Found clinging to submerged surfaces in coastal waters, Ciona is a genus of marine animal known as a sea squirt or tunicate. Its simple, vase-like adult body gives little indication of its complex biology and its unexpected connection to vertebrate life. Sometimes called the vase tunicate, this organism is a translucent, tubular structure that can be greenish or yellowish-white. This filter-feeder belongs to a group called tunicates and offers insights into the origins of more complex animals.
Physical Characteristics and Life Cycle
The adult Ciona has an elongated body that can grow up to 120 millimeters high and is permanently attached to a substrate. Its soft body is protected by a flexible, cellulose-like external layer called a “tunic.” This sessile creature is a living water pump, drawing in water through an incurrent siphon and expelling it through an excurrent siphon. The incurrent siphon is ringed by eight distinct lobes, while the excurrent siphon has six, often marked with small red or orange spots. As water passes through its gill basket, particles of detritus and phytoplankton are trapped in mucus and transported to its U-shaped gut.
The life history of Ciona is defined by a transformation from a mobile larva to a stationary adult. It begins life as a free-swimming larva that bears a striking resemblance to a tadpole. This larval stage possesses several chordate features, including a supportive notochord, a dorsal hollow nerve cord, and pharyngeal slits. Lasting for about 36 hours, it is also equipped with a muscular tail and adhesive papillae at its front end. After this mobile phase, the larva finds a hard surface, attaches itself, and undergoes metamorphosis, absorbing its tail, notochord, and most of the larval nervous system as it reorganizes into the adult form.
Evolutionary Relationship to Humans
Despite the simplicity of its adult form, Ciona’s larval stage reveals its close relationship to vertebrates, including humans. Sea squirts are classified within the phylum Chordata, the same group that encompasses all animals with backbones, and genetic studies affirm that tunicates are the closest invertebrate relatives of vertebrates. The larva’s notochord, dorsal hollow nerve cord, and pharyngeal slits are homologous to structures found in the embryonic stages of all vertebrates. These features are lost during metamorphosis, but their presence in the larva underscores a common ancestry dating back hundreds of millions of years.
Role as a Model Organism
Ciona intestinalis is used as a model organism to understand biological processes because of its practical and genetic advantages. Researchers value its rapid and predictable embryonic development; a fertilized egg becomes a swimming larva in less than 24 hours at room temperature, allowing for quick observation of developmental stages. The transparency of the embryos provides a clear window into cellular processes. The Ciona genome is also small, containing about 16,000 genes. Its genome has not undergone the large-scale duplication events seen in the vertebrate lineage, meaning many genes exist as single copies, which simplifies studying their function. About 80% of its gene families are also found in humans, making it a tool for deciphering the origins of gene networks that regulate vertebrate development and immunity.
Ecological and Economic Impacts
Outside of the laboratory, Ciona is a successful invasive species. It thrives in coastal areas and harbors worldwide, spreading to new regions by attaching to the hulls of ships. In many locations, it forms dense aggregations on submerged artificial structures, including docks, pilings, and aquaculture equipment, a phenomenon known as biofouling. The economic consequences of these invasions can be significant for the shellfish aquaculture industry. Colonies of the tunicate can smother and compete with farmed mussels and oysters for food, reducing their growth and leading to crop loss. The weight of the tunicates can also damage and sink aquaculture gear, requiring resources for removal, as seen in Prince Edward Island, Canada, where invasions have caused economic damage to the mussel farming industry.