Invertebrates
Sea Squirt
Ascidiacea · also called sea squirts, tunicates
A sea squirt starts life as a tiny, free-swimming, tadpole-like larva with a simple nerve cord and a flexible rod running down its back — features it shares, distantly, with every animal that has a spine, humans included. Then it finds a rock, attaches itself permanently, and does something almost no other animal does: it digests its own brain and nervous system, because a life spent filtering seawater through two siphons has no more use for them.
Overview
Sea squirts, class Ascidiacea, are filter-feeding marine animals found on hard surfaces in coastal waters worldwide. Adults draw water through an incurrent siphon and expel it through an excurrent siphon after straining out plankton — a squeeze reaction to disturbance that gives the animal its name.
A surprising relative, briefly
The free-swimming larva’s notochord and simple nerve cord place sea squirts among the closest invertebrate relatives of vertebrates, a relationship visible only before metamorphosis reabsorbs those very structures. Some species live solitarily; others form large shared colonies, and several have become successful, sometimes troublesome, hitchhikers on ship hulls worldwide.
Status
Least Concern — abundant and widespread, with several species thriving well enough as introduced hitchhikers to become a genuine nuisance for shipping and aquaculture.
Sea Squirt: common questions
Why are sea squirts considered close relatives of vertebrates?
Sea squirts belong to the subphylum Tunicata, which sits within the larger group Chordata, the same broad category that includes all vertebrate animals, based specifically on the free-swimming larval stage, which possesses a flexible, rod-like notochord and a simple dorsal nerve cord, both hallmark chordate features shared with early vertebrate embryos. Although the adult sea squirt looks nothing like a fish, reptile, or mammal, this shared larval body plan is strong evidence of a genuine, if distant, evolutionary relationship to backboned animals.
Is it true that a sea squirt eats its own brain?
In a simplified but broadly accurate sense, yes. Once a free-swimming sea squirt larva locates a suitable surface and permanently attaches itself, it undergoes a dramatic internal transformation in which it reabsorbs its swimming tail, notochord, and much of its simple larval nerve cord and sensory structures, since these are no longer needed by a stationary, filter-feeding adult. This process is often popularly described as the sea squirt digesting its own brain, and while its larval nervous system is far simpler than a vertebrate brain, the description captures the striking real biological event fairly well.
Why does a sea squirt squirt water when you touch it?
A sea squirt's simple body is built around two openings, an incurrent siphon that draws seawater in and an excurrent siphon that expels it after the animal has filtered out food particles, and when disturbed or squeezed, the animal's muscular body wall can contract sharply, forcibly expelling a jet of water from these siphons. This defensive squirting reaction, which can occur when a sea squirt is picked up, poked, or exposed at low tide, is the direct source of the animal's common name.
Are sea squirts a problem for ships and harbours?
Yes, several sea squirt species have become notably successful and problematic marine invaders, spreading well beyond their native range by attaching as larvae or adults to ship hulls, aquaculture equipment, and other submerged infrastructure, then establishing dense new colonies in their introduced range. In some regions, invasive sea squirts have caused significant fouling problems, clogging aquaculture equipment, adding drag and weight to ship hulls, and sometimes outcompeting native filter-feeding species for space and food.