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An estimated 97% of all species on Earth are invertebrates, which means they don’t have bones. Some you might be familiar with echinoderms such as starfish and sea urchins, or cnidarians such as corals, sea anemone and jellyfish. Some not so common invertebrates are sponges, mollusks and arthropods, some crustaceans and over 60,000 species of arachnids.

The first animals on the planet were invertebrates, evolving from single-celled microorganisms that worked together to form multicellular organisms. The oldest fossils of invertebrates date back 600 million years.

Although invertebrates have no bones, structural support comes from exoskeletons, as with insects, or from the balance of fluid pressure, as with slugs and jellyfish. The structures of many invertebrates are able to change form as their grow, a process called metamorphosis.

The simplest invertebrates are sponges, which feed by siphoning water through their bodies and filtering out food particles. More complex invertebrates like spiders, snare prey with webs. Most parasites are invertebrates, living on or inside the digestive tract of hosts.

Here are some intriguing invertebrates local to California:

Purple Sea Urchin

The purple sea urchin is an echinoderm and a member of the kelp forest community, used for food by the indigenous people of California. It has the ability to sense its environment, a unique trait for an animal lacking a head structure.

Ochre Sea Star

The ochre sea star is a common starfish found in the waters of the Pacific, considered a keystone species used to assess the health of intertidal regions. The loss of even a few sea stars has a profound effect on nearby mussel bed populations.

Hermit Crab

The hermit crab is a crustacean that can elude predators by retracting into abandoned seashells it carries around; although it must find larger shells as it grows. The habit of living in second-hand shells gave rise to its name.

Rose Anemone

The rose anemone is a species of sea anemone found in the Pacific coast of North America at depths of around 15 feet. Large anemones offer protection to fish like the painted greenling, providing a safe environment to feed on small invertebrates.

Helmet Urchins

Helmet urchins are a species of Echinoderm with a remarkable ability to live in inhospitable wave-battered shorelines.

Two-spotted Octopus

The two-spotted octopus is an octopus species found off the coast of California, identified by a circular blue spot under each eye. Because of their friendly temperament, they are considered to make the best pet octopus.

Sea Lemon Nudibranch

A sea lemon is a sea slugs, or nudibranch, whose name comes from the animal’s visual similarity to a lemon, due to its oval shape and pale yellow coloration.

Invertebrates form a rich topic of study, their high numbers and diversity of forms providing remarkable insight into the full breadth of biodiversity on our planet.

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