Sea Squirt (class Ascidiacea)

InvertebrateTunicateFilter feeder

Overview

Sea squirts (class Ascidiacea) are sac-like marine animals — a kind of tunicate — that live attached to the seabed, rocks, reefs, jetties, and shells. The adult looks like a small, often colourful bag or barrel with two openings (siphons), and it spends its life filtering food from seawater. The name “sea squirt” comes from the way many of them shoot out a jet of water when touched or lifted from the sea.

They may look like simple, sponge-like blobs, but sea squirts hold a big evolutionary surprise: tunicates are among the closest living invertebrate relatives of the vertebrates — the group that includes fish, amphibians, reptiles, birds, and mammals, us included.

Note: sea squirts are a large, varied group, both solitary and colonial; details here cover them broadly. Treat general statements as approximate and verify against authoritative sources.

Habitat & Range

Sea squirts live in oceans all over the world, from shallow tide pools and rocky shores to coral reefs and the deep sea. They attach to hard surfaces — rocks, reefs, shells, pilings, boat hulls, and the seabed — and many grow in clusters or spreading colonies. Some live in cold polar seas, others on tropical reefs.

Diet

Sea squirts are filter feeders. They draw seawater in through one siphon, pass it through a basket-like internal filter that traps tiny particles — plankton, bacteria, and bits of organic matter — and then expel the filtered water out the other siphon. By straining huge volumes of water, sea squirts help keep it clean and recycle nutrients, acting as living water filters of the sea.

Behavior

The adult sea squirt is sessile — fixed in place — and does little but pump and filter water, contracting to squirt water out if disturbed. The real twist is in its life history: the larva is a tiny, tadpole-like swimmer with a nerve cord and a stiffening rod (a notochord) in its tail, the same basic features that mark the ancestors of backboned animals. After a short swim, the larva settles head-down, attaches, and reabsorbs its tail and much of its “brain,” transforming into the stationary adult. This larva is the key clue to the tunicates' close kinship with vertebrates.

Human Interaction & Conservation

Sea squirts are important members of marine communities and useful natural water filters, and they are of great interest to scientists studying the evolution of vertebrates and for compounds in their bodies explored in medicine. Some are eaten in parts of the world, while certain species are problematic foulers of boats and aquaculture or invasive when spread by shipping. They are harmless to people. Consult authoritative sources for details.

More photos of the sea squirt