Tunicate

Tunicate

A tunicate in its natural habitat

Tunicates, also known as urochordates, are a group of marine animals that are part of the subphylum Tunicata. They are notable for their sac-like body structure and are found in oceans worldwide. Tunicates are an important part of the marine ecosystem and are closely related to vertebrates.

Anatomy and Physiology

Bluebell tunicates

Tunicates have a simple body plan that includes a tough outer "tunic" made of a cellulose-like carbohydrate. This tunic protects the animal and gives the group its name. The body of a tunicate is divided into two main regions: the siphons and the pharynx.

The incurrent siphon draws water into the body, where it passes through the pharynx. The pharynx is lined with cilia and mucus, which trap food particles. The filtered water is then expelled through the excurrent siphon.

Life Cycle

Tunicates have a complex life cycle that includes both a free-swimming larval stage and a sessile adult stage. The larval stage resembles a tadpole and possesses a notochord, a dorsal nerve cord, and a post-anal tail, which are characteristic features of the chordates.

Upon settling, the larva undergoes metamorphosis into the adult form, losing its tail and notochord. The adult tunicate is typically sessile, attaching itself to a substrate.

Diversity

Botrylloides violaceus

Tunicates are divided into three main classes: Ascidiacea, Thaliacea, and Appendicularia.

Ascidiacea

Ascidians, or sea squirts, are the most common type of tunicate. They are sessile as adults and can be solitary or colonial. Colonial ascidians, such as Botrylloides violaceus, form large, colorful colonies.

Thaliacea

A colony of salps

Thaliaceans, including salps and pyrosomes, are free-floating and often form large colonies. Salps are barrel-shaped and move by contracting their bodies to pump water through their siphons.

Appendicularia

Oikopleura dioica

Appendicularians, such as Oikopleura dioica, retain their larval characteristics throughout their life. They are small, free-swimming, and build a "house" of mucus to filter food from the water.

Ecological Role

Tunicates play a crucial role in marine ecosystems as filter feeders. They help maintain water quality by removing plankton and other particles from the water column. Some tunicates, like pyrosomes, can form large bioluminescent colonies that are visible at night.

Evolutionary Significance

Appendicularia specimen

Tunicates are of great interest to evolutionary biologists because they are considered the closest living relatives of vertebrates. The study of tunicate development and genetics provides insights into the evolution of chordates.

Related Pages

• Chordate
• Vertebrate
• Marine biology
• Filter feeder