Identifying common sponges and seabed invertebrates

Most seabed animals are easier to name by their body form, texture and location than by their color, since color shifts with depth, light and the water around them. Sponges are fixed and porous, with textures from soft and compressible to firm. Anemones have a soft column ringed with tentacles around a central mouth. Sea stars have a central disc with arms, sea urchins are covered in spines and sea cucumbers are soft and elongated. Start with those features, then check a regional guide for the name.

Identifying these animals is not only about curiosity. Many are slow-growing and easily damaged, and some defend themselves with stings, spines or chemicals. Recognizing what you are looking at helps you avoid touching the wrong animal and gives you a better sense of what the seabed supports.

Sponges: form, texture and pores

Sponges are simple animals that draw water through many small pores and push it out through larger openings. Common shapes include encrusting sheets that cling flat to rock, rounded or barrel-shaped forms, vase-shaped sponges with a wide opening at the top and branching or finger-like growths.

Texture helps separate them. Some sponges feel firm and rough, others are soft and fragile, and some break apart when disturbed. A sponge usually has a surface covered in small openings rather than a smooth skin, and it has no visible eyes, mouth or limbs. An animal that sits in one place and never moves is a sign of a sponge or another fixed animal, so look for those clues before you decide what you have found. Many sponges also contain irritating or toxic compounds, so looking is safer than testing a texture with your fingers.

Several other encrusting animals can look like sponges from a distance. Sea squirts, also called tunicates, are soft-bodied animals with two openings, and they can be bulbous, tube-shaped or brightly colored. Colonial sea squirts form soft crusts, and bryozoans form thin, lacy or fuzzy mats on rock. Look closely at the surface: a sea squirt usually shows small openings that act as tiny siphons, while a bryozoan is built from many tiny chambers that give its surface a fine, grainy texture. Neither should be scraped off the rock to find out what it is.

Anemones and other fixed animals

Sea anemones look like flowers but are animals. They attach to rock or sand through a base, and their tentacles arrange around a central mouth. The tentacles carry stinging cells, and some anemones host small fish that are protected by a mucus layer. Anemones usually stay in one place. Many withdraw their tentacles when touched, and after dark some species pull their tentacles in while others open them to feed.

Other fixed animals are sometimes mistaken for plants. Soft corals have flexible branches or lobes that sway with the current, while stony corals have a hard skeleton.

Mobile seabed animals

Sea stars move slowly across the seabed using many small tube feet, and many have five arms, though some species have more. Feather stars are related to sea stars and have many thin, feathery arms. They cling to high points of the reef, but they can crawl to a new perch, and some species can swim short distances. Sea cucumbers are soft, elongated animals that lie along the bottom, and some release sticky defensive threads when stressed, so give them room. Sea urchins have round bodies covered with long spines that can break off in skin and cause pain.

Location gives useful clues. Sponges and sea squirts favor hard surfaces such as rock walls, overhangs and wrecks, where moving water brings them food. Sea stars and sea cucumbers are more often found on sand, rubble and the bases of reefs, where they can move about and feed on the seabed. Anemones prefer rock and sand that receive a steady flow of water. A creature in an unexpected place is worth a second look, and checking its location against a regional guide often narrows the name quickly.

Nudibranchs, or sea slugs, are small and often brightly colored animals that crawl over rock and coral. Many store chemicals from their food that make them unpleasant to predators, so they should never be handled. A bright color on a sea slug is often a warning rather than an invitation.

Why these animals should be left untouched

Most of these animals grow slowly. A sponge or anemone pulled off a rock, or a sea star crushed underfoot, may take a long time to recover, and some do not recover at all. Touch can also harm you. Urchin spines, anemone stings and the irritating compounds in some sponges are common examples of contact that causes problems for divers and snorkelers.

Many of these animals also do useful work on the seabed. Sponges filter water, sea cucumbers process sediment, and sea urchins graze algae, so removing them can change what lives nearby.

The simplest habit is to look without touching and to keep fins, hands and equipment clear of the seabed. Stay above sand and rock in shallow water, where a stray fin can settle onto animals you cannot see. Take photographs from a distance, and check a field guide after the trip if you want the name. Leaving the seabed as you found it lets these animals grow and recover, which benefits the next swimmer as much as the animals themselves.