surface, such as a rock, and feed by filtering particles from the water.

Common octopusCommon octopus


Molluscs are a group of invertebrates with soft bodies. Many are protected by hard shells. The main groups of molluscs are gastropods, bivalves and cephalopods. Gastropods include sea snails, sea slugs and whelks. They move about on one large foot. Bivalves have two shells joined by an elastic hinge. They include scallops, oysters and mussels. Cephalopods have large heads and a beaked mouth surrounded by tentacles. They include squid, octopuses, cuttlefish and nautiluses.



Echinoderms are a group of marine invertebrates with spiny skins and skeletons made of chalky calcium plates. They include starfish, sea urchins and sea cucumbers. Most echinoderms also have the ability to regrow body tissue, including organs and limbs. All have tube feet, small, tube-shaped projections along their arms that allow them to move around and pass food to their mouths in the centre of their bodies.

Sea pens living on the abyssal plainSea pens living on the abyssal plain


Cnidarians are a group of 8000–9000 species, most of which live in the sea. They include sea anemones, corals, sea pens, jellyfish, box jellies and the Portuguese man-o'-war. A cnidarian's body consists mostly of a jelly-like substance contained inside a thin covering of tissue. Its mouth (which is also its anus) is surrounded by stinging tentacles, which it uses to capture prey.

The Atlantic sea nettle The Atlantic sea nettle Click to play videoA cnidarian's tentacles have special cells, called cnidocytes, which fire tiny "harpoons", called nematocysts, containing venom at great speed at its prey. Humans feel these as stings. The nematocysts are used mainly to capture prey by paralyzing it before the tentacles draw it into the creature's mouth.



Christmas tree wormChristmas tree worm

Marine worms

Marine worms are a large group of long, thin, soft-bodied creatures. They include segmented worms, flatworms and nematode worms (roundworms). There are around 9000 species of segmented worm, including earthworms and leeches. These worms consist of a number of segments, each of which contains the same set of organs. Polychaetes (pronounced "polly-keets") are a group of segmented worms that are mostly marine. Each segment has fleshy tentacle-like parts with bristles, hence their common name, bristle worms.



A colourful flatwormA colourful flatwormFlatworms are simple, soft-bodies creatures that breathe and eat by allowing oxygen and nutrients to pass through their flattened bodies. Most species live in water or soil, but some species are parasites, living in or on other animals. The tapeworm, a parasitic flatworm, lives in animal intestines and, in extreme cases, can grow up to 37 metres (120 feet) long.

Nematode worms seen under a microscopeNematode worms seen under a microscope


Nematodes, or roundworms, are one of the most diverse groups of all animals, with possibly more than one million species. They are to be found living in nearly every ecosystem from the polar regions to the tropics, from mountains to the oceans—even inside rocks several kilometres deep inside the Earth's crust. Free-living or parasites, they have thin, tapered bodies with openings at both ends. Many are so small that they can only be seen through a microscope.

Tube spongeTube sponge


Sponges have no mouths, hearts, brains or distinct body parts. Like cnidarians, their bodies consist of non-living, jelly-like masses surrounded by thin layers of living tissue. They are sessile: they live permanently attached to solid surfaces. They feed by filtering food from the sea water. Water passes through the thousands of tiny pores in their bodies from which the sponges strain out any food particles, including bacteria and plankton.

 Chris Jarvis

See also in Life

The octopus has the largest brain of any invertebrate.

Most invertebrates have symmetrical bodies, meaning that each half looks exactly the same. The only exception are sponges. Echinoderms have five-fold symmetry: their bodies can be divided into five identical pieces.

The sea wasp, a kind of box jellyfish, is probably the most venomous marine animal. A person may die within three minutes of being stung. The amount of venom in one creature is thought to be enough to kill 60 adult humans.

A jellyfish's or sea anemone's tentacles shoot out "harpoons" (nematocysts) at explosive speed—probably the fastest action in nature. They may take just 700 nanoseconds (millionths of a second) to reach their target, an acceleration of about 5,410,000 g (a bullet, for comparison, has an acceleration of 190,000 g).

Horseshoe crabs, which are related not to crabs but to spiders and scorpions, are almost identical to species that lived 230 million years ago—before the Age of Dinosaurs.

Some deep-sea marine worms escape their predators' clutches by launching glowing "bombs". These give off an intense light which lasts for several seconds—time enough for the worms to make their getaway.

There are 15,000 known species of nematode worm, and many more to be discovered. They have colonized every available habitat on the planet, including under beer coasters in Germany (Panagrellus redivivus), in the placentas of sperm whales (Placentonema gigantissima) and the right kidneys of minks (Dioctophyme renale).

The 5-cm (2-inch) long pistol shrimp may be the loudest animal in the sea. To stun its prey of fish, it makes a bubble by snapping one of its claws shut. The noise this makes can reach 200 decibels (a jet aircraft taking off a few metres away registers 150 decibels).

The pressure created by a pistol shrimp's bubble is strong enough to stun or kill small fish for the shrimp to eat. As it collapses, the bubble reaches temperatures of over 4700°C (8500°F).

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