Urochordata: Definition, Meaning, Classification, characteristics, Examples

What Are Urochordata?

Tunicata also called Urochordata is a subphylum of chordates and these are animals which develop a notochord at some point in their life cycle. This subphylum has characteristics like, dorsal hollow nerve cords, pharyngeal slits and post-anal tail, species in this subphylum include vertebrates and cephalochordates and urochordates.

Urochordates, more specifically, are members of this phylum that are mostly invertebrates and these chordate characteristics are mostly evident during the larval stage. In their adult form, they become sessile and live encased in a tunic which is made up of a cellulose-like material. This subphylum comprises’ fascinating animals like sea squirts, salps, and larvaceans that paint a very rich picture of the versatility and richness of Chordata.

Taxonomy and Classification Of Urochordata

The classification is described below-

Hierarchy of Classification

• Kingdom: Animalia

• Phylum: Chordata

• Subphylum: Urochordata

• Classes: Ascidiacea, Thaliacea, Larvacea

Characteristics of Each Class

The subphylum includes three primary classes:

Ascidiacea (Sea Squirts): Laid on the sea bed as adults with cylindrical bodies protected by a tunic, with two openings, incurrent and excurrent siphons through which the organism feeds on particulate matter. Adults are also bipedal with lungs; however, larvae are free-swimming and shed their notochord and tail during metamorphosis.

Thaliacea (Salps): Salps are floating animals, long, barrel-shaped, and gelatinous, and they can be colonial also. It presents itself in a life cycle with both single-celled and colonial forms that allow for both sexual and asexual means of reproducing while at the same time filtering plankton with their bodies.

Larvacea (Appendicularia): Larvaceans are small planktonic animals, which never lose their notochord or tail and build mucous houses to filter food. They look like tadpoles to some extent, particularly to retain the basic forms in the freeloader behavioural mode.

General Characteristics Of Urochordata

The characteristics are explained-

Body Structure

• Urochordates are enclosed in a kind of cuticle called the tunic, which is made up of tunicin which is similar to cellulose.

• This tunic offers both physical protection and rigidity from predators to the wearer.

• In regards to their physical anatomy, urochordates show dramatic metamorphosis from their larval stage to their adult stage.

• During the larval stage, the organisms are motile and possess a notochord, a dorsal nerve cord, and a tail, during the adult stage however many of these species are sessile, especially sea squirts and have very few structures.

• Chordates possess a notochord which exists in the larva stage and helps in giving support and movement.

• It is usually dissolved during the process of metamorphosis to adapt to the adult form of the organisms.

Life Cycle

• Urochordates in their life cycle pass through a motile larva phase to a generally sessile adult phase which may be much more developed.

• The development includes the involution practically of the notochord and tail in many species and the evolution from swimming or crawling to a sedentary or floating existence.

• The adult life stage of urochordates, the class, which comprises adults that gave rise to tunicates, is quite diverse.

• Members of Ascidiacea (sea squirts) are mostly sessile and normally attach themselves to a substrate/substratum while those of Thaliacea (salps) and Larvacea (appendicularians) are more or less free-moving, floating or swimming in the midwater or epipelagic zone of the ocean.

Anatomy And Physiology

The anatomy and physiology are described-

External Features

• The tunic is a strong, outer garment made up of a cellulose-like protein called tunicin, formed as an extra layer of protection.

• It gives the more or less the skeletal frame and protection from enemies, covering the whole body.

• Urochordates are said to have a simple circulatory system characterised by the presence of two siphons; incurrent (oral) and excurrent (atrial) siphons.

• The incurrent siphon takes in water into the body for food and oxygen, on the other hand, the excurrent siphon ejects the food-filtered water and waste products.

Internal Features

Digestive system

• The organism’s digestion process includes a mouth, pharynx, oesophagus, stomach, intestine, and anus.

• Cereals are stuck in the pharynx and enclosed in mucus to the stomach which breaks down the food particles.

Circulatory system

• In this kind of chordate, there is an open circulatory system present.

• The blood circulates through vessels that antis into body cavities so that they are in contact with the tissue.

Nervous system

• The nervous system is quite small and delimited by a cerebral ganglion situated just behind the incurrent siphon and nerve cords that spread throughout the organism.

Reproductive system

• The majority of urochordates represent hermaphrodites, they are organisms which have both male and female reproductive habits.

• Fishes can reproduce sexually by shedding eggs and sperm into water and the process of external fertilization.

Respiration And Feeding

The respiration and feeding process is described below-

Filter feeding mechanism

• Urochordates are filter-feeding animals.

• Water that contains the food is ingested through the incurrent siphon and is lined with a mucous-pharyngeal basket that filters out the food particles.

• They move these particles through cilia and into the digestive system.

Role Of Pharyngeal Slits In Respiration

• The pharyngeal slits which are situated in the pharyngeal basket act as an apparatus for feeding as well as respiration.

• Because of the slits that are present in the surrounding walls of the tubes, water is allowed to flow with diminished oxygen levels while at the same time releasing carbon dioxide in the process of gas exchange.

Habitat And Distribution

The habitat and distribution are explained-

Marine Habitats

• Urochordates are strictly aquatic dwelling animals that are distributed within different marine habitats such as the coastal area, abyssal zone, and pelagic zone.

• Ascidiacea is a sessile lifestyle living in intertidal and subtidal environments while Thaliacea and Larvacea float in the water column at various depths.

• Urochordates filter parasites and debris since the organisms are filter feeders and this expelled water is clean.

• They are key elements of the marine ecosystem as they are the food to many species and participate in nutrient recycling and energy transfers, which in turn support marine species’ survival and the overall health of the ocean ecosystems.

Reproduction And Development

The reproduction and development are listed below-

Asexual Reproduction

Budding in colonial forms

• Most of the colonial urochordates especially in Ascidiacea have a feature of asexual reproduction which involves budding.

• This process entails reproduction that is asexual where several individuals are produced from the body of a single organism, essentials for the colony’s growth and expansion.

Sexual Reproduction

Hermaphroditic nature

• In the majority of urochordates, such animals are hermaphroditic, this means that they contain elements of both the male and female reproductive systems.

• This makes them able to lay eggs as well as develop a sperm chamber.

Fertilization process

• In urochordates, there is internal and external fertilization.

• Frequently, male and female gametes are poured into the water where the external development of gametes takes place.

• In others, it is internal within the body cavity, specifically, the process of fertilization.

Larval development stages

• Obtaining larval form is very important for urochordates.

• The larvae are mobile with a notochord and tail and with a dorsal nerve cord.

• These features are important for movement and to search for a proper surface to attach to.

• At the time of settlement, most of the larvae undergo a direct metamorphosis in which all of the tail and notochord are resorbed and a sessile or a pelagic adult is constructed depending on the class.

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