The genus Ascidia includes sea squirts, sessile marine chordates in the subphylum Tunicata (Urochordata). The main characteristics of Ascidia are a sac-like body enclosed in a tunic, a large pharyngeal basket for filter feeding, and a tadpole larva that shows the chordate body plan. Siphons in Ascidia, one oral and one atrial, draw water in and expel it after filtering.
Because their larvae have a notochord and a dorsal nerve cord, tunicates are now regarded as the closest living relatives of vertebrates.
Ascidia, which is frequently found in shallow coastal waters, is distinguished by its sac-like body structure that is covered in a tunic composed of a substance that resembles cellulose.
Ascidians are essential to marine ecosystems because they act as filter feeders, eliminating organic matter and plankton from the water. By giving microorganisms a place to live and acting as food for other marine creatures, they also support the ecological equilibrium.
They have also been the focus of extensive scientific study because to their capacity to produce bioactive chemicals and regenerate tissue.
For Practical Study
The following notes are arranged in a practical examination format for easy study and identification.
Classification of Ascidia
| Rank | Classification | Characteristic |
|---|---|---|
| Kingdom | Animalia | Multicellular animals |
| Phylum | Chordata | Animals showing fundamental chordate features during development |
| Subphylum | Tunicata | Marine chordates enclosed by a protective tunic |
| Class | Ascidiacea | Adult tunicates commonly known as sea squirts |
| Order | Phlebobranchia | Ascidians with a relatively simple branchial sac organization |
| Family | Ascidiidae | Family containing the genus Ascidia |
| Genus | Ascidia | Mostly solitary, sac-like marine ascidians |
Geographical Distribution
Ascidia species occur in marine environments in different parts of the world, particularly in coastal and shelf waters.
Distribution varies among species.
Habit and Habitat
Ascidia is a sessile marine animal that commonly remains attached to rocks, shells, piers or other firm substrates.
The adult body is enclosed in a tunic and bears two siphons used for water circulation and feeding.
Comments
- Commonly called as sea squirts.
- Shape of body short and cylindrical with a broad base attached to rocky substratum.
- Test translucent, wrinkled and brownish in colour. Below test is mantle.
- Anteriorly body contains eight-lobed terminal branchial siphon and six-lobed sub-terminal atrial.
- The branchial (oral) siphon, eight-lobed and terminal, leads to the mouth. The atrial siphon, six-lobed and sub-terminal, opens from the atrial cavity, into which the anus discharges.
- Mouth is surrounded by 50-100 tentacles.
- Branchial siphon leads into pharynx which is perforated by stigmata.
- Pharynx contains endostyle.
- A dorsal tubercle, nerve ganglion, neural gland and membranous dorsal lamina are present.
- Alimentary canal consists of mouth pharynx, stomach, intestine, rectum and anus.
- Gonads, gonoduct and gonopore distinct.
- Monoecious. Ovary, testes in the same animal. Reproduction sexual. Oviparous. Metamorphosis retrogressive.
Special Features
There are some compound ascidians in which individuals are buried in a common test.
The development includes tailed larva exhibiting typical chordate characters having notochord, nerve cord, etc. Kowalewsky (1886) carefully studied the development of larval ascidians and brilliantly demonstrated true position of the group among chordates.
The tunicates are best understood by studying first free-living larva of an ascidian, than the adult.
The larva shows chordate characters but same are absent in adult and others are modified to sessile mode of life.
Compound ascidians reproduce asexually by gemmation and also produce eggs and sperms for sexual reproduction.
Identification
A sac-like, sessile marine animal enclosed in a tunic and provided with two siphons, a large pharyngeal basket and a characteristic filter-feeding system is identified as an ascidian such as Ascidia.
Quick Identification of Ascidia
Ascidia is a sessile marine tunicate commonly known as a sea squirt. The adult has a sac-like body enclosed within a protective tunic and remains attached to a firm substrate.
- Common name: Sea squirt
- Scientific name: Ascidia
- Body form: Sac-like and sessile
- Main body regions: Tunic, pharyngeal region and visceral region
- Siphons: Oral (incurrent) siphon and atrial (excurrent) siphon
- Feeding: Filter feeding
- Respiration: Through the pharyngeal basket
- Important structure: Endostyle
- Larval feature: Notochord, dorsal nerve cord and post-anal tail
- Key identifying character: Sessile tunicate with two siphons and a large pharyngeal basket
The adult Ascidia is highly modified for a stationary filter-feeding lifestyle, whereas its tadpole larva displays the characteristic chordate body plan.
Physical Characteristics of Ascidia
Ascidia has a simple sac-like body adapted for a permanently attached marine life. The adult body is enclosed by a protective tunic and is usually attached to a firm underwater surface.
Body Form
The body is generally rounded, cylindrical or somewhat elongated, depending on the species. The outer tunic protects the animal and provides structural support. The adult does not have the streamlined body form seen in free-swimming chordates.
Siphons
Two prominent openings are present:
- Oral or incurrent siphon: draws water into the pharyngeal basket.
- Atrial or excurrent siphon: releases filtered water and carries out waste products and reproductive materials.
Pharyngeal Basket
The pharynx forms a large branchial basket occupying much of the body. It contains numerous small openings called stigmata through which water passes.
Endostyle
The endostyle is located on the ventral side of the pharyngeal basket. It produces mucus that helps trap suspended food particles during filter feeding.
Digestive System
The digestive tract proper (oesophagus, stomach, intestine) lies beside the pharyngeal basket. Food passes from the pharynx into the oesophagus, stomach and intestine before reaching the anus.
Tunic
The body is surrounded by a protective tunic or test. It is secreted by the animal and provides protection while allowing the adult to remain attached to the substrate.
Chordate Features
Although the adult is highly modified and sessile, the larva possesses important chordate characteristics, including a notochord, dorsal hollow nerve cord and post-anal tail. These structures are largely lost or modified during metamorphosis.
Feeding and Filter-Feeding Adaptations in Ascidia
Ascidia is a suspension feeder that obtains food by filtering small particles from seawater. Continuous water currents through the pharyngeal basket make feeding and respiration possible.
Water Current
Water enters through the oral siphon and passes into the large pharyngeal basket. Ciliary activity maintains the water current through the numerous pharyngeal stigmata.
Food Capture
The endostyle secretes mucus that spreads over the pharyngeal surface. Suspended particles such as microscopic plankton and organic matter become trapped in this mucus.
Transport of Food
Cilia move the mucus and trapped food particles toward the digestive tract. The food then passes through the oesophagus to the stomach and intestine, where digestion and absorption occur.
Main Feeding Adaptations
- Oral siphon draws water into the body.
- Pharyngeal basket provides a large filtering surface.
- Numerous stigmata allow water to pass through the pharynx.
- Endostyle produces mucus for trapping food particles.
- Cilia maintain water flow and transport food-containing mucus.
- The atrial siphon expels filtered water.
This highly specialized filter-feeding system allows the sessile adult to obtain food without actively searching for prey.
Respiration and Circulation in Ascidia
Respiration in Ascidia is closely associated with its large pharyngeal basket. Water entering through the oral siphon passes through numerous openings in the pharyngeal wall, allowing respiratory gas exchange.
Respiration
The pharyngeal basket contains a dense network of blood vessels. As water passes through the stigmata, oxygen diffuses into the blood while carbon dioxide diffuses out.
The continuous water current produced by ciliary activity therefore serves two important functions:
- Respiration by bringing oxygenated seawater into contact with the pharyngeal surface.
- Feeding by carrying suspended food particles into the filtering apparatus.
Circulation
Ascidia has a simple circulatory system consisting of a tubular heart and blood vessels. The heart periodically reverses the direction of blood flow, a distinctive feature of tunicates.
Tunicates have an open circulatory system.
The circulatory system distributes absorbed nutrients and respiratory gases throughout the body and helps maintain the metabolic activities of the sessile animal.
Nervous System and Sensory Features in Ascidia
The nervous system of adult Ascidia is greatly simplified compared with that of free-swimming chordates. This reduction is associated with the sessile lifestyle of the adult.
Adult Nervous System
A cerebral ganglion is located between the oral and atrial siphons. It acts as the main coordinating centre of the adult nervous system. Nerves extend from the ganglion to different regions of the body.
Larval Nervous System
The tadpole larva has a more clearly organized chordate nervous system. It possesses a dorsal hollow nerve cord, which is associated with the notochord and post-anal tail.
During metamorphosis, the larva settles on a suitable substrate and undergoes major structural changes. The tail and associated locomotory structures are lost as the animal becomes sessile.
Sensory Features
Sensory cells associated with the siphons and body surface help the animal respond to changes in its surroundings. These responses are particularly important for regulating water flow through the siphons and coordinating feeding and other activities.
Reproduction and Development in Ascidia
Ascidia species are generally hermaphroditic, possessing both male and female reproductive organs. Reproduction is associated with the marine environment, where gametes can be released into the surrounding water.
Fertilization
The gametes are released through the atrial region, and fertilization generally occurs in seawater. The resulting embryo undergoes early development before forming a characteristic tadpole larva.
Tadpole Larva
The tadpole larva is free-swimming and differs greatly from the sessile adult. It possesses important chordate features:
- Notochord
- Dorsal hollow nerve cord
- Post-anal tail
- Pharyngeal structures
The larva uses its muscular tail for swimming and dispersal.
Settlement and Metamorphosis
After a period of free swimming, the larva attaches to a suitable substrate. It then undergoes metamorphosis, during which the tail and larval locomotory structures are lost.
The body reorganizes into the sessile adult form, with the siphons and pharyngeal basket becoming prominent.
Life Cycle
Adult → Gametes → Fertilization → Embryonic development → Tadpole larva → Settlement → Metamorphosis → Young adult → Adult
The striking difference between the free-swimming chordate-like larva and the sessile adult is one of the most important features of ascidian development.
Tunic and Siphons in Ascidia
The tunic and siphons are prominent structural features of the adult Ascidia. Together, they support protection, attachment, water circulation and filter feeding.
Tunic
The body is enclosed within a protective outer covering called the tunic or test. It is secreted by the animal and contains a cellulose-like substance called tunicin, along with other extracellular materials.
The tunic:
- Protects the soft body from mechanical damage.
- Provides structural support.
- Helps the animal remain attached to the substrate.
- Forms the outer covering around the adult body.
Oral Siphon
The oral siphon is the main incurrent opening. It draws seawater into the pharyngeal basket, bringing oxygen and suspended food particles into the animal.
Atrial Siphon
The atrial siphon is the excurrent opening. Water that has passed through the pharyngeal basket leaves through this siphon. The atrial opening also provides a route for the release of digestive waste and reproductive products.
Functional Significance
The coordinated action of the tunic, oral siphon, pharyngeal basket and atrial siphon allows Ascidia to maintain a continuous water current while remaining permanently attached to its substrate.
Evolutionary Significance of Ascidia
Ascidia is important in zoology because tunicates are chordates, even though the adult animal has a highly modified sessile body.
The free-swimming tadpole larva clearly displays several fundamental chordate characteristics, including the notochord, dorsal hollow nerve cord, post-anal tail and pharyngeal structures. These features demonstrate the chordate affinity of tunicates.
During metamorphosis, the larva loses its tail and becomes a sessile adult specialized for filter feeding. This dramatic transformation provides an important example of how body organization can change during development.
Modern molecular and phylogenetic studies have also placed Tunicata within Chordata, making ascidians important organisms for studying chordate evolution and the evolutionary relationships among tunicates, cephalochordates and vertebrates.
Why Ascidia is important
Sea squirts contributes to marine ecosystems through its filter-feeding activity, nutrient processing and role in marine food webs.
Filter Feeding
By continuously filtering seawater, Ascidians removes suspended organic particles and microorganisms from the surrounding water. This makes ascidians important suspension feeders in many benthic communities.
Nutrient Cycling
The processing and digestion of organic particles contributes to the movement and recycling of nutrients between the water column and benthic environment.
Role in Food Webs
Ascidians can serve as prey for various marine organisms and therefore form part of coastal marine food webs.
Marine Communities
Sessile ascidians may occur on rocks, shells and other submerged surfaces alongside sponges, bryozoans, algae and other sessile organisms. Their presence contributes to the biological diversity of these benthic communities.
Key Takeaways
- Ascidia is a marine tunicate belonging to Phylum Chordata and Class Ascidiacea.
- It belongs to Order Phlebobranchia and Family Ascidiidae.
- The adult is a sessile, sac-like sea squirt enclosed within a protective tunic.
- Two siphons, the oral siphon and atrial siphon, maintain water circulation.
- The large pharyngeal basket is specialized for filter feeding and respiration.
- The endostyle produces mucus that helps capture suspended food particles.
- The adult has a simplified nervous system, while the tadpole larva shows distinct chordate organization.
- The larva possesses a notochord, dorsal hollow nerve cord and post-anal tail.
- Metamorphosis transforms the free-swimming larva into a sessile adult.
- Ascidia is important for understanding tunicate biology, chordate evolution and marine filter-feeding ecosystems.
Frequently Asked Questions
What is Ascidia?
Ascidia is a genus of marine tunicates commonly known as sea squirts. The adults are sessile and filter-feed on suspended particles.
Why is Ascidia included in Chordata?
Its tadpole larva possesses characteristic chordate features, including a notochord, dorsal hollow nerve cord and post-anal tail.
What is the habitat of Ascidia?
Ascidia is a marine animal that commonly lives attached to firm underwater substrates such as rocks and shells.
What is the function of the endostyle in Ascidia?
The endostyle secretes mucus that helps trap food particles during filter feeding.
What are the two siphons of Ascidia?
The two siphons are the oral or incurrent siphon and the atrial or excurrent siphon.
Does the adult Ascidia have a notochord?
The adult does not retain the prominent notochord found in the tadpole larva. The notochord is primarily a larval structure.
What is the larva of Ascidia like?
The larva is a free-swimming tadpole that possesses a notochord, dorsal hollow nerve cord and post-anal tail.
Why is Ascidia important in zoology?
Ascidia is important for studying tunicate biology, chordate development, metamorphosis and the evolutionary relationships of chordates.
Recommended Study Resource
A Manual of Practical Zoology – Chordates
by P S Verma (Author)
A Manual of Practical Zoology Chordates : For Undergraduate, Postgraduate Students | 12th Edition | Basic Lab Methods Book | Experimental Biochemistry, Physiology, Osteology, Embryology
by Dr. P S Verma (Author)
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References
- Integrated Taxonomic Information System (ITIS). Ascidia and Ascidiacea taxonomy.
- National Center for Biotechnology Information (NCBI) Taxonomy. Ascidia.
- World Register of Marine Species (WoRMS). Ascidia and Ascidiacea.
- Ascidiacea World Database. Classification and biodiversity information for sea squirts.
- Ruppert, E. E., Fox, R. S., & Barnes, R. D. Invertebrate Zoology: A Functional Evolutionary Approach. Brooks/Cole.
- Brusca, R. C., Moore, W., & Shuster, S. M. Invertebrates. Sinauer Associates.
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