Ascidia - Zoology - exam point of view
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Ascidiacea (commonly known as the ascidians or sea squirts) is a paraphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders Despite their plant-like appearance, sea squirts are actually more closely related to vertebrates than they are to invertebrates such as sponges and coral. There are more than 3,000 known sea squirt species found on the seabed around the world, with the majority of sea squirt species being found in the warmer, nutrient-rich tropical waters. Sea squirts can vary from just 3cm to 30cm in length depending on the species of sea squirt and its habitat.
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Ascidia - Zoology - exam point of view
- 2. • Ascidiacea (commonly known as the ascidians or sea squirts) is a paraphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders • Despite their plant-like appearance, sea squirts are actually more closely related to vertebrates than they are to invertebrates such as sponges and coral. • There are more than 3,000 known sea squirt species found on the seabed around the world, with the majority of sea squirt species being found in the warmer, nutrient-rich tropical waters. • Sea squirts can vary from just 3cm to 30cm in length depending on the species of sea squirt and its habitat. ASCIDIACEA
- 4. Kingdom: Animalia Phylum: Chordata Class: Urochordata Common Name: Sea Squirt Scientific Name: Ascidia Found: Worldwide Size: 3-30cm (1.2-11.8in) Weight: 100-200g (3.5-7oz) ASCIDIACEA
- 5. • Ascidians are characterized by a tough outer "tunic" made of polysaccharide cellulose. • sea squirts are sessile animals: they remain firmly attached to substratum, such as rocks and shells. • Sea squirts feed by taking in water through the oral siphon. The water enters the mouth and pharynx, flows through mucus-covered gill slits (also called pharyngeal stigmata) into a water chamber called the atrium, then exits through the atrial siphon. ASCIDIACEA
- 7. • three main types: solitary ascidians, social ascidians that form clumped communities by attaching at their bases, and compound ascidians that consist of many small individuals (each individual is called a zooid) forming colonies up to several meters in diameter. • Unlike the shells of molluscs, the tunic is composed of living tissue, and often has its own blood supply. • In some colonial species, the tunics of adjacent individuals are fused into a single structure. ASCIDIACEA
- 10. • The upper surface of the animal, opposite to the part gripping the substratum, has two openings, or siphons. • When removed from the water, the animal often violently expels water from these siphons, hence the common name of "sea squirt". • The body itself can be divided into up to three regions, although these are not clearly distinct in most species. • The pharyngeal region contains the pharynx, while the abdomen contains most of the other bodily organs, and the postabdomen contains the heart and gonads. • In many sea squirts, the postabdomen, or even the entire abdomen, are absent, with their respective organs being located more anteriorly. ASCIDIACEA
- 11. • the pharyngeal region is occupied mainly by the pharynx. The large buccal siphon opens into the pharynx, acting like a mouth. • The pharynx itself is ciliated and contains numerous perforations, or stigmata, arranged in a grid-like pattern around its circumference. • The beating of the cilia sucks water through the siphon, and then through the stigmata. • A long ciliated groove, or endostyle, runs along one side of the pharynx, and a projecting ridge along the other. • The endostyle may be homologous with the thyroid gland of vertebrates, despite its differing function. ASCIDIACEA
- 12. • The pharynx is surrounded by an atrium, through which water is expelled through a second, usually smaller, siphon. • Cords of connective tissue cross the atrium to maintain the general shape of the body. • The outer body wall consists of connective tissue, muscle fibres, and a simple epithelium directly underlying the tunic. ASCIDIACEA
- 13. • The pharynx forms the first part of the digestive system. • The endostyle produces a supply of mucus which is then passed into the rest of the pharynx by the beating of flagella along its margins • . The mucus then flows in a sheet across the surface of the pharynx, trapping planktonic food particles as they pass through the stigmata, and is collected in the ridge on the dorsal surface. • The ridge bears a groove along one side, which passes the collected food downwards and into the oesophageal opening at the base of pharynx. DIGESTIVE SYSTEM
- 14. • The oesophagus runs downwards to a stomach in the abdomen, which secretes enzymes that digest the food. • An intestine runs upwards from the stomach parallel to the oesophagus and eventually opens, through a short rectum and anus, into a cloaca just below the atrial siphon. • In some highly developed colonial species, clusters of individuals may share a single cloaca, with all the atrial siphons opening into it, although the buccal siphons all remain separate. DIGESTIVE SYSTEM
- 15. • The heart is a curved muscular tube lying in the postabdomen, or close to the stomach. • Each end opens into a single vessel, one running to the endostyle, and the other to the dorsal surface of the pharynx NERVOUS SYSTEM
- 16. • The vessels are connected by a series of sinuses, through which the blood flows. • Additional sinuses run from that on the dorsal surface, supplying blood to the visceral organs, and smaller vessels commonly run from both sides into the tunic. • Nitrogenous waste, in the form of ammonia, is excreted directly from the blood through the walls of the pharynx, and expelled through the atrial siphon. CIRCULATORY SYSTEM
- 17. • The ascidian central nervous system is formed from a plate that rolls up to form a neural tube. • The number of cells within the central nervous system is very small. • The neural tube is composed of the sensory vesicle, the neck, the visceral or tail ganglion, and the caudal nerve cord. • The anteroposterior regionalization of the neural tube in ascidians is comparable to that in vertebrates. NERVOUS SYSTEM
- 18. • Although there is no true brain, the largest ganglion is located in the connective tissue between the two siphons, and sends nerves throughout the body. • Beneath this ganglion lies an exocrine gland that empties into the pharynx. • The gland is formed from the nerve tube, and is therefore homologous to the spinal cord of vertebrates. • Sea squirts lack special sense organs, although the body wall has numerous individual receptors for touch, chemoreception, and the detection of light. NERVOUS SYSTEM
- 19. • Almost all ascidians are hermaphrodites and conspicuous mature ascidians are sessile. • The gonads are located in the abdomen or postabdomen, and include one testis and one ovary, each of which opens via a duct into the cloaca. • Broadly speaking, the ascidians can be divided into species which exist as independent animals (the solitary ascidians) and those which are interdependent (the colonial ascidians). • Different species of ascidians can have markedly different reproductive strategies, with colonial forms having mixed modes of reproduction. REPRODUCTION
- 20. • Solitary ascidians release many eggs from their atrial siphons • External fertilization in seawater takes place with the coincidental release of sperm from other individuals. • A fertilized egg spends 12 hours to a few days developing into a free-swimming tadpole-like larva, which then takes no more than 36 hours to settle and metamorphose into a juvenile. • Sexual maturity can be reached in as little as a few weeks. Since the larva is more advanced than its adult, this type of metamorphosis is called 'retrogressive metamorphosis'. • This feature is a landmark for the 'theory of retrogressive metamorphosis or ascidian larva theory'; the true chordates are hypothesized to have evolved from sexually mature larvae. REPRODUCTION
- 21. COMPARISON OF ASCIDIAN WITH FROG
- 22. • Colonial ascidians reproduce both asexually and sexually. • SEXUAL REPRODUCTION • Different colonial ascidian species produce sexually derived offspring by one of two dispersal strategies- Colonial species are either broadcast spawners (long-range dispersal) or philopatric (very short-range dispersal). • Broadcast spawners release sperm and ova into the water column and fertilization occurs near to the parent colonies. • Resultant zygotes develop into microscopic larvae that may be carried great distances by oceanic currents. • The larvae of sessile forms which survive eventually settle and complete maturation on the substratum- then they may bud asexually to form a colony of zooids. COLONIAL SPECIES
- 23. • For the philopatrically dispersed ascidians, sperm from a nearby colony (or from a zooid of the same colony) enter the pharyngeal siphon and fertilization takes place within the atrium. • Embryos are then brooded within the atrium where embryonic development takes place: this results in macroscopic tadpole-like larvae. • When mature, these larvae exit the atrial siphon of the adult and then settle close to the parent colony (often within meters). SEXUAL REPRODUCTION
- 24. • Many colonial sea squirts are also capable of asexual reproduction, although the means of doing so are highly variable between different families. • In the simplest forms, the members of the colony are linked only by rootlike projections from their undersides known as stolons. • Buds containing food storage cells can develop within the stolons and, when sufficiently separated from the 'parent', may grow into a new adult individual. ASEXUAL REPRODUCTION
- 25. • In other species, the postabdomen can elongate and break up into a string of separate buds, which can eventually form a new colony. • In some, the pharyngeal part of the animal degenerates, and the abdomen breaks up into patches of germinal tissue, each combining parts of the epidermis, peritoneum, and digestive tract, and capable of growing into new individuals. • In yet others, budding begins shortly after the larva has settled onto the substrate. • In the family Didemnidae, for instance, the individual essentially splits into two, with the pharynx growing a new digestive tract and the original digestive tract growing a new pharynx. ASEXUAL REPRODUCTION
- 26. • Various ascidians are used as food. • Sea pineapple is cultivated in Japan and Korea .When served raw, they have a chewy texture and peculiar flavor likened to "rubber dipped in ammonia" which has been attributed to an unsaturated alcohol called . • The tunicate Styela clava is farmed in parts of Korea where it's known as mideoduk and is added to various seafood dishes such as the fish stew agujjim. Sea squirt bibimbap is a specialty of Geojae island, not far from Masan.". • Microcosmus sabatieri and several similar species from the Mediterranean Sea are eaten in France Italy and Greece for example just raw with lemon or in salads with olive oil, lemon and parsley. • The piure is used as food in the cuisine of Chile, consumed both raw and used as ingredients in seafood stews like bouillabaisse. • Pyura praeputialis is known as cunjevoi in Australia. It was once used as a food source by Aboriginal people living around Botany Bay, but is now used mainly for fishing bait. IMPORTANCE
- 27. Sea pineapples with sausages CULINARY
- 29. • A number of factors make sea squirts good models for studying the fundamental developmental processes of chordates, such as cell-fate specification. • The embryonic development of sea squirts is simple, rapid, and easily manipulated. Because each embryo contains relatively few cells, complex processes can be studied at the cellular level, while remaining in the context of the whole embryo. • The embryo's transparency Is ideal for fluorescent imaging and its maternally-derived proteins are naturally pigmented, so cell lineages are easily labeled, allowing scientists to visualize embryogenesis from beginning to end. IMPORTANCE
- 30. • Sea squirts are also valuable because of their unique evolutionary position: approximation of ancestral chordates, they can provide insight into the link between chordates and ancestral non- chordate deuterostomes, as well as the origination of vertebrates from simple chordates. • The sequenced genomes of the related sea squirts Ciona intestinalis and Ciona savignyi are small and easily manipulated; comparisons with the genomes of other organisms such as flies, nematodes, pufferfish and mammals provides valuable information regarding chordate evolution. • A collection of over 480,000 cDNAs have been sequenced and are available to support further analysis of gene expression which is expected to provide information about complex developmental processes and regulation of genes in vertebrates. • as an IMPORTANCE