Scientific names: Styela clava, didemnum vexillum, and ciona savignyi

What Is It?

Tunicate, commonly called a sea-squirt, is a primitive marine animal that spends most of its life attached to docks, rocks, or the undersides of boats. A tunicate is built like a barrel. The name “tunicate” comes from the firm, but flexible body covering, called a tunic. Most tunicates live with their posteriors, or lower ends of the barrel, attached to fixed objects, and two openings, or siphons, projecting from the other. Tunicates eat plankton and live by drawing seawater through their bodies.

Is It Here Yet?

Yes. While our marine waters are home to several native species of tunicates, there are three invasive tunicate species present: ciona savignyi, styela clava, and didenmun. Ciona savignyi is present throughout Hood Canal and the Puget Sound, from Olympia to Whidbey Island. Huge infestations appeared and suddenly disappeared in southern Hood Canal. Styela clava is at Pleasant Harbor Marina in Hood Canal, and at Blaine, Semiahmoo, and Elliott Bay Marinas. A few were found among heavy infestations of didemnum on Maury Island, but the Washington Department of Fish and Wildlife eradicated them. A local diver sited a single styela clava just south of Maury Island. Didemnum is present in various densities throughout Puget Sound with extremely heavy infestations in Vancouver, British Columbia.

Why Should I Care?

Didemnum and styela clava have been invading coastal waters in several countries, where they spread rapidly. Mats of these species can smother other sea life, and nothing eats them because they are toxic to other species. In some areas of the country, invasive tunicates are becoming a major threat to aquaculture operations because they compete with native filter feeders such as clams, mussels, and oysters.

How Can We Stop It?

Although didemnum mats can break apart and spread by currents or storms, the primary way that tunicates spread is through the ballast water of ships or by attaching themselves to boats that are moved from one water body to another. Clean, drain, and dry all watercraft and equipment before using them in another water body to prevent spreading.

The Washington Department of Fish and Wildlife works with commercial divers to remove tunicates from heavily-infested marinas. Various marine resource areas and recreational dive groups also have undertaken control measures. You also can participate with volunteer divers surveys.

What Are Its Characteristics?


  • Color can vary from cream to white, yellow, or tan.
  • Dense, blob-like (look like pancake batter) colonies.
  • May form long hanging, rope-like lobes, like dreadlocks or beard-like colonies on hard substrates.
  • May form flat, undulating mats with bumps or small lobes on the sea floor.

Styela clava

  • May reach densities of 500-1,500 per square meter. The juveniles do not move far before settling out of the water and becoming attached.
  • Club-shaped with two siphons; anchored to substrate by a stalk.
  • Tough, leathery, bumpy exterior; often covered with other organisms.
  • Up to 8 inches with stalk about a third of its total length.
  • Found in shallow, sub-tidal waters on hard surfaces.

Ciona savignyi

  • Whitish to almost clear. It can be so transparent that you can see its organs.
  • It is generally tube-shaped, has two siphons of unequal length that are slightly scalloped at the edges with small yellowish to orange flecks forming at the rim.
  • It is usually found in depths of 40-75 feet, but also may be found undercover in protected waters on hanging aquaculture rafts and in marinas under docks, pilings, boat hulls, and other structures.

Additional Photographs