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The evolutionary history of vertebrate animals begins with special organisms that do not appear to be animals, but they do. It's about the sea squirts. In these small sessile animals, barely a few centimeters in size, the information that gave way to advanced vertebrate animals is stored. Although they do not have a vertebral column, they do have the structure that originates it, called the notochord. In this interesting article by Ecologist Verde we will explain what are sea squirts, their characteristics and species that exist.
What are sea squirts
Sea squirts are immobile animals found in all types of marine ecosystems, from temperate waters to considerable depths. They belong to the phylum of chordates, but differ from vertebrates by have no backbone. They are classified within the subphylum of Urochordates or Tunicates, since their body has a tunic to give it support. Finally, these animals are classified within the Ascidacea class, with particular characteristics that we will see below.
If you want to know more details, we recommend that you take a look at this other article about Invertebrate Animals: examples and characteristics.
Characteristics of sea squirts
In the same way that there are different types of animals, for example, there are also different types of sea squirts. Even so, there are some characteristics of sea squirts that all of them share and that we are going to detail below:
Basic structure
The basic structure of sea squirts comprises a discopedium that joins them to the ground with water pumping siphons. They have a pharyngeal region within the large mouth siphon, an abdomen that groups the organs, a postabdomen with gonads, and the heart. They have no vertebrae or skeleton. If you want to know more, visit this post about 20 curiosities of invertebrate animals.
Another important characteristic that makes them belong to the category of Urochordates is the presence of the endostyle on the ventral wall of the pharynx. This structure is made up of mucous glands and connective tissue to capture food and carry it to the esophagus.
Feeding
The feeding of the sea squirts is quite showy. Inhale water with particles of plankton and algae with its mouth siphon, which filters large particles with the help of cilia. It sends this filtrate through the pharynx covered by stigmata, to empty the content over the main cavity. It then releases the excess water by jets through its atrial siphon. Sea squirts of greater depths can even consume fish. These animals are also known as marine jets for their particular way of feeding.
If you want to know more, you can visit these Green Ecologist posts about What is plankton and its importance or Green Algae: what they are, characteristics, types and examples.
Types of sea squirts
The types of sea squirts they are classified as social, compound or solitary. Its size depends on this, being able to be from just one centimeter, up to 30 centimeters in case of grouping. The robe that surrounds them can come in different shapes and a variety of bright colors. It is made up of cellulose and consists of a living tissue. Under it there is muscle tissue, connective tissue and epithelium.
Reproduction
The reproduction of sea squirts can be sexual or asexual, although most are hermaphrodites. Here's how sea squirts reproduce:
- Asexual reproduction: It can occur through the development of buds in discopedia that contain storage cells capable of generating another sea squirt. Another form of asexual reproduction is through the rupture of the postabdominal region, which generates buds for new organisms. It may also happen that the larva divides in two during its development. Here you can find more information about asexual reproduction: what it is, types and examples.
- Sexual reproduction: consists of the release of eggs and sperm into the water, where they must meet to be fertilized. Here the larva is generated, from which the metamorphosis of the sea squirts begins. They begin their life in a mobile larval stage similar to a tadpole, with the embryonic structure common to chordates: the notochord. They use this mobility to find a prosperous habitat, where it settles with the help of adhesive papillae and begins its metamorphosis into an adult, during a period of approximately 36 hours. As an adult it loses its mobility and the notochord to form sessile sea squirts.
You can also read more about Hermaphrodite Animals: reproduction and examples, here.
Sea squirt species
There are more than 2,300 species of sea squirts. Here are some examples of them with interesting characteristics.
Tunicates (Ecteinascidia turbinata)
Tunicates inhabit associated with mangroves or shipwrecked ships in hot and acidic waters, creating spherical nuclei of up to 500 sea squirts in the same tissue. Its color is translucent, varying slightly in coloration depending on the location. If you want to know more about mangroves, read this article by Ecologist Verde on What is a mangrove and its characteristics.
Sea vase (Ciona intestinalis)
This sea squirt has a transparent yellow tunic and its organs are visible through it. Around its siphons it has a striking yellow ring. Mouth siphons have 8 lobes and atrial siphons have 6. He is lonely and measures 15 centimeters. It inhabits all the warm areas of the world at maximum depths of 500 meters with low waves. They have been observed associated with human surfaces, such as ports or underwater cement plates. Its genome was sequenced and it has been very useful for development research.
Ascidia bulb (Clavelina lepadiformis)
The ascidian bulb groups several individuals in transparent colonies, through which their eggs and larvae can be observed when they are in the reproductive stage. It is 2 centimeters tall and around the pharynx they have a white ring-shaped coloration, as well as a white endostyle. The whole set reminds of a light bulb. In cold areas where winter is evident, colonies can retreat and re-emerge by spring. They inhabit from southern Norway to the Mediterranean Sea, associated with rocks and algae.
Ascidia soccer ball (Diazona violacea)
Of colonial association, these sea squirts are accommodated in a globular structure of almost 100 individuals translucent green, united in a common discopedium. They are found at sub-littoral depths, lower than most tunicates. It can measure between 30 and 40 centimeters in diameter and 20 in height. If you want to read more information about Colonial Associations, visit this article that we recommend.
Sea squirt currant (Distomus variolosus)
It groups in colonies to form a type of mat in the marine substrate. They can be joined at the base or by the sides, but they are usually free. They are red and they measure just 1 centimeter Tall. Both siphons, exhaling and inhalants, are located at the top. In the British Isles to Portugal, supported by rocks and macroalgae, is where the sea squirts are found.
If you want to read more articles similar to Sea squirts: what are they, characteristics and species, we recommend that you enter our Biodiversity category.
Bibliography- The Marine Life Information Network (2008). Be squirt. Available at: https://www.marlin.ac.uk/search?q=sea+squirt
- Prieto, M. (1979). Sea squirts (urochordata-tunicata-ascidacea) of the continental coast of Venezuela and the Leeward Antilles. Caracas, Venezuela: CESUSIBO
