Tunicates are marine invertebrates of the subphylum Tunicata characterized by an outer exoskeleton made of tunicin (a type of cellulose). These animals possess dorsal nerve cords and notochords and are thus part of the phylum Chordata. Around 3,051 described species of Tunicates are divided into three classes: sea squirts (Ascidacea), thaliaceans (Thaliacea), and larvaceans (Appendicularia).
Since the subphylum was once called Urochordata, this group is also called Urochordates. Unlike other chordates, they have lost their myomeric segmentation, except for doliolids (an order of thaliaceans), which show segmentation in muscle bands.
Most tunicates live as colonies, comprising units called zooids formed by budding; however, some are completely solitary. While adult sea squirts are sessile and remain attached to some substrate, thaliaceans and larvaceans swim freely in the pelagic zone of the sea.
The largest tunicate is a stalked sea tulip, Pyura pachydermatina, which measures over 1 meter (3.3 ft), whereas the smallest, Minipera pedunculata, reaches only 0.02 inches (0.5 mm).
Tunicate colonies exhibit different physical forms depending on how the individual members (zooids) are integrated. While the individuals are distinctly separated (though linked by horizontal connections or stolons) in the simplest colonies, they are often clustered together and share a common base in others.
In most advanced colonies, the zooids are clubbed into a single structure surrounded by the tunics. They are organized into larger systems with multiple star-shaped units. These tiny zooids often appear like large matted patches on the seabed.
Tunicates, being members of the phylum Chordata, possess a notochord during their larval stage but lose it as soon as the metamorphosis is over. Their body coelom is restricted to the pericardial, renal, and gonadal cavities, and their organs develop from the three basic germ layers – the ectoderm, mesoderm, and endoderm. All organs (except the heart, pharynx, and gonads) are covered by a membranous epicardium, which, in turn, is surrounded by a jelly-like matrix called mesenchyme.
Their body coat, or tunic, is made of complex organic molecules, like tunicin (a variety of cellulose) and proteins. The tunic makes these chordates unique because they continue to grow proportionately as the animals grow, thus eliminating the necessity of molting.
The body wall, or mantle, comprises connective tissue, muscle fibers, blood vessels, and nerves. It has two openings: the buccal siphon on top (for water flow into the interior) and the atrial siphon on the ventral side (for water expulsion). A large muscular tube called the pharynx occupies most of the body’s interior.
The pharynx contains a ciliated groove called endostyle on its ventral surface and a gullet at its lower end. The pharynx wall has slits, known as stigmata, which allow water to fill the surrounding cavity, the atrium.
These free-swimming pelagic individuals are filter-feeders, using their pharyngeal mucous net to catch prey. The different groups of thaliaceans exhibit different body shapes. For example, pyrosomes (genus Pyrosoma) are colonial tunicates with a hollow cylindrical structure characterized by external buccal and internal atrial siphons. Similarly, doliolids (order Doliolida) are small, having barrel-shaped bodies and two siphons at the opposite ends.
They resemble the tadpole larvae of amphibians, and their tails are characteristically tilted at right angles to the body.
They have an open circulatory system characterized by a well-developed, U-shaped heart that periodically reverses the direction of blood flow. The blood vessels are simple tubes circulating pale green blood throughout the body.
They have a U-shaped, ciliated gut with an enlarged stomach at the lowest part. Glands in the stomach produce digestive enzymes that break down the ingested food materials. Following digestion in the stomach, the food enters the intestine for absorption. It is then passed to the rectum, where undigested remains form fecal pellets or strings. Finally, the excreta is expelled through the anus that opens into the cloaca of the peribranchial cavity.
Unlike most deuterostomes, tunicates lack the kidney-like metanephridial organs. Instead, they typically excrete through a renal organ comprising a mass of clear-walled vesicles in the rectal loop. The vesicles accumulate nitrogenous waste from the circulating blood in the form of urate crystals.
Those without these excretory organs expel waste by diffusing ammonia across their tissues, similar to gills.
The nervous system of adult tunicates comprises a hollow cerebral ganglion and a hollow neural gland, both located behind the siphons. A series of nerve fibers spread out from the ganglion, with the anterior nerves innervating the buccal siphon while those from the posterior end conduct impulses to the rest of the body, including the atrial siphon, organs, gut, and body wall. Many sensory cells are also found in the siphons, the buccal tentacles, and the atrium.
Jean-Baptiste Lamarck introduced the subphylum Tunicata in 1816. A few years later, in 1881, the British Biologist Francis Maitland Balfour proposed another name for the group, Urochordata. However, the scientific community opted for the term ‘Tunicata’ simply because of its precedence, as all marine invertebrates with tunics were called tunicates.
They are considered more closely related to the craniates (hagfish, lampreys, and jawed vertebrates) than lancelets, echinoderms, hemichordates, and other invertebrates.
Based on the current classification, 3,051 described species of Tunicates are divided into three classes: Ascidiacea or Sea squirts (the largest class), Thaliacea (Thaliaceans), and Appendicularia (Larvaceans), which are further grouped into subsequent orders.
Although this traditional classification system is universally accepted, recent molecular studies suggest that Ascidiacea are a paraphyletic group. Moreover, they are found to be closely related to Thaliacea, resulting in both classes being grouped under a group, Acopa.
Tunicate fossils are rare because their bodies decay soon after death, but some possess microscopic spicules that are preserved as microfossils. The earliest fossils have been found to possess a stiffening rod or notochord in their larval stages, thus suggesting their close relationship to vertebrates.
They are widely distributed in oceans from the polar regions to the tropics. While some tunicates are sessile and remain attached to rocks, shells, corals, seaweeds, mangrove roots, docks, or a ship’s hull, others are free-moving and inhabit pelagic zones. Fewer than 100 species of sea squirts are found at depths of more than 200 m (660 ft).
Around ten species of pyrosomes are concentrated in tropical waters, while 40 species of salps inhabit the surface waters of both warm and cold seas.
Although most tunicates feed on plankton, some deepwater species, like ghostfish (Megalodicopia hians), consume crustaceans, nematodes, and other tiny invertebrates. Members of the family Didemnidae derive nutrition from symbiotic green algae or cyanobacteria, such as Prochloron, that live in their tunics.
Sea squirts inhabiting the sea floor are detritivores and feed on detritus found within the sediments.
Tunicates resort to suspension-feeding, sieving planktonic particles into their bodies. They draw water into their bodies through the siphons, assisted by the cilia lining the gill slits. The water is then filtered using a net (secreted by the endostyle) lining the pharynx.
Deepwater species are sit-and-wait predators that stealthily wait for the right moment to capture their prey unalarmed.
Like all chordates, tunicates develop a notochord during their early development stages and gradually transform into adults through metamorphosis. During this process, they develop the three germinal layers typical of all coelomates: the ectoderm, mesoderm, and endoderm, eventually losing their notochord. As the tunicate reaches the end of its metamorphosis, the body cavity or coelom gets reduced to the pericardial, renal, and gonadal regions of its body.
Almost all sea squirts are hermaphroditic. In some solitary species, the sperm and eggs are released directly into the sea for fertilization, while in most colonial forms, only the sperm is released into the water, which is drawn in with the help of siphons for fertilizing the eggs.
The larvae formed after fertilization are planktonic and resemble small tadpoles with a notochord and a simple eye or ocellus. They swim by undulating their tails, assisted by a balancing organ called the statocyst.
The larva attaches to a substrate and metamorphoses into an adult in sessile ascidiaceans. The larvae are usually lecithotrophic (non-feeding) and gradually undergo a reduction of the cerebral ganglion (controls movement).
Some members of this group give rise to mobile larvae, while others (families Styelidae and Molgulidae) completely bypass the metamorphic state and develop directly into adults.
Botryllus schlosseri (Golden Star tunicate) is the only sea squirt reproducing sexually and asexually. It is a sequential (protogynous) hermaphrodite, in which the eggs ovulate about two days before the sperm is spurted into the surrounding water. However, in this self-fertilization type, fewer larvae successfully metamorphose into adults. In contrast, some sea squirts, like Ciona intestinalis, release sperm and eggs simultaneously.
In some thaliaceans groups, like pyrosomes, the colonies start to bud off new zooids, following which the egg is fertilized internally within their body. The fertilized egg then develops into an oozooid without any intermediate larval form. The ooozoid continues to bud, forming four blastozooids that detach later.
Members of the order Doliolida have a unique reproductive process. Each sexually reproducing member (gonozooid) is a hermaphrodite that releases eggs to be fertilized by the sperm of other individuals. Following fertilization, an embryo forms directly (viviparous), which feeds on the yolk sac before being released as a free-swimming, tadpole-like larva. These larvae undergo metamorphosis to form oozooids, which, just like pyrosomes, bud off asexually. Some form phorozooids (help in transportation), while others form trophozooids (nutritive in function). With time, these zooids develop into gonozooids, which separate and start the life cycle all over again.
Some thaliacean families, like salps, switch between alternating generations during reproduction. In the asexual phase, the oozooids bud off hundreds of zooids, which develop into blastozooids and remain aggregated (aggregate phase) while feeding and swimming. In the next sexual phase, the blastozooids are internally fertilized by sperm of other colonies. The egg continues to develop in a brood sac inside the blastozooid’s body until they are ready to become independent.
They are protandrous hermaphrodites (except Oikopleura dioica, which is gonochoric) that switch from male to female and reproduce sexually. After fertilization of the sperm and eggs, the larvae are released into the surrounding water. These larvae resemble those of sea squirts and grow by enlarging their body cells instead of dividing. As the trunk reaches its full length, the tail shifts from a dorsal to a ventral position, rotating 90° in the process.
In contrast, larvaceans, like Oikopleura dioica, are semelparous, reproducing only once in their lifetime. Its unique reproductive strategy involves the female germline growing within a single ovary in a giant multinucleate cell called the coenocyst.
Being an animal group, some researchers were surprised to find cellulose (a plant polymer) in the exoskeleton of tunicates. Later, it was confirmed that they produce cellulose using a functional cellulose-synthesizing enzyme encoded by a gene transferred horizontally from a bacterium through evolution.
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