Invertebrates: 25 Examples of Animals Without a Backbone

What is an invertebrate?

Animals are often split into two camps. Vertebrates (fish, amphibians, reptiles, birds and mammals) have a backbone made of vertebrae. Everything else is an invertebrate. That makes “invertebrates” a group defined by what its members lack rather than a single family tree: a bee is more closely related to a lobster than to a jellyfish.

Scientists have described roughly 1.5 million animal species, and only about 75,000 of them are vertebrates. Invertebrates range from microscopic parasites to the colossal squid, and they live everywhere from deep-sea vents to mountain peaks, deserts and your garden soil.

How do invertebrates keep their shape without bones?

Invertebrates have found several ways to stay in shape:

• Exoskeleton: insects, spiders, crustaceans and other arthropods wear a hard outer skeleton made largely of chitin. It cannot stretch, so they must molt (shed it) to grow.
• Hydrostatic skeleton: earthworms, sea anemones and many soft-bodied animals use fluid trapped under pressure inside the body, like a water balloon, for muscles to push against.
• Shells: most mollusks build a shell of calcium carbonate, while squid keep only a thin internal stiffener.
• Internal plates and spicules: sea stars and sea urchins have a skeleton of small chalky plates under the skin, and sponges are stiffened by tiny needles of silica or calcium carbonate.

The main invertebrate groups at a glance

Insects

1. Western honey bee

The western honey bee lives in colonies of up to about 80,000 workers ruled by a single queen. Foragers tell their nestmates where to find flowers with the “waggle dance,” and the species pollinates many of the world’s crops.

2. Monarch butterfly

Eastern North American monarchs migrate up to about 4,800 km (3,000 mi) to spend the winter in fir forests in central Mexico. The butterflies that fly south have never made the trip before; the last monarchs to do so lived several generations earlier.

3. Leafcutter ant

Leafcutter ants of the American tropics do not eat the leaves they carry. They chew them into a mulch to grow a fungus, which feeds the colony, making them farmers millions of years before humans.

4. Hercules beetle

Males of this rainforest beetle from Central and South America can measure about 17 cm (7 in) including their long horn, making it one of the longest beetles in the world. Males use their horns to wrestle rivals.

5. Atlas moth

This giant silk moth of Asian forests has a wingspan of about 24–28 cm (9.5–11 in). The adults have no working mouthparts and live only on fat stored as caterpillars, surviving about one to two weeks.

Arachnids

Arachnids have eight legs and two main body parts, while insects have six legs and three body parts; see spider vs insect for the full comparison. Ticks and mites belong here too.

6. European garden spider

This common orb weaver of European and North American gardens spins a new wheel-shaped web almost every day, often eating the old silk to recycle its protein.

7. Zebra jumping spider

This small black-and-white spider does not build a web to catch prey. It stalks insects on walls and fences, using its large front eyes to judge distance before leaping.

8. Scorpions

Scorpions grab prey with their pincers and can use the venomous stinger on their tail. Their exoskeleton glows blue-green under ultraviolet light, and mothers carry their newborn young on their backs.

Crustaceans

9. American lobster

The American lobster of the North Atlantic is the heaviest crustacean known. The record, caught off Nova Scotia, weighed 20.1 kg (44.4 lb).

10. Japanese spider crab

This deep-water crab from Japan has the longest legs of any arthropod, with a leg span of up to about 3.7 m (12 ft).

11. Antarctic krill

These shrimp-like animals grow only about 6 cm (2.4 in) long, but they swarm in such numbers that their combined weight is estimated at hundreds of millions of tonnes. Whales, seals, penguins and fish all depend on them.

12. Pill bug

Pill bugs, or roly-polies, are not insects but land crustaceans related to crabs. They need damp places because they breathe with gill-like organs, and they roll into a ball when disturbed.

Myriapods

13. Giant African millipede

The largest living millipede can grow over 30 cm (12 in) long. Despite the name, millipedes never have a thousand legs; this species usually has around 250 or more, two pairs on most body segments.

14. House centipede

This fast hunter has 15 pairs of long legs and chases down insects and spiders in houses across Europe and North America.

Mollusks

15. Common octopus

The common octopus has eight arms, three hearts and blue blood. It changes color in a fraction of a second and is one of the most intelligent invertebrates.

16. Giant squid

The giant squid lives in the deep ocean and was first photographed alive in its natural habitat in 2004. Its close rival, the colossal squid, is the heaviest invertebrate known, at about 495 kg (1,091 lb).

17. Garden snail

This land snail carries a coiled shell, glides on a layer of mucus and scrapes plants with a ribbon of tiny teeth called a radula. It is native to the Mediterranean region and western Europe but has spread to North and South America, southern Africa, Australia and New Zealand.

18. Ocean quahog

This North Atlantic clam is one of the longest-lived animals known. A specimen collected off Iceland in 2006, nicknamed Ming, was 507 years old.

Worms

“Worm” is not one group but several unrelated ones.

19. Common earthworm

The common earthworm is an annelid, or segmented worm, usually 20–25 cm (8–10 in) long when stretched. By burrowing and eating soil, earthworms mix and enrich it, as Charles Darwin described in 1881.

20. Caenorhabditis elegans

This roundworm is only about 1 mm long, and an adult hermaphrodite has exactly 959 body cells. In 1998 it became the first multicellular animal to have its whole genome sequenced.

21. Planarian

These small freshwater flatworms can regrow a complete body, including a new head and brain, from a small fragment.

Cnidarians

Cnidarians, including sea anemones, catch prey with stinging cells on their tentacles.

22. Moon jellyfish

This common jellyfish of coastal seas has no brain, heart or bones. Its body is mostly water, and its sting is too weak to trouble most people.

23. Staghorn coral

Each coral colony is made of thousands of tiny polyps that build a shared limestone skeleton. Staghorn coral, once one of the main reef builders in the Caribbean, is now listed as Critically Endangered.

Echinoderms

24. Ochre sea star

The ochre sea star of the Pacific coast of North America walks on hundreds of tiny tube feet and eats mussels by pushing its stomach out into their shells. Studies of this species in the 1960s gave ecology the idea of a keystone species.

Sponges

25. Venus’ flower basket

This deep-sea glass sponge from the Pacific Ocean builds a delicate lattice skeleton of silica. Like all sponges, it has no organs and feeds by filtering seawater.

Why invertebrates matter

Invertebrates run much of the living world. Bees, butterflies and beetles pollinate plants, earthworms and millipedes recycle dead leaves, corals build reefs that shelter thousands of fish species, and krill feed whales. Many invertebrates are declining because of habitat loss, pollution, pesticides and warming seas.