Kingdom: Animalia Phylum: Chordata Class: Urochordata (aka Tunicata)
Sea squirts have a body that is basically a sack with two siphons in which water enters and exits through. However, many tunicates also have a small animal larva (like a tadpole) and they are free-swimming and have characteristics of an invertebrate chordate. These are still considered tunicates but have a notochord, dorsal nerve cord, pharyngeal slits and post-anal tail (6). The notochord is a rodlike structure that helps form the main support in the body of sea squirts. The dorsal nerve cord is a hollow cord that has nerves that connect to the muscles of the organism. Pharyngeal slits are openings that help filter out nutrients out of the water. The post-anal tail is a tail that runs past the anal opening and usually is only present during embryonic development (13). Many will lose their tail as it eventually disintegrates, and therefore loses their ability to be mobile (6).
Relationship to humans
The sea squirt is believed to have been the first invertebrate to have a vasculature heart system, similar to that in humans, with blood cells traveling through blood vessels. What is incredible about sea squirt though, is that they have the ability to regrow everything just with their blood vessels. Using this special ability and the fact that humans and the sea squirt share about 77% of the same genes, scientists have found genes in the sea squirt that are actually critical to the development and function of the vertebrate heart, eye, and our ability to hear. Closer studies of sea squirts are potentially going to lead scientists to be able to find ways for humans grow back their own new limbs! (9).
Habitat and niche
Sea squirts live in marine environments. They can be found in marine seabeds all over the world, but tend to prefer warmer tropical waters that are rich in nutrients. Sea squirts attach themselves to objects at a very young age and remain there for their entire lives, feeding on plankton and nutrients in the water (7).
Predator avoidance
Due to the fact that sea squirts attach themselves to objects from a very young age, they are often sitting targets for predators. Snails, crustaceans and eels are the most common predators of the sea squirt along with some species of larger fish. Sea squirts contain a cavity called a tunic that has two openings, an inhalant (water comes in) siphon and an exhalent (the water goes out) siphon. The tunic is surrounded by muscle that, when contracted, can cause a jet of water to leave the exhalent siphon which can help deter predators (2).
Nutrient acquisition
Sea squirts are filter feeders, so they feed by absorbing sea water through their incurrent and excurrent siphons and filtering them through slits and taking in food while releasing waste as well as excess water. They mostly feed on plankton and nutrients along with the algae that grows on them. Larger sea squirt species even have tentacle-like appendages that allow them to catch food particles when they pass by (1). Fish and jellyfish are trapped and ingested by large species of deep ocean sea squirts(1).
Reproduction and life cycle
Sea squirts have both male and female reproductive organs, but self-fertilization is highly unlikely. They release into the water their eggs and sperm, which become fertilized and also a part of the plankton. The larvae then hatches out of the egg and makes it way to the ocean floor to find a place to anchor itself. (1) After the larvae attaches itself to a hard surface, it's nervous system degrades to a small mass of nerve tissue, and in 3-4 days the circulatory, digestive, and reproductive systems develop (8).
Growth and development
(4).
In the early stages of development for a tunicate, the tunicate is in the form of a larva after it hatches out of the egg. The larva is basically that of a tunicate "tadpole" and has a tail that allows it to move around. Tadpoles tunicates' primary focus is to find a place a good living environment for their adult life. Once they have found a place to anchor themselves, there is a sticky secretion that allows them to attach to the spot that was chosen. The tail disintegrates and the adult body forms. The adult body of a tunicate, along with all its features, are enlarged significantly. As shown by the picture above, the siphons are a lot larger in the adult tunicate, and the pharynx with slits becomes huge. The image below shows how tunicates go from tadpoles to the adult form after a location is chosen (15).
(14).
Integument
The tunic of a tunicate is made of a polysaccharide called tunicin, with a few living but well spaced cells as part of it. There are two opening in this shell, which are the siphons. Inside, there is a thin epidermis, which secretes the tunic, and muscles to squeeze water out of the tunic to repel predators (2).
Movement
Most are sessile, or they don't move. In early development tadpole tunicates have the ability to move until they attach to a hard substrate and lose their tail. At this stage, the nervous system also begins to disintegrate (6).
Sensing the environment
Sea Squirts have a very basic nervous system. A small ganglion, or cluster of neurons, usually resides between the two siphons and above the atrium. Due to the lack of full nervous system, this small group of neural bodies responds to the stimulus in the environment (5).
When in developmental stages of the larva form, they have an eyespot, which helps detect light, and an otolith, which helps the animal orient the pull of gravity (15).
Gas exchange
Gas exchange in tunicates occurs by diffusion over the surfaces of the body. The circulatory system is open so gases are exchanged relatively freely over the body surfaces (12).
Waste removal
Water and waste is expelled from the anus by an internal cavity called the exhalant siphon, with the water flow being sustained by beating cilia (10).
Environmental physiology (temperature, water and salt regulation)
Tunicates have adapted to live in water. They are able to survive in water because of their incurrent and excurrent siphons that filter nutrients (usually plankton) from the water that allows them to survive (1). In their early development of a larvae they are able to find a good place to live their adult life and that benefits their survival in the water (15). They are also able to undergo gas exchange through diffusion over their body surface and this shows their adaptation to survival in marine environments (12).
Internal circulation
Possess an open circulatory system that doesn't have many vessels. The blood generally flows through large spaces in the tissue or through blood channels in the tissues. There are no arteries, veins, or capillaries. Sea squirts do have a rather simplistic heart, a small tube with walls that contract, forcing the blood through it. What is quite unique about this organism's heart is its ability to reverse its beat. Typically, the heart beats around one hundred times in one direction, then it stops for a moment and beats around one hundred times in the other direction (5).
Chemical control (i.e. endocrine system)
Sea squirts contain genes that are very similar to the gene that produces thyroid hormones in humans. This suggests that sea squirts have a similar chemical signalling system to spur stages of development (11).
Review Questions:
1) How does their circulatory system work? 2) How are sea squirts similar to humans?
By Kevin Zhang
Table of Contents
Classification/Diagnostic characteristics
Kingdom: AnimaliaPhylum: Chordata
Class: Urochordata (aka Tunicata)
Sea squirts have a body that is basically a sack with two siphons in which water enters and exits through. However, many tunicates also have a small animal larva (like a tadpole) and they are free-swimming and have characteristics of an invertebrate chordate. These are still considered tunicates but have a notochord, dorsal nerve cord, pharyngeal slits and post-anal tail (6). The notochord is a rodlike structure that helps form the main support in the body of sea squirts. The dorsal nerve cord is a hollow cord that has nerves that connect to the muscles of the organism. Pharyngeal slits are openings that help filter out nutrients out of the water. The post-anal tail is a tail that runs past the anal opening and usually is only present during embryonic development (13). Many will lose their tail as it eventually disintegrates, and therefore loses their ability to be mobile (6).
Relationship to humans
The sea squirt is believed to have been the first invertebrate to have a vasculature heart system, similar to that in humans, with blood cells traveling through blood vessels. What is incredible about sea squirt though, is that they have the ability to regrow everything just with their blood vessels. Using this special ability and the fact that humans and the sea squirt share about 77% of the same genes, scientists have found genes in the sea squirt that are actually critical to the development and function of the vertebrate heart, eye, and our ability to hear. Closer studies of sea squirts are potentially going to lead scientists to be able to find ways for humans grow back their own new limbs! (9).Habitat and niche
Sea squirts live in marine environments. They can be found in marine seabeds all over the world, but tend to prefer warmer tropical waters that are rich in nutrients. Sea squirts attach themselves to objects at a very young age and remain there for their entire lives, feeding on plankton and nutrients in the water (7).Predator avoidance
Due to the fact that sea squirts attach themselves to objects from a very young age, they are often sitting targets for predators. Snails, crustaceans and eels are the most common predators of the sea squirt along with some species of larger fish. Sea squirts contain a cavity called a tunic that has two openings, an inhalant (water comes in) siphon and an exhalent (the water goes out) siphon. The tunic is surrounded by muscle that, when contracted, can cause a jet of water to leave the exhalent siphon which can help deter predators (2).Nutrient acquisition
Sea squirts are filter feeders, so they feed by absorbing sea water through their incurrent and excurrent siphons and filtering them through slits and taking in food while releasing waste as well as excess water. They mostly feed on plankton and nutrients along with the algae that grows on them. Larger sea squirt species even have tentacle-like appendages that allow them to catch food particles when they pass by (1).Fish and jellyfish are trapped and ingested by large species of deep ocean sea squirts(1).
Reproduction and life cycle
Sea squirts have both male and female reproductive organs, but self-fertilization is highly unlikely. They release into the water their eggs and sperm, which become fertilized and also a part of the plankton. The larvae then hatches out of the egg and makes it way to the ocean floor to find a place to anchor itself. (1) After the larvae attaches itself to a hard surface, it's nervous system degrades to a small mass of nerve tissue, and in 3-4 days the circulatory, digestive, and reproductive systems develop (8).Growth and development
In the early stages of development for a tunicate, the tunicate is in the form of a larva after it hatches out of the egg. The larva is basically that of a tunicate "tadpole" and has a tail that allows it to move around. Tadpoles tunicates' primary focus is to find a place a good living environment for their adult life. Once they have found a place to anchor themselves, there is a sticky secretion that allows them to attach to the spot that was chosen. The tail disintegrates and the adult body forms. The adult body of a tunicate, along with all its features, are enlarged significantly. As shown by the picture above, the siphons are a lot larger in the adult tunicate, and the pharynx with slits becomes huge. The image below shows how tunicates go from tadpoles to the adult form after a location is chosen (15).
Integument
The tunic of a tunicate is made of a polysaccharide called tunicin, with a few living but well spaced cells as part of it. There are two opening in this shell, which are the siphons. Inside, there is a thin epidermis, which secretes the tunic, and muscles to squeeze water out of the tunic to repel predators (2).Movement
Most are sessile, or they don't move. In early development tadpole tunicates have the ability to move until they attach to a hard substrate and lose their tail. At this stage, the nervous system also begins to disintegrate (6).Sensing the environment
Sea Squirts have a very basic nervous system. A small ganglion, or cluster of neurons, usually resides between the two siphons and above the atrium. Due to the lack of full nervous system, this small group of neural bodies responds to the stimulus in the environment (5).When in developmental stages of the larva form, they have an eyespot, which helps detect light, and an otolith, which helps the animal orient the pull of gravity (15).
Gas exchange
Gas exchange in tunicates occurs by diffusion over the surfaces of the body. The circulatory system is open so gases are exchanged relatively freely over the body surfaces (12).Waste removal
Water and waste is expelled from the anus by an internal cavity called the exhalant siphon, with the water flow being sustained by beating cilia (10).Environmental physiology (temperature, water and salt regulation)
Tunicates have adapted to live in water. They are able to survive in water because of their incurrent and excurrent siphons that filter nutrients (usually plankton) from the water that allows them to survive (1). In their early development of a larvae they are able to find a good place to live their adult life and that benefits their survival in the water (15). They are also able to undergo gas exchange through diffusion over their body surface and this shows their adaptation to survival in marine environments (12).Internal circulation
Possess an open circulatory system that doesn't have many vessels. The blood generally flows through large spaces in the tissue or through blood channels in the tissues. There are no arteries, veins, or capillaries. Sea squirts do have a rather simplistic heart, a small tube with walls that contract, forcing the blood through it. What is quite unique about this organism's heart is its ability to reverse its beat. Typically, the heart beats around one hundred times in one direction, then it stops for a moment and beats around one hundred times in the other direction (5).Chemical control (i.e. endocrine system)
Sea squirts contain genes that are very similar to the gene that produces thyroid hormones in humans. This suggests that sea squirts have a similar chemical signalling system to spur stages of development (11).Review Questions:
1) How does their circulatory system work?2) How are sea squirts similar to humans?
References:
1. http://a-z-animals.com/animals/sea-squirt/2. http://www.earthlife.net/inverts/ascidiacea.html
3. http://www.seasky.org/reeflife/assets/images/seasquirt_common.jpg (picture)
4. http://science.kennesaw.edu/~jdirnber/Bio2108/Lecture/LecBiodiversity/34_05Tunicate-L.jpg
5.http://www.reefkeeping.com/issues/2005-03/rs/
6. http://www.ucmp.berkeley.edu/chordata/urochordata.html
7. http://a-z-animals.com/animals/sea-squirt/
8. http://www.chesapeakebay.net/fieldguide/critter/sea_squirt
9. http://abcnews.go.com/Technology/sea-squirt-grow-limbs/story?id=20057998
10. http://www.pznow.co.uk/marine/squirts.html
11. http://www.genomenewsnetwork.org/articles/01_03/seasquirt.shtml
12. http://biology.unm.edu/ccouncil/Biology_203/Summaries/Deuterostomes.htm
13. http://www.nhptv.org/wild/chordata.asp
14. http://depts.washington.edu/fhlk12/links/StudentProjects/Images/CionaTunicateBiology/group_low.gif
15. http://depts.washington.edu/fhlk12/links/StudentProjects/Tun.biology.html