Cionidae

Cionidae is a family of tunicates, commonly called sea squirts, within the subphylum Urochordata and the class Ascidiacea. The best-known members belong to the genus Ciona, including species such as Ciona intestinalis and Ciona savignyi; taxonomic boundaries have varied, and some classifications include a small number of additional genera.

Members of Cionidae are sessile, filter-feeding animals that live attached to hard substrates in coastal waters, from intertidal zones to deeper habitats. Adults are typically vase- or barrel-shaped and possess two siphons—the incurrent and excurrent siphon—through which seawater enters and exits the body. The exterior is covered by a tunic, a protective layer composed largely of cellulose-like polysaccharide tunicin.

During their life cycle, Cionidae undergo a free-swimming larval stage that bears a notochord and dorsal nerve cord, reflecting their chordate ancestry. The larva settles, undergoes metamorphosis, and becomes a sedentary adult. Reproduction is usually sexual, with eggs and sperm released into the surrounding water; many species are hermaphroditic.

Ecology and distribution: Cionidae tunicates occur in temperate and boreal oceans worldwide, frequently on natural substrates or artificial structures such as docks and ships. They play a role in their ecosystems as filter feeders and can be indicators of coastal water quality.

Significance: Ciona species are important model organisms in developmental biology and genomics. Their relatively simple body plan and rapid life cycles have facilitated studies of chordate evolution, gene regulation, and developmental processes. The genomes of Ciona intestinalis and Ciona savignyi have been sequenced and analyzed.

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