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Bluebell tunicates. [1]
Strawberry tunicates. [2]

Forms of Tunicates

There are three forms of tunicates:

  • Solitary Tunicates – some tunicates live alone, not in a colony with other tunicates
  • Colonial Tunicates – tunicates are able to bud off individual tunicates and grow colonies. These colonies are similar in resemblance to encrusting marine animals like sponges, but upon closer look they have the same structures as solitary tunicates.
  • Drifting Tunicates – Some tunicates never attach to an object or surface, but live as planktonic drifters. They use their siphons for movement.

There are over 2000 species of Ascidians along with 72 species of Thaliaceans and 20 Appendicularians. They are often referred to as " sea squirts" because when touched they tense up and then squirt water from their siphons.

The most common types of tunicates found in the Caribbean, in areas around St.John, are the bluebell tunicate and the strawberry tunicate.

Comparison of Tunicates and Vertebrates

Tunicates belong to the phylum Urochordata, which is closely related to phylum Chordata that includes all vertebrates.

Tunicates and vertebrates both have the features: notochord, dorsal hollow, and nerve cord. These characteristics are only seen in embryonic development of tunicates. As tunicates are sedentary, they lose some parts of their skeletal anatomy. This allows them to attach to their chosen spot, this will then lead them to grow the necessary structures to live an adult life. [4]

Tunicates during their larval stage also contain gravity and light-sensitive sensory vesicles, this gives them the ability to orient themselves as they swim. However, later in life the organism reabsorbs the tail, notochord and dorsal nerve cord as a source of food.

Anatomy of Tunicates

Basic anatomy of a tunicate.[4]
  • Tunic- tissue that can be gelatinous or hard, it provides protection from predators and it also contains cellulose most of the time.
  • Heart- pumps blood in a different manner, for a few minutes it will pump in one direction and then pump in reverse for another few minutes.
  • Inhalant Siphon- This opening allows the tunicate to feed as well as take in water.
  • Exhalant Siphon- the second opening allows the tunicate, after eating and filtering what came in through the first siphon, to expel any waste or water.
  • Pharynx- the equivalent of a mouth. The pharynx is inside a larger compartment called the atrium. It is porous to allow water to move out, it also connects the inhalant siphon to the digestive system.

The heart of a tunicate is complex in the way it pumps blood; blood flow will only move in one direction at a time. Movement of the blood begins at the anterior region of the heart ad then moves to the pharyngeal vessels then to the mid dorsal vessel where digestion occurs and it ends up on the dorsal part of the heart. After the blood build up, pressure also builds up and this allows for blood flow to continue in the opposite direction.[4]

Reproduction and Life Cycle

Tunicates have a brief larval stage in which their main goal is to find a place to live out their adult life. This happens in a matter of hours. During the larval stage, the tunicates does not have a mouth and doesn't feed. After several days, the larval will attach to a surface using anterior adhesive papillae. As the larva metamorphoses into an adult, the larval tail is resorbed, providing food to the adult tunicate. Free-swimming tunicates metamorphose without attaching to a surface. [5]

For the most part, tunicates are hermaphrodites, they have an ovary and a single testis. They can reproduce both sexually and asexually by budding. Tunicates, like most hermaphroditic animals, try to avoid self-fertilizing. Fertilization often occurs in the water surrounding the tunicates, and development occurs in both the surrounding water and in the female tunicates. To avoid self-fertilization the tunicate makes sure to not produce a sperm or an egg a the same time. Embryos stat in the mothers atrium until the larval has developed. During the larval stage the organism looks like a tadpole. The whole body is mostly made up of a tail, which is where the notochord and nerve cord are located. Michael T. Ghiselin

Diet and Digestive System

Tunicates mainly eat phytoplankton, zooplankton, detritus and other small organisms in the water, these organisms are filtered through the tunicate. They have cilia in the pharynx and they also produce mucus. Their food gets stuck in the mucus and with the help of beating cilia the mucus moves toward the digestive system. There are also predatory tunicates, which eat small creatures that get pulled through their siphon and get stuck on their mucous.[6]

Distribution and Abundance

Tunicates can be found in ocean waters from polar regions to the tropics. They can be found floating in the ocean water or attached to rocks, docks, ship hulls, and other hard surfaces, usually in the pelagic zone of the water. [7] They can become invasive because colonies can settle on top of other stationary creatures, push them off and compete for food.


Tunicates do not have many predators because the tunic that surrounds does not have a pleasant taste. Thaliaceans, a type of tunicate, are mostly transparent and this allows them to escape predators as they are also free-swiming, and are difficult to detect. Some tunicates are also bioluminescent which distracts the predator. [8]

Tunicate immune systems can be compromised due to antifouling agents used on boats to prevent organisms such as this to attach, this is lethal to tunicates because they can not protect themselves. They are susceptible to toxins such as tributyltin, creosote and copper.[9]

Evolution of Tunicates

There are three classes of tunicates: Ascidiacea, Appendicularia and Thaliacea.

The class Ascidiacea are the one most often referred to as sea squirts, they are also mostly benthic.The mobile and pelagic groups are under the class Thaliacea and the Appendicularia are the ones, in structure during their adult stage,that look most similar to a tadpole larvae. [10] Tunicates are seen in a variety of colors. It is unusual for animals to contain cellulose so it is thought that ancestral tunicates were able to gain this characteristic through gene transfer from a bacteria. Through time they also developed an asexual and sexual way of reproduction which allows them to reproduce in different ways depending on their environments and whether or not they live in colonies or alone.Tunicates have evolved much faster then relatives. Gene sequencing supports the idea that tunicates are a sister group of vertebrates. During their tadpole like larval stage they show features that can be seen other vertebrates, this allows tunicates to be model organisms for scientist to understand the development of vertebrates and immunity of vertebrates. Tunicates are not able to leave any fossils because of their outer tunic that is too soft because of this reason it is hard to understand how vertebrate evolution occurred in tunicates, there is no ancestor to compare any of the characteristics that are present today in tunicates and other invertebrates. There is a debate as to how the ancestor would have looked and how it managed to progress into the characteristics that are visible today. [11]


  1. http://marinebio.org/species.asp?id=2069
  2. http://www.snorkelstj.com/strawberry-tunicate.html
  3. http://a-z-animals.com/animals/sea-squirt/
  4. 4.0 4.1 4.2 "What's a Tunicate?" What's a Tunicate? N.p., n.d. Web. 21 Apr. 2016. [1]
  5. Ghiselin, Michael T., "tunicate". Encyclopædia Britannica. Encyclopædia Britannica Online Encyclopædia Britannica Inc., 2016. Web. 20 Apr. 2016. [2]
  6. Harris, Rob. "What Does a Tunicate Eat?" Animals. N.p., n.d. Web. 21 Apr. 2016. [3]
  7. "SERC - All in the Family: The Tunicates." SERC - All in the Family: The Tunicates. N.p., n.d. Web. 21 Apr. 2016. [4]
  8. "Result Filters." National Center for Biotechnology Information. U.S. National Library of Medicine, n.d. Web. 21 Apr. 2016. [5]
  9. "Tunicates." Tunicates. N.p., n.d. Web. 21 Apr. 2016. [<http://academics.smcvt.edu/dfacey/AquaticBiology/Coastal%20Pages/Tunicates.html>.]
  10. Ghiselin, Michael T., "tunicate". Encyclopædia Britannica. Encyclopædia Britannica Online Encyclopædia Britannica Inc., 2016. Web. 20 Apr. 2016. [6]
  11. Holland, LZ. "tunicates." CURRENT BIOLOGY 26.4 (2016): R146-52. Web.