The ocean is filled with millions of different species, with many that have yet to even be discovered or studied. Even some more common creatures are not well known, and I find these understudied animals fascinating.
You may have never heard of a tunicate before now, and you may believe that they are of little importance. The reality, however, is that Tunicates (more commonly known as sea squirts) are extremely important to the human society, even being studied to find a possible cure for cancer.
What is a Tunicate?
A tunicate is a marine animal (thought they may look like a plant!) that most commonly lives in colonies of other individuals of its species. They live in shallow waters, and there are around 3,000 different species, each one of them unique. It spends the majority of its life sessile, attached to rocks, coral, sunken ships, or other surfaces. They are invertebrates, but scientists have actually found that they are our closest invertebrate relatives, sharing many traits with vertebrates themselves.
Tunicates look like a barrel, enveloped in a firm, protective structure called a “tunic,” which is where they get their name from. They are filter feeders, having two siphons that are used to filter in plankton and water from one siphon and expel the excess water from the other. They get their more common name, Sea Squirt, from the fact that when they are taken out of the water, their main defense mechanism is to squirt water at whoever or whatever took them away from their home.
Life cycle and Anatomy
Despite their odd appearance as an adult, a Tunicate larvae looks much more like a tadpole. At this stage, the tunicate seems much more like an animal because it has mobility. It has a notochord structure in its tail, which acts similarly to the spine of vertebrates. Along its back, it has a nerve cord . In their head-shaped structure, they have a cerebral vesicle that acts as a brain, which has an eyespot to detect light and an otolith, which orients the larva to the current flow of gravity.
However, the Tunicate does not spend much time as a larva. It does not have a mouth at this stage of its life, so it quickly finds a surface to attach itself to. Then, they proceed to digest their own brain structure, eye sensors, spinal structure, and tail! What is left behind is the adult body of the Tunicate surrounded by the Tunic.
The anatomy of the adult tunicate is more simple. Within the tunic, the animal consists of a simple digestive system: an inhalant siphon (used to take in food and water), and exhalant siphon (used to expel waste and excess water). The tunic is secreted by the epidermis, which is on the inside of the tunic. The inside of the epidermis has a thicker dermis and a ring of muscle which allows the tunicate to squeeze itself to expel water.
Inside that structure, the tunicate is comprised mostly of an atrium, which is a large cavity, and a large pharynx with pores that allow water to pass through. The pharynx is connected to the digestive system at one end and the inhalant siphon on the other end.
Although they do not have many predators, they are normally eaten by sharks and skates. Many tunicates also have toxic skin to ward off any potential threats.
Reproduction
Nearly all Tunicates are hermaphrodites, meaning they have both male and female reproductive organs. Self-fertilization is possible, but it is avoided, because sexual reproduction allows for more variability in individuals, increasing survivability. Since tunicates live in groups or clusters, eggs are retained within the body while sperm is released into the surrounding water, falling onto the neighboring tunicates and beginning the fertilization process. The eggs are incubated inside the body until they hatch.
Abilities of the Tunicate
The most incredible thing about the tunicate is its regenerative ability. For example, its inhalant siphon: when it is damaged or amputated, the tunicate is able to regenerate the siphon without the use of cell division. Instead, it is rebuilt through direct differentiation of sac stem cells, or through the migration of cells created by the sac stem cells. As the tunicate ages, however, its regenerative properties decrease and eventually completely stop working.
Why are they important?
The reason the tunicate is so important to society is due to two things: the fact that they are our closest invertebrate ancestor and their incredible regenerative ability. These two factors have given scientists reason to believe that tunicates may be a key to finding a cure to cancer. Though there has been little success so far, understanding how tunicates are able to regenerate themselves without the need to create new cells is the first step in searching for a way to make this effect possible in humans. Since they are our closest invertebrate relatives, scientists believe that it may be possible to somehow find a link to how this regeneration occurs, and may be able to discover a way to replicate this process. Cancer cells cause exceptional cellular loss and cause mutated replication, so a sort of regeneration that has no need for cellular replication is especially interesting. Who knew that such a simple looking creature could be so vital to study?
It might not seem like much at a glance, but it's astounding that tunicates can regenerate such major injuries without even using cell division. I'm so curious how they do it.
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