didemnid

Didemnids are a family of ascidians, commonly known as sea squirts or tunicates. They belong to the order Aplousobranchia and are a diverse group found in marine environments worldwide, particularly in tropical and subtropical waters. These colonial filter feeders are characterized by their tunic, a tough, leathery outer covering that encases the individual zooids. A distinctive feature of many didemnid species is their ability to incorporate foreign materials into their tunic, such as sand grains, spicules from other organisms, or even fragments of other sessile invertebrates. This incorporation can serve various purposes, including defense against predators, camouflage, or structural support.

Didemnids exhibit a range of colony forms, from encrusting sheets to more complex, lobed structures. The individual zooids within a colony are small and typically share a common cloacal aperture through which filtered water is expelled. Reproduction in didemnids is primarily sexual, with the release of gametes into the water column, followed by a planktonic larval stage. Some species also reproduce asexually through budding. They play a role in marine ecosystems by filtering water and serving as a food source for some organisms. Certain didemnid species are also known to host symbiotic algae, such as cyanobacteria, within their tissues, which can contribute to their nutritional needs.