Not everything on a coral reef has fins and a backbone. In fact, some of its most fascinating inhabitants have no backbone at all.

Crabs, lobsters, octopuses, sea stars, sea urchins, jellyfish, marine worms and countless other animals all belong to the enormous and incredibly diverse world of the invertebrates.

What Is an Invertebrate?

Quite simply, an invertebrate is an animal without a backbone.

That definition includes an astonishing variety of animals. A delicate jellyfish, an intelligent octopus, a spiny sea urchin and a heavily armored lobster may look completely unrelated, yet all are invertebrates.

In fact, the vast majority of animal species on Earth are invertebrates.

No Backbone, Many Solutions

Without an internal skeleton like ours, invertebrates have evolved all kinds of other ways to support and protect their bodies.

Crabs and lobsters wear a hard external skeleton. Sea urchins are protected by spines. Mollusks may build shells, while an octopus relies on a soft, flexible body that can squeeze through remarkably small openings.

Some simply defend themselves with venom, stinging cells, camouflage or chemicals that make them extremely unpleasant to eat.

The Art of Staying Hidden

Some of the best camouflage artists on the reef are invertebrates.

Octopuses can change color and skin texture, decorator crabs attach pieces of their surroundings to their bodies, and many sea slugs use brilliant colors either to disappear into their habitat or to warn predators to stay away.

Sometimes the strange little object you have been staring at really is staring back.

From Tiny to Enormous

Invertebrates range from animals so small they are almost impossible to see to some of the largest creatures without backbones.

Even within the Caribbean reef community, their variety is extraordinary. Some spend their entire lives attached to one spot, while others crawl, swim, burrow or drift through the ocean.

The Reef's Hidden Workforce

Invertebrates perform countless important jobs. They graze on algae, filter the water, recycle dead material, turn over the sand and provide food for many other animals.

Corals themselves are invertebrates, and the tiny coral polyps are responsible for building the physical foundation of the reef on which so many other species depend.

A Closer Look Is Always Rewarded

Invertebrates are often easier to miss than colorful reef fish. Many hide beneath rocks, inside crevices or among corals, while others are so well camouflaged that you can look straight at them without noticing.

But once you start paying attention to the smaller inhabitants of the reef, an entirely new world appears.

And that is when a coral reef becomes much more than a place full of fish.