Ascidians (Ascidiacea) are a class belonging to the subphylum of tunicates, with about 2,300 species, all of which are found in marine habitats and are characterized by being species fixed to rocky or hard substrates, that is to say they are sessile species. They are distributed throughout the world’s seas and oceans, generally in coastal waters.
The body of the ascidians is characterized by being covered with an external protective layer of tunicine. One end of the body is fixed to the rocky bottom. At the opposite end, they have two siphons, one that acts as a mouth and through which water enters, and a second siphon located more laterally and called atrial siphon through which the ingested water is expelled. Through this water circuit, and by means of a ciliated pharynx with numerous gill slits forming an elaborate filtering basket, ascidians are able to capture suspended particles from the water. Because of the way they feed, filtering significant amounts of water, they have the ability to accumulate pollutants, bacteria, viruses or toxins, if these are present in the water. This makes ascidians excellent bioindicators of the quality of the water in which they live.
There are both solitary and colonial species. Solitary species are usually spherical or cylindrical in shape. Colonial species, all the individuals that form the colony, called zooids, are usually grouped under a common cover, and the zooids may or may not be interconnected by basal stolons. The sizes of the ascidians are usually between 5 mm and 10 cm. The different species that make up the class of ascidians are classified in 3 orders (Aplousobranchia, Phlebobranchia and Stolidobranchia), 25 families and 176 different genera.
Mostly hermaphrodites, we find species with both sexual reproduction (predominant in solitary species) and asexual reproduction (predominant in colonial species). Sexual reproduction, as in many sessile species, consists of the release of gametes into the water column, with fertilization occurring externally.
