Ascidiacea

Lore & Background

Ascidians are rounded or cylindrical animals, ranging from about 0.5 to 10 cm in size. One end is firmly fixed to rock or coral, and the body wall is covered by a smooth, often rigid tunic composed of cellulose, proteins, and calcium salts. The tunic is living tissue with its own blood supply. The upper surface has two siphons; when removed from water, the animal violently expels water, giving it the common name 'sea squirt'. The body can be divided into pharyngeal, abdominal, and postabdominal regions, though many species lack the postabdomen or entire abdomen. They feed by taking in water through the oral siphon, which passes through mucus-covered gill slits into the atrium, then exits through the atrial siphon. The endostyle produces mucus that traps planktonic food particles. The heart is a curved muscular tube that alternates the direction of blood flow every three to four minutes. Blood contains four cell types: lymphocytes, phagocytic amoebocytes, nephrocytes, and morula cells, with some species having pigmented cells containing iron or vanadium.

Reader's Guide

Ascidiacea are significant as a paraphyletic class within Tunicata, illustrating the evolutionary transition from free-swimming larvae to sessile adults. Their larval form possesses a notochord and dorsal nerve tube, clearly indicating chordate affinities and providing insight into vertebrate origins. The presence of an endostyle, homologous to the vertebrate thyroid gland, further underscores their evolutionary importance. Some authors now include thaliaceans in Ascidiacea, making it monophyletic, though this remains a point of dispute. Their unique tunic, composed of cellulose—a material rare in animals—and their ability to accumulate metals like vanadium, make them subjects of biochemical and biomedical interest. Colonial species exhibit complex reproductive strategies, including mixed modes, and their shared cloacal systems demonstrate advanced social organization.

Did You Know?

• The heart of sea squirts alternates the direction of blood flow every three to four minutes.
• The tunic is made of cellulose, a polysaccharide, and is living tissue with its own blood supply.
• Some ascidians have blood cells containing vanadium, giving the blood a green color.
• The endostyle in the pharynx is homologous to the thyroid gland of vertebrates.