UROCHORDATES: CHARACTERISTICS, TYPES, HABITAT, FOOD - BIOLOGY - 2025

The urochordates or tunicates (subphylum Tunicata) are a group of non-vertebrate chordates that live exclusively in the sea. They are called urochordates, since in most of them the notochord is restricted to the caudal region of the larvae.

The name tunicates, meanwhile, comes from the fact that its body is protected by a polysaccharide covering called a tunic. This polysaccharide, called tunicin, has a chemical composition similar to cellulose.

Styela canopus, sea squirt. Photo Carlos Lira.

Some species of tunicates are pelagic, but most are benthic. They can live alone or form colonies. Some sea squirts can also be compound, that is, several individuals share the same exhaling siphon (a structure through which the water exits the organism or organisms).

characteristics

Tunicates are chordates, therefore they share with other members of the phylum the characteristics that define them as such. These are present at least in the embryonic stage and are:

-They have an internal support structure called notochord. This rod-shaped structure is made up of cells wrapped in a sheath of connective tissue. It is present in all chordates, at least during their larval phase.

-They have a hollow dorsal nerve cord. In invertebrates that have a nerve cord, it is located ventrally. In chordates, during the larval phase, it is located dorsally with respect to the digestive tract. In some groups it may be reduced to a single ganglion in adulthood.

-They have pharyngeal gill slits. These are openings that communicate the pharynx with the outside. In amniotic vertebrates (embryonic tetrapods with four extra-embryonic membranes), these slits may not open, but they form and remain as simple slits.

-The presence of an endostyle or a thyroid gland is also an exclusive characteristic of chordates.

Other characteristics, which urochordates present, but not the rest of the chordates, are:

-Body covered with tunicin.

-Notochord only in the caudal region and generally only in the larval stage. In appendicularia, however, this structure remains in adults.

-The digestive tract is shaped like a "U".

-The number of pharyngeal branchial clefts is high.

-The dorsal nerve cord is present only in the larval stages.

-They have two siphons, one for the entry of water, called the inhalant or incurrent siphon, and the other for its expulsion, called the exhaling or excurrent siphon.

Taxonomy and classification (types)

The Tunicata taxon was erected in 1816, by the famous French naturalist Jean-Baptiste Lamarck, to group the chordates that presented the body covered by a tunic. In 1881, the British biologist Francis Maitland Balfour erected the taxon Urochordata to group the same organisms.

Perhaps because of Balfour's fame, his classification of the group was accepted by many scientists. Both names have long been used by different researchers.

However, the International Code of Zoological Nomenclature states that in similar cases the older name should prevail. In this case, the name erected by Lamarck must take precedence and therefore be considered valid.

Tunicates were traditionally divided into four classes: Appendicularia (= Larvacea), Ascidiacea, Thaliacea and Sorberacea. The latter class was erected in 1975 to host a group of sea squirt-like organisms that inhabit deep waters.

The sorberáceans had previously been grouped in a family (Molgulidae) within the sea squirts. They were then relocated to the family Hexacrobylidae, where they had remained until elevated to class level.

However, molecular analyzes showed their proximity to other sea squirts, despite the morphological differences they showed. Due to this, currently the taxon Sorberacea is not considered valid.

According to the current classification, the valid classes of tunicates are:

Appendicularia

Also known as Larvacea. They are planktonic and solitary, they do not form colonies. The adults retain characters of the larvae, including the notochord and the tail, which is why they are believed to have suffered neoteny.

Ascidiacea

It is the most diverse within the group. Representatives of this class are benthic organisms that live fixed to the substrate. They can be solitary, colonial or composite. The two siphons are directed away from the substrate. The nerve cord is restricted to the larval stage.

Thaliacea

They are tunicates of pelagic habits, also known as salps. The siphons are located in opposite directions and serve to generate streams of water that help organisms in swimming. Adults lack a tail, but retain gill slits.

Habitat

Tunicates are exclusively marine organisms. Appendicular and thalliaceans are pelagic, while ascidiaceae (or ascidians) are benthic. Regarding their bathymetric distribution, they are mainly present in shallow waters, however some species are exclusively abyssal.

Thaliaceans inhabit all seas, from the equator to the poles, but are more frequent in warm waters. Likewise, they prefer shallow waters, but some specimens have been found at a depth of 1500 meters.

The appendicularia are part of the plankton. They are solitary and inhabit gelatinous structures secreted by themselves. They are found in the surface waters of all oceans.

Ascidiates are sessile and live attached to almost any type of substrate. They are found in all seas and oceans. They are more frequent in rocky substrates, although there are species that live on muddy bottoms. They inhabit from the intertidal zone to the abyssal bottoms.

Feeding

Tunicates feed mainly by filtration, generating water currents that penetrate the interior of the organism through the oral or incurrent siphon, in this way they trap plankton organisms and particulate organic matter.

Some species of deep-sea ascidians are invertebrate-feeding predators. They catch the prey that touch them, using the oral siphon. Other species live in muddy substrates and feed on the organic matter present at the bottom.

Reproduction

Asexual

It occurs in thaliaceae and sea squirts. This type of reproduction occurs by budding. Two types of budding are recognized: propagation and survival.

Budding spread

It generally occurs when environmental conditions are suitable. In the case of ascidians, it helps a rapid colonization of the substrate. It also serves to increase the size of the colony.

Survival budding

When environmental conditions are unfavorable, the colonies produce buds called potentials. They will not grow while conditions are adverse. When conditions improve, the buds grow rapidly.

Sexual

Most tunicates are simultaneous hermaphrodites (that is, an individual possesses both male and female organs). In sea squirts, fertilization can be external or internal and produces an egg that hatches into a larva called a tadpole. However, in some species development is direct, which means that there is no larval stage.

In thaliaceae, unlike in ascidiates, there is no free-living larva, there are species that present alternation of sexual and asexual generations, presenting internal fertilization during sexual reproduction.

The appendicularia only present sexual reproduction, but in these the fertilization is external. They present larval development and the organisms mature retaining larval characters (neoteny), that is, they undergo pedomorphosis.

Ascidium tadpole larva. Photo: Carlos Lira.

Importance

Economical

Although the consumption of sea squirts is very localized, in some countries, mainly Asian, these organisms are highly desired. In Korea, the Halocinthya roretzi species is used for cultivation purposes, generating sales for the year 2000, profits of over 18 million dollars.

In recent years, interest in the production of these organisms in both crops and fisheries has increased, due to their potential for the production of bioactive substances of pharmacological importance.

Other species of tunicates, on the contrary, are potentially harmful. Due to their high capacity to colonize substrates, some species of ascidians become pests in bivalve cultures, mainly oysters and mussels.

Pharmacological

Tunicates are capable of biosynthesizing numerous substances with high potential for the pharmaceutical industry, among which are linear and cyclic peptides, alkaloids, terpenoids, as well as isoprenoids and hydroquinones. Because of this, at the beginning of this century, more than 5% of all natural marine products came from tunicates.

Among the properties of the compounds obtained from tunicates is a moderate to high cytotoxicity on tumor cells, they have also shown to possess antiplasmodic and antitripanosomal activities.

Lepadines, marine alkaloids, have shown activity against neuronal acetylcholine receptors, which are related to Parkinson's and Alzheimer's diseases. Substances with antibacterial, antifungal, antiviral, anticancer, immunosuppressive, and immunostimulatory properties have also been isolated.

References

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2. CP Hickman, LS Roberts & A. Larson (1997). Integrated principles of zoology. Boston, Mass: WCB / McGraw-Hill.
3. P. Castro & ME Huber (2003). Marine Biology. 4th Edition, McGraw-Hill Co.
4. RC Brusca, W. Moore & SM Shuster (2016). Invertebrates. Third Edition. Oxford University Press.
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7. J. Petersen (2007). Ascidian suspension feeding. Journal of Experimental Marine Biology and Ecology.