All About Tunicates

A photograph of a Gold-Mouth Sea Squirt (Polycarpa aurata)

What Are Tunicates?

Tunicates (Tunicata) are marine invertebrates. If you are a vertebrate, these creatures are your closest invertebrate relatives! Both tunicates and vertebrates have a notochord in embryo, a rod-like structure that is eventually replaced by secondary structures. In humans, all rememnants of the notochord are usually replaced by cartilaginous matter at the age of four. The name "Tunicate" comes from the tunic-like covering encasing Tunicates that acts as an exoskeleton.

Many tunicates are filter feeders, consuming plankton for sustenance. They siphon water into their bodies and capture edible matter with a net made of mucus connected to their pharynx. The net is then rolled into the pharynx, dropping captured plankton down the esophagus and into the stomach to be digested.

Types of Tunicates

There are over 3,000+ species of tunicates so far described! They are categorized into three classes:

  1. Ascidiacea: The largest collection tunicates, Ascidians (also known as Sea Squirts) are unique in that they become sessile after their larval phase. Ascidian larvae are born with the ability to swim using a long tail, almost resembling a tadpole.

    A diagram of a sea squit's larval stage
    Soon after birth they will latch onto a hard surface and begin metamorphosing into their adult stage. Once attached to a secure surface, the Ascidian's pharynx grows in size and envelops the interior body. Additionally, some of its larval stage body parts are digested.

  2. Thaliacea: Consisting of Salps, Pyrosomes, and Doliolids, these tunicates are pelagic free-floaters. They can either be solitary (a single organism) or colonial (many organisms working together as a system).

    A Pyrosoma atlanticum specimen, a colonial tunicate A microscopic Doliolum species of Tunicate


  3. Appendicularia: Known as Appendicularians, and more commonly as Larvaceans, these tunicates retain their notochord and tadpole-like shape into adulthood. The smallest of tunicates, these species range from 2 millimeters to 8 millimeters (although the Giant Larvaceans, also known as Bathochordaeus, can reach between 3 to 10 centimeters in size).

    a microscopic Fritillariidae species of tunicate

Educational Videos

Deep Sea Marine's YouTube videos on different tunicate classes are an excellent introduction to learning about these amazing creatures!




Further Resources

If you would like to learn more about Tunicates, check out some of the resources listed below:


  • Bone, Q (ed.), The Biology of Pelagic Tunicates (Oxford, 1998; online edn, Oxford Academic, 31 Oct. 2023), https://doi.org/10.1093/oso/9780198540243.001.0001.

  • Cole, Linda. (2018, June). Tunicates-Not So Spineless Invertebrates. Smithisonian Ocean.

  • Hirose, E., Nakashima, K., & Nishino, A. (2011). Is there intracellular cellulose in the appendicularian tail epidermis? A tale of the adult tail of an invertebrate chordate. Communicative & integrative biology, 4(6), 768–771. https://doi.org/10.4161/cib.17757

  • Holland, Linda Z. Tunicates. Current Biology, Volume 26, Issue 4, R146 - R152

  • Lemaire, Patrick et al. Ascidians and the Plasticity of the Chordate Developmental Program. Current Biology, Volume 18, Issue 14, R620 - R631

  • Lemaire, P., & Piette, J. (2015). Tunicates: exploring the sea shores and roaming the open ocean. A tribute to Thomas Huxley. Open biology, 5(6), 150053. https://doi.org/10.1098/rsob.150053

  • Wikipedia contributors. (2024, September 9). Tunicate. Wikipedia, The Free Encyclopedia. https://en.wikipedia.org/w/index.php?title=Tunicate&oldid=1244808141