Life of marine animals
The sea has salt water, waves, pressure and little light deeper down. Animals fit the sea in different ways. Scientists group them by how they move:
- Nekton: strong swimmers (fish, whales, squid).
- Plankton: weak swimmers that drift (jellyfish, tiny animals).
- Benthos: live on or in the sea floor (crabs, shellfish, starfish).
Fish and sea mammals
Fish breathe with gills that take oxygen from water, have fins and scales, and most lay eggs. They are cold-blooded.
Whales, dolphins and seals are mammals. They breathe air with lungs, are warm-blooded, give birth to young and feed them milk. A whale is not a fish; its tail moves up and down, a fish tail moves side to side.
Crustaceans, molluscs and others
- Crustaceans (crab, prawn, lobster): hard outer shell, jointed legs, two pairs of feelers.
- Molluscs (oyster, snail, squid, octopus): soft body, many have a hard shell. Squid and octopus are fast, clever swimmers with no outer shell.
- Cnidarians (jellyfish, sea anemone, coral): soft body with stinging tentacles. Coral builds reefs.
- Echinoderms (starfish, sea urchin, sea cucumber): spiny skin, five-part body, tube feet.
Main marine animals and what they eat
Plankton eaters: sardine, whale shark, blue whale. Meat eaters: tuna, shark, dolphin. Grazers on plants: sea urchin, some turtles. Cleaners of the floor: crabs and sea cucumbers. Together they form food chains, with plankton at the start.
Many animals live in the sunlit top layer; fewer live in the dark deep sea, where some make their own light.
Marine animals and people
We take fish, prawns, squid and shellfish as food, and pearls, oils and medicines. Overfishing, plastic, pollution and warm seas harm them. Rules on catch size and fishing seasons help animals to recover.
Try it
In the 3D, pick each animal and read its clues. Sort them into swimmers, drifters and bottom dwellers. At home, sort pictures of sea animals into three piles: swim, drift, crawl.
Key formulas and definitions
- Key term: nekton = swim; plankton = drift; benthos = live on the floor
- Fish: gills, fins, cold-blooded. Whale: lungs, warm-blooded, milk
- Crustacean: hard shell + jointed legs
- Mollusc: soft body, often shell
- Echinoderm: spiny skin, five arms, tube feet
Worked examples
1. Put these in groups: tuna, crab, jellyfish.
Tuna swims strongly: nekton. Crab lives on the floor: benthos. Jellyfish drifts: plankton.
2. How can you prove a dolphin is not a fish?
It breathes air through lungs, is warm-blooded and feeds milk to its baby. Fish use gills and lay eggs.
3. A sea animal has five arms and tube feet. What group is it?
Echinoderm, like a starfish.
Common mistakes
- Calling a whale or dolphin a fish. They are mammals and must breathe air.
- Thinking jellyfish are fish. They have no bones, no gills and drift.
- Thinking all shellfish are crabs. Crabs are crustaceans; oysters and snails are molluscs.
- Thinking the sea floor is empty. Benthos is rich with life.