17.+chordata-+tunicate


 * Chordata- Tunicate (ascidians) - Sea Squirts ** specific species: //Halocynthia roretzi (Sea Pineapple)// by: Esha Uddin





Classification/Diagnostic Characteristics
Sea Squirts are classified as ascidians, which fall in the subphylum Urochordata, also called Tunicata. The full classification of the Sea Pineapple (//Halocynthia roretzi//) is **Animalia, Chordata, Ascidiacea, Pleurogona, Pyuridae, Halocynthia, //Halocynthia roretzi//** //[|(1)]//. Animalia means that the species is a part of the kingdom Animalia, and chordata means that the organism must have each of the following characteristics:
 * 1) **Gill slits**, which are small slits that allow water to pass through and deliver oxygen to the body
 * 2) A nerve chord along the back of the body, known as a **notochord (pictured below)**
 * 3) Backbone of a firm rod of cells beneath the nerve chord

 Ascidians are the evolutionary link between invertebrates and vertebrates, so from this classification we can tell that tunicates are in between both types of organisms. Most tunicates produce both egg and sperm, but although they have both types of sex organs, they cannot self-fertilize and rely on sexual reproduction [|(2)]. They are called tunicates because their bodies are covered by a protective leathery sac, similar to a tunic. Sea Pineapples also have two distinct pores; one of the pores guides the water into its body for filter feeding whereas the other ones serves as an exit port [|(4)].

Relationship to Humans
The Sea Pineapple is known as the **Meongge** in Korea and the **Hoya** or **Maboya** in Japan. In Korea, this specimen is mostly eaten raw with vinegary gochujang, a red chili paste, or it can be pickled in jeotgal, a sour dish. In Japan, it is commonly eaten raw as sashimi and served with soy sauce. It is also often salted, smoked, grilled, deep-fried or dried. In China, Japan, and other Asian countries, the Sea Pineapple is farmed and mass produced. They are an important part of the food chain in the marine world, so they are also indirectly a source of food for humans. Outside of food, the Sea Squirt is mostly used for observation and research by humans.

Sea squirts and humans share about 77% of the same genes, making them useful subjects for study. These genes are crucial for the development of the spine, eyes, heart, and hearing ability. Any mutation in these genes in humans can result in heart disease, cataracts, and deafness, so studying similar genes in humans can help find potential cures. They are able to regrow their entire body with simply blood vessels, which also makes them important organisms to study in the field of regenerative medicine. [|(16)]

Habitat and Niche
Tunicates like the Sea Squirt/Sea Pineapple are marine animals and live underwater. They are predicted to have evolved from a species that lived in **estuarine** environments, which is where freshwater meets saltwater. The Sea Pineapple live in the shallow, rocky sea bottom and can be found in the northern seas of Japan and China, as well as the eastern and southern coastal waters of Korea. In these environments, the Sea Pineapple prefers cold water environments at a temperature at about twelve degrees Celsius. Although they are plentiful in this Asian region, tunicates and Sea Squirts of all kinds can be found in all seas from the intertidal zone (seashore) to the greatest depths. The most common types of tunicates are found on pier pilings, rocks, hulls seashells, and on the backs of crabs. They can live individually or in larger colonies. [|(5)]

Predator Avoidance
<span style="font-family: Georgia,serif; font-size: 12pt;">Sea squirts have been given their common name due to the fact that they squirt water when removed from their environment. Like other Chordata, tunicates have sensory tentacles. They primary method of predator avoidance used by sea squirts, also known as tunicates, is a thick ecternal skin that contains heavy layers of cellulose, and, in some species, contains high concentrations of toxins that can harm potential predators [|(6)]. For example, the //Styela plicata//, another species of tunicate, has concentrations of vanadium in their outer layer. Sea squirts attach themselves to other animals at a very young age, and unfortunately this can make them sitting targets for other animals to eat them [|(7)].



<span style="color: #db5e17; font-family: Georgia,serif;">Nutrient Acquisition
<span style="font-family: Georgia,serif; font-size: 12pt;">Tunicates filter feed. This means that their pharynx (the part of the throat behind the mouth) doubles as a feeding sac. They suck water in through their gill slits, digest their prey and nutrients, then release the excess water back out. Tunicates have an organ called endostyle which secretes an internal mucus sheet that traps particles and transfer them to the digestive tract [|(8)]. Some tunicates trap small animals whereas others trap and digest small plants. In deep water, their primary food source is plankton, but near the coast, debris from other dead organisms are a major secondary food source.

<span style="color: #db5e17; font-family: Georgia,serif;">Reproduction and Life Cycle
<span style="font-family: Georgia,serif; font-size: 12pt;">Ascidians can form colonies through asexual reproduction, which means they can bud and clone themselves. Most tunicates have both male and female sexual reproductive organs, and while they can reproduce asexually through budding, they cannot self-fertilize. Typically, the eggs are released into the water near the tunicate where they are fertilized and develop. However, in some species the fertilized eggs remain inside the female until they are fully developed. [|(9)]

<span style="font-family: Georgia,serif; font-size: 12pt;">The tunicate's larval stage for non-free swimming individuals is typically short as the larva focuses on locating a place to live, rather than fixating on feeding. Over the course of three to five days the tunicate larva of the non-free swimming tunicates will transform into an adult, digesting its tail as a source of nutrients in the process.

<span style="color: #db5e17; font-family: Georgia,serif;">Integument
<span style="font-family: Georgia,serif; font-size: 12pt;">As seen in the labeled diagram in the Classification section of this page, a tunicate, though seemingly simple in body structure, has many complex parts. Tunicates have a **dorsal (back) hollow nerve chord**, and a notochord which is a back supporting rod that is fluid-filled, so it's firm but flexible [|(10)]. Sea Squirts also have a **tail** extending past the anus, and a **dorsal vertebral column** that switches with the notochord as a primary supporting structure when the organism becomes an adult.

<span style="font-family: Georgia,serif; font-size: 12pt;">The body of a tunicate is made of bands of muscle that surround the notochord and build the body's tube-like structure. The entire sac-like body is covered in a tunic or outer covering, usually made of proteins and polysaccharides (chains of sugars) that are secreted by their skin or epidermal cells. Their pharynx develops in adulthood and it is where oxygen is taken in and carbon dioxide is expelled. The full body of a tunicate can range from sizes of one millimeter to sixty centimeters. As larvae they resemble tadpoles. [|(11)]

<span style="color: #db5e17; font-family: Georgia,serif;">Movement
<span style="font-family: Georgia,serif; font-size: 12pt;">Sea Squirts are generally sessile, which means that they don't move and grow where they land as adults. However, in their early stages, the notochord rod in the tail makes for more efficient swimming.When they are young organisms, like tadpoles, they swim around and find a place to land, but once they are planted they rarely move. In this manner, Sea Squirts have spread across many seas and oceans and developed into many different species. [|(12)]

<span style="color: #db5e17; font-family: Georgia,serif;">Sensing the Environment
<span style="font-family: Georgia,serif; font-size: 12pt;">Like other Chordata, Sea Squirts have sensory tentacles surrounding their opening that allow them to sense their surroundings. Their nervous system in general is very rudimentary, but it's more complex when they are younger larvae. Tunicates have a dorsal hollow nerve chord above the notochord with small nerves branching out all over.



<span style="color: #db5e17; font-family: Georgia,serif;">Gas Exchange
<span style="font-family: Georgia,serif; font-size: 12pt;">In Sea Squirts, water is sucked in using cilia to draw off the flow into the animal and distribute the water through various channels and pores, including the gills and lining of the atrium [|(13)]. These respiratory surfaces of the sea squirt make use of the fast-flowing water to quickly defuse gas in and out of the animal, almost resembling a quick breath performed by an animal that has lungs. The quick expulsion (squirt) removes the oxygen depleted water from the body. [|(14)]

<span style="color: #db5e17; font-family: Georgia,serif;">Waste Removal
<span style="font-family: Georgia,serif; font-size: 12pt;">Due to their method of filter feeding, Sea Squirts remove their waste by taking in water to digest nutrients, then releasing waste water and nutrients into their surroundings thorough slits in their membrane. Water and waste exit the tunicate through the anus by an internal cavity called the exhalant siphon (exhalant as in exhale). The water is propelled out by cilia that wiggle along the sides and control the water flow. [|(18)]

<span style="color: #db5e17; font-family: Georgia,serif;">Environmental Physiology
<span style="font-family: Georgia,serif; font-size: 12pt;">Since tunicates live in water, they are unable to regulate their temperature through the water that flows through their sacs. For the Sea Squirts that like in saltwater environments, salt regulation is just another part of the internal regulatory systems.

<span style="color: #db5e17; font-family: Georgia,serif;">Internal Circulation
<span style="font-family: Georgia,serif; font-size: 12pt;">Tunicates have hearts and large blood vessels which circulate blood and carry nutrients and gasses throughout the body, but they do not have arteries, veins, or capillaries. The blood of a tunicate is pumped throughout its body using small spaces called sinuses, which act like blood vessels. They have an open circulatory system. The tunicate is able to reverse its heartbeat; the heart will beat 100 times while the blood flows in one direction, then stop for a short amount of time, and then beat 100 times while the blood flows in the opposite direction. A Tunicate's blood is clear and has a high concentration of vanadium. [|(14)]

<span style="color: #db5e17; font-family: Georgia,serif;">Chemical Control
<span style="font-family: Georgia,serif; font-size: 12pt;">There is a good possibility that these organisms do have endocrine systems that produce and use hormones to send signals throughout the body similar to humans in that they have genes similar to the ones that produce thyroid hormones in humans. Since thyroid hormones are an integral part of the human endocrine system, this can signify that tunicates, in an earlier stage of life, could have a similar chemical control system to that of humans. [|(19)]

<span style="font-family: Georgia,serif; font-size: 12pt;">Describe the circulatory system of these organisms. Compare it to that of humans.
<span style="font-family: Georgia,serif; font-size: 12pt;">Similar to humans, tunicates use blood to carry gases and nutrients throughout their bodies. Both circulatory systems have hearts and large blood vessels and use their circulatory systems to carry out similar functions. However, the tunicate is able to reverse its heartbeat and beat 100 times in each direction.

<span style="font-family: Georgia,serif; font-size: 12pt;">What is the notochord? Explain it's function in the evolutionary history of the tunicate.
<span style="font-family: Georgia,serif; font-size: 12pt;">The notochord is a hollow nerve chord that runs along the back of the Sea Squirt's body. Since tunicates fall in the evolutionary category as a transitional animal between invertebrates and vertebrates, the notochord signifies a structure similar to a spinal chord.

<span style="font-family: Georgia,serif;">Why might a Sea Squirt be a good organism to study for regenerative medicine.
<span style="font-family: Georgia,serif; font-size: 12pt;">A Sea Squirt shares 77% of similar genes with humans, making them a good model for genes that are crucial in the development of eyes, ears, heart, and other systems. Due to the fact that Sea Squirts can regenerate any part of their body fully through their blood vessels, they can be especially helpful in studying regenerative biology and the possibility of regrowing human organs.

<span style="font-family: Georgia,serif;">Why is a Sea Squirt called a Sea Squirt?
<span style="font-family: Georgia,serif; font-size: 12pt;">When taken out of their natural environment and when responding the touch, Sea Squirts squirt out liquid from their second siphon. Since they are filter feeders, they filter water through their gill slits and absorb nutrients, then store the water water in the second siphon until they expel it naturally or squirt it out as means of predator avoidance.

<span style="font-family: Georgia,serif; font-size: 12pt;">How do tunicates form colonies on the sea floor?
<span style="font-family: Georgia,serif; font-size: 12pt;">Tunicates can reproduce sexually and asexually. Through budding, some tunicates can clone themselves and build a colony of genetically identical individuals on the ocean floor. Tunicates also have both male and female reproductive organs, so they can sexually reproduce without many limitations. Sexual reproduction usually takes place externally as sperm and egg are released and fertilized in the open water.

<span style="color: #db5e17; font-family: Georgia,serif;">Sources

 * 1) <span style="background-color: #ffffff; font-family: Georgia,serif; font-size: 12pt;">[|www.lib.noaa.gov/retiredsites/korea/main_species/sea_squirt.html]
 * 2) <span style="background-color: #ffffff; font-family: Georgia,serif; font-size: 12pt;">[|www.britannica.com/animal/sea-squirt.]
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