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Jellyfish are composed of more than 90% water.

The Sea - Invertebrates

Sea invertebrates include crabs, starfish, and jellyfish. Discover how these spineless animals move, feed, and defend themselves in different marine habitats, then try the quiz.

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Fascinating Fact

Sea anemones look like flowers but are animals, and they use stinging cells on their tentacles to catch small prey.

Sea invertebrates are animals without backbones, and they make up most marine species. You can find them clinging to rocks, burrowing in sand and mud, drifting in the water column, or hiding in seaweed and reefs. Many are identified by body shape, shells, spines, legs, and the way they move, such as crawling, swimming, jetting, or filtering food from seawater.

Key Terms

Invertebrate: An animal without a backbone.
Exoskeleton: A hard outer covering that supports and protects the body, common in crabs and lobsters.
Filter feeder: An animal that eats by straining tiny food particles from water.

Frequently Asked Questions (Click to see answers)

What is a marine invertebrate?

A marine invertebrate is an ocean animal that has no backbone. This group includes creatures such as crabs, starfish, mussels, squid, worms, and many types of plankton.

What are common examples of sea invertebrates?

Common sea invertebrates include crabs and shrimps, snails and clams, starfish and sea urchins, jellyfish, sponges, and octopus or squid. They live from shorelines to deep seas.

How do sea invertebrates protect themselves?

Sea invertebrates may protect themselves with shells, spines, tough skin, camouflage, or hiding in crevices and sand. Some release ink or mucus, and many rely on staying still to avoid detection.

Try These Related Quizzes

To see a larger image, click on the picture.

1 .

What colour is the blood of a lobster?

Photograph courtesy of www.flickr.com/photos/simon999/

Yellow

Green

Red

Blue

Order: Decapoda
Suborder: Pleocyemata
Infraorder: Astacidea
Family: Nephropidae
Lobsters have 10 walking legs; the front three pairs bear claws, the first of which are larger than the others.
They are found in all oceans.
In the main, they live on their own in crevices or in burrows under rocks.

2 .

An octopus became internationally famous during the 2010 FIFA World Cup. What was his name?

Photograph courtesy of MatthiasKabel

Ollie

Oscar

Paul

Peter

Octopuses are among the most intelligent and behaviourally flexible of all invertebrates.
The photo shows an octopus opening a container with a screw cap!
They have numerous strategies for defending themselves against predators, including the expulsion of ink, the use of camouflage and deimatic displays, their ability to jet quickly through the water and their ability to hide.
Though octopuses can be difficult to keep in captivity, some people keep them as pets. Octopuses often escape even from supposedly secure tanks, due to their problem-solving skills, mobility and lack of rigid structure.

His feeding behaviour was used to predict the winner of each of Germany's seven matches, as well as the outcome of the final. He was 100% correct!

3 .

Dried scallop is known in Cantonese Chinese cuisine as what?

Photograph courtesy of Svdmolen

Congee

Pakchoi

Conpoy

Dimsum

Order: Ostreoida
Suborder: Pectinina
Superfamily: Pectinoidea
Family: Pectinidae
Found in all of the world's oceans.
Unusually, some scallops are male and female; some are both sexes in the same individual and yet others are males when young then change to females.

4 .

What is the collective name for a group of jellyfish?

Photograph courtesy of www.flickr.com/photos/dan90266/

Smack

Sting

Slap

Smart

Jellyfish are found in every ocean, from the surface to the deep sea.
They are composed of more than 90% water.
Most species don't have a brain or central nervous system.
The box jellyfish has 24 eyes, two of which are capable of seeing colour, and four parallel brains which act in competition.
It is thought to be one of the only creatures that has a 360 degree view of its environment.

5 .

What does a starfish NOT have?

Photograph courtesy of Nick Hobgood

Blood

Eyes

Mouth

Stomach

Most starfish typically have five rays or arms, which radiate from a central disc, however some species can have up to 50 arms.
On the end of each arm or ray there is a microscopic eye.
Some species reproduce asexually by fragmentation, often with part of an arm becoming detached and eventually developing into an independent individual starfish!

6 .

Which astrological sign is named after the crab?

Photograph courtesy of Patrick Verdier

Capricorn

Cancer

Leo

Pisces

Order: Decapoda
Suborder: Pleocyemata
Infraorder: Brachyura
Crabs vary in size from the pea crab, a few millimetres wide, to the Japanese spider crab, with a leg span of up to 4 metres (13 ft)!
Not all crabs walk sideways - some walk forwards and backwards.
Some crabs are also capable of swimming.

7 .

Opening oysters requires skill. What is the alternative name for 'opening' them?

Photograph courtesy of David Monniaux

Clacking

Shucking

Teasing

Squeezing

A group of oysters is commonly called a bed or oyster reef.
Some tropical oysters grow on mangrove roots. Low tides expose them, making them easy to collect.
In the West Indies, tourists are amazed when told that 'oysters grow on trees'.
Whitstable is noted for oyster farming from beds on the Kentish Flats that have been used since Roman times.

Oyster shucking competitions are held all over the world

8 .

Cuttlefish ink was formerly a much used dye called what?

Photograph courtesy of Nick Hobgood

Indigo

Manganese

Sepia

Sienna

They are sometimes referred to as the chameleon of the sea because of their remarkable ability to rapidly alter their skin colour at will.
Cuttlefish eyes are among the most developed in the animal kingdom.
The pupil is a smoothly-curving 'W' shape.
Its blood is an unusual shade of green-blue.
The blood is pumped by three separate hearts.

Artificial dyes have mostly replaced natural sepia these days

9 .

Mussels are an excellent source of which vitamin?

Photograph courtesy of Wilson44691

B12

The external shell is composed of two hinged halves or 'valves'. The valves are joined together on the outside by a ligament, and are closed when necessary by strong internal muscles.
Mussels are filter feeders; they feed on plankton and other microscopic sea creatures which are free-floating in seawater.
In Belgium, the Netherlands and France, mussels are consumed with french fries ('moules frites) or bread.

'Excellent source' means that mussels provide at least 20% of the recommended daily value

10 .

Clam Chowder is a type of what?

Photograph courtesy of Marlith

Bread

Soup

Cake

Cocktail

In October 2007 a clam, caught off the coast of Iceland, was discovered to be at least 405 years old, and was declared the world's oldest living animal by researchers from Bangor University.
The word 'clam' has given rise to the metaphor 'clam up', meaning to refuse to talk or answer, based on the clam behaviour of quickly closing its shell when threatened.
Clams have also inspired the phrase 'happy as a clam', short for 'happy as a clam at high tide'.

Author: Sarah Garratty

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