Can Sea Anemones Move? Gliding, Swimming and Hitchhiking

Gliding on the pedal disc

Most sea anemones spend their lives attached to rock, shells or other hard surfaces by a muscular, sticky foot called the pedal disc. They often stay in the same spot for weeks or months, which is one reason people mistake them for plants (see Is a sea anemone a plant or an animal?).

When an anemone does decide to move, it creeps. A wave of muscle contraction runs along the pedal disc from back to front. The front edge loosens its grip and slides forward, then reattaches, much like the foot of a snail. The movement is so slow that it is almost impossible to see with the naked eye, and it usually shows up only in time-lapse photography.

Anemones can also stretch, shrink, bend and twist their bodies quickly, pulling in their tentacles in a moment when touched. They have sheets of muscle fibers running lengthwise, around and diagonally through the column, and they use seawater held inside their body as a skeleton to push against.

Why do sea anemones move?

An animal that can catch food without moving still has good reasons to change places:

• Better conditions. An anemone may creep toward a spot with more food, better water flow or, for species that host algae, more light.
• Escaping danger. Sea stars and sea slugs eat anemones, and some anemones flee when they detect them.
• Avoiding burial or exposure. Shifting sand, strong waves and low tides can make a spot unlivable.
• Fights with neighbors. Some anemones, such as the beadlet anemone of European shores, sting rivals with special structures packed with stinging cells, and the loser may move away.

Letting go: drifting, tumbling and rolling

Many sea anemones can release their grip completely and let currents carry them to a new spot, then reattach when they land somewhere suitable. Some species inflate their bodies before letting go, which helps them float or roll.

The sea onion (Paranthus rapiformis), which lives buried in shallow mud flats, has an unusual trick. If strong currents wash it out of its burrow, it contracts into a glistening, pearly ball that rolls along the seabed until it can dig in again.

Anemones are not drifters by lifestyle, though. Unlike the Portuguese man o’ war, a floating colony that spends its whole life at the sea surface, a sea anemone uses drifting only to get from one attached life to the next.

Anemones that swim

A few sea anemones can swim, usually to escape predators. The best-studied is Stomphia coccinea, a small reddish anemone of the North Atlantic, North Pacific and Arctic Ocean. When it is touched by certain predators, such as the leather star (Dermasterias imbricata) or the sea slug Aeolidia papillosa, it detaches from its rock or shell and swims by repeatedly bending its column back and forth. Chemicals from the leather star alone can set off the response, and researchers have traced the swimming to a slow-conducting signal system in the anemone’s tissues. After escaping, it settles and attaches again.

Gonactinia prolifera can both walk and swim. It swims by beating its tentacles together in rapid strokes, like oars.

Walking anemones

Some anemones move in steps rather than gliding. Gonactinia prolifera walks by attaching its tentacles to the surface ahead and pulling its base forward, in looping steps like a caterpillar.

The walking anemone (Preactis millardae) of South Africa, found only around the Cape Peninsula, creeps along rather like a leech. It is a predator of sea fans, which it eats down to their skeletons, and its ability to move lets it travel from one meal to the next.

Hitchhikers on hermit crabs

Some anemones solve the transport problem by riding on other animals. The hermit anemone (Calliactis parasitica) of European waters usually lives on snail shells inhabited by hermit crabs, often several anemones on one shell. In British waters its usual partner is the common hermit crab. The anemone seems to start the relationship itself, performing a complex series of movements to climb onto the shell while the crab stays still.

Both partners gain. The crab is protected by the anemone’s stinging cells, and octopuses have been seen avoiding shells that carry these anemones. The anemone gets a free ride and catches food stirred up by the crab. Some hermit crabs even transfer their anemones to a new, larger shell when they move house.

The cloak anemone (Adamsia palliata) goes further. It wraps its base completely around the hermit crab’s shell, and as it grows it adds a tough membrane that extends the shell, so the crab can keep the same home for longer. In the tropics, small boxer crabs (Lybia) carry a tiny anemone in each claw, waving them at predators like stinging pom-poms and using them to gather food.

Do clownfish anemones move?

Large tropical host anemones, such as the magnificent sea anemone, usually stay in one place for long periods, which is good news for the clownfish that depend on them. They can still creep or detach when conditions change. Whether an anemone stays or goes, it still has to feed, which it does with its stinging tentacles, as described in How do sea anemones eat?