Tunicates are marine invertebrates that inhabit oceans worldwide. Despite their simple appearance, these organisms hold a surprising place within the animal kingdom as members of the Phylum Chordata. Their unique life cycles and diverse forms contribute to their ecological significance. About 3,000 species exist globally.
Defining Features and Place in the Animal Kingdom
Tunicates belong to the subphylum Tunicata, also known as Urochordata, within the Phylum Chordata. Their classification as chordates stems from specific characteristics during their larval stage. These include a flexible notochord for support, a dorsal hollow nerve cord, pharyngeal slits or pouches, and a post-anal tail. The larval form resembles a tiny tadpole, exhibiting these chordate features.
Upon metamorphosis, most tunicates transition into a sessile adult form, attaching to various underwater surfaces. During this transformation, they lose their notochord, nerve cord, and tail, while retaining the pharyngeal slits for feeding. Adult tunicates are filter feeders, drawing water in through an incurrent siphon and expelling it through an excurrent siphon. Their bodies are encased in a protective outer layer called a “tunic,” which gives them their name. This tunic is a flexible covering composed of tunicin, a cellulose-like carbohydrate material, an unusual animal substance.
Diverse Forms and Life Stages
Tunicates exhibit diverse forms and life strategies, categorized into three main classes. The most recognized are the Ascidiacea, commonly known as sea squirts. These are sessile as adults, appearing sac-like and found attached to rocks, coral, or ship hulls. Sea squirts use their incurrent and excurrent siphons to filter plankton and small particles from the water. Their life cycle involves a free-swimming, tadpole-like larva that settles and undergoes metamorphosis into the adult.
Another class is Thaliacea, which includes free-swimming, pelagic tunicates such as salps and doliolids. These organisms possess transparent, barrel-shaped bodies and propel themselves through the water by jet propulsion, expelling water from their siphons through muscular contractions. Salps can form long, chain-like colonies, reaching several meters, while doliolids exhibit complex solitary and colonial life cycles. They are filter feeders, using a mucous net to capture food as water passes.
The third class, Appendicularia, or larvaceans, are tiny, free-swimming tunicates that retain their tadpole-like characteristics throughout life. They build external mucus “houses” that can be larger than their bodies, up to several feet in diameter. These houses contain filters that allow larvaceans to capture microscopic particles from the water. When the filters become clogged or the animal is disturbed, the larvacean abandons its house and constructs a new one, a process occurring every few hours.
Ecological Contributions
Tunicates play a role in marine ecosystems through filter-feeding. By drawing in large volumes of seawater, they remove plankton, detritus, and suspended particles, contributing to water clarity and quality. A single tunicate can filter many liters of seawater daily, making them purifiers of aquatic environments. This helps maintain planktonic community balance.
Tunicates also participate in the marine food web, as consumers of microscopic organisms and a food source for other marine animals. Their waste products, especially discarded mucus houses and fecal pellets, contribute to nutrient cycling. These organic materials sink to deeper ocean layers, contributing to the biological carbon pump and transferring particulate carbon and nutrients to the deep sea.
Human Interactions
Tunicates interact with human activities through biofouling. Many sessile tunicate species, especially sea squirts, attach to submerged structures such as ship hulls, docks, and aquaculture equipment. This attachment can lead to economic impacts by increasing drag on vessels, requiring removal, and interfering with aquaculture operations. Some tunicate species are also invasive, spreading to new locations via ballast water, and competing with native filter feeders for food and space.
Tunicates are valuable in scientific research in developmental biology. Their chordate lineage, combined with simple body plans and transparent embryos, makes them models for studying chordate evolutionary development. Researchers use tunicates to investigate biological processes, including how developmental mechanisms contribute to evolutionary changes and genetic networks influence structure formation.