What are Tunicates?

Tunicates are a common subphylum of marine sac-like filter feeders found commonly in all the oceans of the world. Though usually found attached to the sea floor, the most familiar tunicates are free floating, including pyrosomes (bioluminescent tunicates), salps (which live in long linear colonies), and doliolids (tunicates with large siphons). Along with countless other organisms, tunicates make up the plankton. They are also called "sea squirts" or "sea pork" and come in a variety of colors, especially among the benthic (bottom-dwelling) species.

Though the simple barrel and sac-like bodies of tunicates make them reminiscent of simple animals like sponges or jellyfish, tunicates are actually more closely related to the vertebrates, including human beings. This is evidenced by their larvae, bilateral tadpole-like animals with a simple nerve cord. Based on this, tunicates are classified as chordates, the phylum that contains all vertebrates, in subphylum Urochordata.

Many paleontologists consider tunicates a window into what the earliest chordates may have looked like. Chordates would have branched off from our closest relatives, the acorn worms, sometime in the Late Ediacaran or Early Cambrian period (about (542 million years ago). However, modern tunicates barely resemble acorn worms, suggesting there may have been a number of intermediate stages. Because all these animals are soft-bodied, however, they have a very poor fossil record. Fossilized tunicates often leave behind only indirect signs, such as imprints left by their holdfasts, stalks of tissue they use to attach themselves to surfaces.

Tunicates are so named for their tunic, a tough fleshy shield that surrounds the barrel-shaped body of the animal. Prior to becoming adult tunicates, larval tunicates float in the water column, building filter-equipped protein "houses" (tests) for themselves that help concentrate food particles prior to their being drawn into the pharyngeal slits. As the larva grows and the filters on the test become clogged, they are discarded. The larva are the only tunicates that are able to discard their tunics in this fashion. Though these tests are very small, the number of tunicate larva in the oceans is so great that discarded tests make up a substantial portion of all carbon that descends into the ocean's deep.

Tunicates, also known as sea squirts, are a group of marine invertebrates that belong to the subphylum Tunicata. They are most commonly found attached to various surfaces in the ocean, from shallow waters to the deep sea. Tunicates are characterized by their tough outer covering, or 'tunic,' which is made of a cellulose-like substance called tunicin.

Tunicates are filter feeders; they draw in water through an incurrent siphon, filtering out plankton and other small particles for sustenance. Their diet primarily consists of phytoplankton and zooplankton. After extracting nutrients, they expel the filtered water through an excurrent siphon, which also helps to keep them anchored to their substrate.

Surprisingly, tunicates are closely related to vertebrates, sharing a common ancestor. This relationship is evident during their larval stage, which possesses a notochord and a dorsal nerve cord, features characteristic of chordates. According to evolutionary studies, tunicates diverged from the lineage that led to fish and ultimately to humans.

Tunicates play a crucial role in marine ecosystems. They are an important food source for a variety of predators, including fish, sea stars, and crustaceans. Additionally, their filter-feeding activities help to maintain water clarity and quality, which is vital for the health of coral reefs and other marine habitats.

Yes, tunicates have the ability to reproduce both sexually and asexually. Sexual reproduction occurs when they release sperm and eggs into the water, leading to external fertilization. Some species can also reproduce asexually through budding, where a part of the tunicate grows into a clone of the original individual.

Tunicates are of significant interest in scientific research due to their unique evolutionary position and simple body plan. They are used as model organisms in developmental biology and genomics. Studies on tunicates have provided insights into the origins of vertebrates and the evolution of key biological processes, making them invaluable to our understanding of life's complexity.