Invertebrates
0 of 3 hands-on
01 Introduction
Invertebrates are animals without a backbone, and they make up the great majority of animal species in the sea. On any dive, if you look closely at a rock, a sponge or a patch of sand, you will see more invertebrates than fish.
Objectives
- Explain what an invertebrate is and why they dominate the ocean
- Describe the main groups of marine invertebrates and an example of each
- Recognise which invertebrates are hazardous to touch
02 Watch
Life without a backbone
5 min · Video in production
- 1
A slow zoom across a reef wall covered in sponges, corals, sea squirts and worms.
- 2
A series of close-ups: a nudibranch, an octopus changing colour, a sea star walking on tube feet.
- 3
A coral polyp extending its tentacles at night to catch plankton.
- 4
A diver's hand held back from a lionfish, a fire coral and an urchin, with warning labels.
03 Learn
Most animals have no backbone
An invertebrate is an animal without a backbone. Together they make up the vast majority of animal species on Earth, and the sea is crowded with them. They range from sponges that look like rocks to octopuses that solve puzzles.
The main groups
| Group | Key features | Examples |
|---|---|---|
| Sponges (Porifera) | Simple animals with no organs; filter water through pores | Barrel sponges, tube sponges |
| Cnidarians | Soft bodies with stinging cells (nematocysts) and tentacles | Corals, jellyfish, sea anemones |
| Molluscs | Soft bodies, often with a shell; some have a muscular foot | Snails, clams, nudibranchs, octopus, squid, cuttlefish |
| Crustaceans | Hard exoskeleton, jointed legs, which they shed as they grow | Crabs, lobsters, shrimps, barnacles |
| Echinoderms | Spiny skin, five-part symmetry, tube feet | Sea stars, urchins, sea cucumbers, feather stars |
| Worms | Soft, segmented bodies, many in tubes | Christmas tree worms, fire worms |
| Tunicates | Filter feeders with a sac-like body, relatives of vertebrates | Sea squirts |
Cnidarians: corals and stingers
Cnidarians have stinging cells that fire a tiny harpoon to catch prey. A coral colony is made of thousands of small polyps, each with a ring of tentacles, living on a hard skeleton of limestone that they build over time. Many reef-building corals also host algae that give them food and colour.
Molluscs: from snails to octopuses
Molluscs include the sea snails and clams with shells, and the cephalopods (octopus, squid and cuttlefish), which have lost their shells and are among the most intelligent invertebrates. They can change colour and texture in a fraction of a second to blend in or signal.
Crustaceans and echinoderms
Crustaceans are armoured. To grow, they moult, leaving the old shell behind, and are soft and vulnerable until the new one hardens. Echinoderms move with hundreds of tiny water-filled tube feet. Sea cucumbers clean the sand by eating it and extracting food.
Predict first
An animal has a soft body, no backbone, eight arms and can change colour in a flash. What is it most likely to be?
Which ones to be careful with
Many invertebrates defend themselves with stings, spines or venom. They are not aggressive, they are simply protected, and you will avoid trouble by keeping your hands to yourself.
| Animal | Hazard |
|---|---|
| Fire coral and some hydroids | A painful sting on contact |
| Jellyfish and their relatives | Tentacles sting. Some species are very dangerous |
| Sea urchins | Sharp spines can break off in the skin |
| Cone shells | Some have a venomous harpoon and can be dangerous |
| Blue-ringed octopus | Small but highly venomous. Never touch |
| Stonefish and scorpionfish (fish, but camouflaged on the bottom) | Venomous spines. Look before you put a hand down |
04 Explore
Try it yourself
Match each invertebrate group with its example.
Match each group with an example.
- Cnidarian
- Mollusc
- Crustacean
- Echinoderm
- Sponge
05 Apply
Scenario
On a night dive, a diver sees a small, beautiful octopus with bright blue rings, sitting in a shell on the sand. They want to pick it up for a photograph.
What should they do?
06 Check
07 Summary
Key points
- Invertebrates have no backbone and make up the vast majority of animal species.
- Main groups: sponges, cnidarians, molluscs, crustaceans, echinoderms, worms and tunicates.
- Corals are colonies of polyps, and octopuses are intelligent molluscs.
- Many invertebrates sting, spike or poison. Look but do not touch.
Before you apply this in the water
- Have I been trained by a qualified instructor for this?
- Am I medically fit to dive?
- Do I have a buddy and a plan?
- Am I diving within my training and limits?
Answer every question in the Check section to mark this lesson complete.