What's a Tunicate?

 Tunicates covering rope

 Tunicates, commonly called sea squirts, are a group of marine animals that spend most of their lives attached to docks, rocks or the undersides of boats. To most people they look like small, colored blobs. It often comes as a surprise to learn that they are actually more closely related to vertebrates like ourselves than to most other invertebrate animals.

Tunicates are part of the phylum Urochordata, closely related to the phylum Chordata that includes all vertebrates. Because of these close ties, many scientists are working hard to learn about their biochemistry, their developmental biology, and their genetic relationship to other invertebrate and vertebrate animals.

  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 the posterior, or lower end of the barrel attached firmly to a fixed object, and have two openings, or siphons, projecting from the other. Tunicates are plankton feeders. They live by drawing seawater through their bodies. Water enters the oral siphon, passes through a sieve-like structure, the branchial basket that traps food particles and oxygen, and is expelled through the atrial siphon.

Diagram of adult solitary tunicate

Are they really our cousins?
One clue that tunicates are related to vertebrates is found in the tunicate larva, or tadpole. It even looks like a tiny tadpole, and has a nerve cord down its back, similar to the nerve cord found inside the vertebrae of all vertebrates. The Cerebral Vesicle is equivalent to a vertebrate's brain. Sensory organs include an eyespot, to detect light, and an otolith, which helps the animal orient to the pull of gravity.

 Diagram of tunicate "tadpole" larva
The tail also has a semi-rigid rod called a notochord, which can be compared to the spine of true vertebrates.

Tunicate tadpoles mature extremely quickly, in a matter of just a few hours. Since the tadpoles do not feed at this stage of their lives, they have no mouths. Their sole job is to find a suitable place to live out their lives as adults. When ready to settle, a sticky secretion helps them attach head first to the spot they have chosen. They then reabsorb all the structures within their tail and recycle them to build new structures needed for their adult way of life.

Diagram of tunicate metamorphosis

Other tunicate forms

 The colonial tunicate, Botrylloides violaceus

The colonial tunicate, Distaplia occidentalis
 Some kinds of tunicates live alone, and are called solitary tunicates. Others, including the two forms shown here, have the ability to bud off additional individuals from the first to arrive, and these grow into colonies. At first glance, these colonial tunicates look much like other encrusting marine animals, such as sponges. If you look closer, you can see that they have the same structures as solitary tunicates, only much tinier.

Still other members of this group never attach to objects, but live out their entire lives as planktonic drifters. These include thaliaceans, strange gelatinous animals that use their siphons to jet-propel themselves gently through the water. The two photos immediately below were taken of the same animal. Pyrosoma atlanticum. The the image on the left was filmed with artificial light, while the one on the right shows the light, or bioluminescence, produced by the animal itself.

 The thaliacean, Pyrosoma atlanticum. a planktonic Urochordate

 Pyrosoma atlanticum showing bioluminescence

Another planktonic group of Urocordates includes the larvaceans. These animals live inside intricate mucous "houses" and retain their larval tail throughout their lives. This tail drives a gentle current of water through the house, propelling the organism through the water. The photograph below shows the organism, Oikopleura vanhoeffeni, inside its house, creating a current by movements of its tail.

 Oikopleura vanhoeffeni, a larvacean Urochordate

Acknowledgements to Dr. Euichi Hirose of University of Ruykyus, Japan, and to Dr. Alexander Bochdansky, Queens University, Canada, for the above pictures of thaliacean and larvacean Urochordates.

How will we recognize the non-native tunicate species, Ciona savignyi when it appears? What do we know about this species? For more information, go to C. savignyi.


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