Tunicates, or sea squirts (phylum Urochordata), are marine invertebrates often mistaken for sponges or plants due to their simple, sac-like appearance as adults. They are found in all of the world’s oceans, typically anchored to solid surfaces like rocks, docks, or ship hulls. Tunicates are classified within the phylum Chordata, the same group that includes vertebrates, making them much closer relatives to humans than to most other invertebrates. The name “tunicate” is derived from the tough, protective outer covering of their body.
Anatomy and Their Place in the Animal Kingdom
The adult tunicate has a barrel-shaped body encased in a protective layer called a tunic. This tunic is unique because it is composed largely of tunicin, a polysaccharide chemically similar to cellulose, a material usually associated with plants. The body features two distinct siphons that regulate water flow for feeding and respiration. Water is drawn in through the incurrent siphon into a large pharyngeal basket, where a mucus net traps plankton particles, before the filtered water is expelled through the excurrent siphon.
Tunicates maintain their classification in the phylum Chordata due to features present only in their larval stage (Subphylum Tunicata). While the sessile adult lacks a backbone, the transient larval form displays the defining characteristics of all chordates. This includes a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail, linking them directly to the evolutionary lineage of vertebrates.
The Tunicate Life Cycle: From Tadpole to Sessile Adult
The tunicate life cycle begins with a distinct, free-swimming larval stage, often called the “tadpole larva” due to its resemblance to a small amphibian tadpole. This motile stage is relatively short, typically lasting only a few hours to a few days, and its primary function is dispersal rather than feeding. The larva possesses a muscular tail, which allows it to swim and search for an appropriate location for its adult life.
Once the larva locates a suitable substrate, it attaches itself head-first using specialized adhesive papillae. This attachment triggers a rapid process of retrogressive metamorphosis. The larva entirely reabsorbs its tail, notochord, and most of its nervous system within a short period. This transformation changes a motile, neurologically complex organism into a stationary, simplified filter feeder.
During metamorphosis, the internal organs rotate, and the siphons reorient to support the sessile lifestyle. The pharyngeal filtering apparatus enlarges considerably as the dispersal-focused larval features are rapidly dismantled. This biological switch highlights the tunicate’s unique evolutionary position, possessing chordate traits only when movement and substrate selection are required.
Ecological Impact and Scientific Significance
Tunicates play a significant role in marine ecosystems as highly efficient filter feeders, pumping large volumes of seawater to strain out plankton and particulate matter. This action helps clarify the water column and contributes to nutrient cycling by consolidating organic matter into fecal pellets. Their ability to colonize surfaces quickly makes certain species a major concern in human industry, contributing to biofouling on structures like ship hulls, aquaculture cages, and underwater pipes.
Invasive tunicate species can rapidly cover natural substrates and man-made structures. They often outcompete native filter feeders, causing significant economic damage.
Conversely, their unique biology makes them invaluable in scientific research, particularly in developmental biology. Scientists study the tunicate embryo and larval form to gain insights into the early development and evolution of the chordate body plan.
Tunicates are prolific producers of marine natural products, leading to intense interest from the pharmaceutical industry. These organisms and their associated symbiotic microbes yield numerous bioactive compounds. Many of these compounds exhibit anti-tumor, anti-microbial, and anti-inflammatory properties, offering a source of novel molecules for new human therapies.