Chordate Zoology - textbook - Y. V. Tsaryk - 2013

Chapter 2. SUBPHYLUM TUNICATA, or UROCHORDATA, TUNICATA, seu UROCHORDATA

2.2. DISTRIBUTION AND LIFESTYLE

Some tunicates live on the seabed, while others inhabit the Water Column. About half of the known species are colonial. Sessile forms attach themselves firmly to rocks or other hard substrates by their tunic. Sometimes, the body may be elevated above the bottom on a slender stalk. This body form is characteristic of certain deep-sea ascidians.

In the tropics, tunicates are ten times more abundant than in temperate and polar latitudes, and they exhibit extraordinary diversity. At the same time, larger specimens occur in cold seas, where their biomass per unit area is also higher. The maximum depth at which the ascidian Situla pelliculosa has been found is 8 340 m, while the optimal depth is down to 200 m. Tunicates do not inhabit brackish areas of seas and oceans. Most species require normal oceanic salinity (35%). They are capable of forming large aggregations. Some ascidians can cause damage to water supply systems by colonizing pipes; their dead bodies can clog filtration systems, etc. Conversely, their ability to form aggregations can be utilized for human benefit. It is known that ascidians accumulate vanadium, which can account for up to 1% of the ash weight of an individual, or 0,65% of its dry body weight.

This value is ten times higher than the concentration of vanadium in seawater. Ascidians also accumulate tantalum, niobium, and titanium in their bodies. The tunic of ascidians contains tunicin, which includes Cellulose (2-3 mg/cm³). In certain areas of the sea, ascidian biomass can reach up to 140 kg/m², meaning that 'marine farming' could be practiced to extract vanadium, cellulose, etc.

Pelagic tunicates—appendicularians, pyrosomes, and salps—live mainly in tropical waters. However, due to warm Atlantic currents, salps also appear in the North Sea. Only three species penetrate these waters: Salpa fusiformis, Ihlea asymmetrica, and Thalia democratica.

Two species of salps have been found off the coast of Antarctica: Salpa thompsoni and Salpa gerlachei. Appendicularians are more common in cold waters; in particular, ten species have been found in the seas of the Arctic Ocean. Pelagic tunicates live at salinity levels of 34-36% at depths of up to 200 m. Some species occur at depths exceeding 1 000 m, but they have not been found in waters with a salinity of 40% or higher.