Chordata- Urochordates and Cephalochordates

Ayesha Sundaram
tunicate.jpg
A group of tunicates, one member of the urochordate subphylum

In understanding the evolution of vertebrates, one must first understand their predecessors: the two primary invertebrate subphyla, urochordates and cephalochordates. The urochordates and cephalochordates evolved as one branch of the ancestral deuterostome and, stemming from the two invertebrate subphyla, vertebrates emerged as more complex organisms. Members of the two invertebrate subphyla- most prominently, tunicates and lancelets- are sessile organisms that illustrate the essential body plan of a chordate.







Structure and Diagnostic Characteristics


chordate_tree.jpg
Cladogram depicting the Evolution of Vertebrates from Basic Chordates
Members of the chordate subphyla are distinguished by 4 certain characteristics:
  • The notochord consists of a long, flexible tube located between the nerve cord and the digestive tube. Made of large, fluid-filled cells that are covered with fibrous tissue, the notochord is vital in providing skeletal support for the respective chordate. The structure is found in all chordate embryos but, in most vertebrates, disappears upon reaching adult stage and is instead replaced by a more complex, jointed skeleton.
  • Developed from a rolled-up piece of ectoderm, the nerve cord of a chordate embryo eventually develops into the central nervous system, including the brain and spinal cord.
  • In the process of digestion and food intake, the pharyngeal slits, located posterior to the mouth, function in suspension feeding in invertebrate chordates. In more complex vertebrates, the pharyngeal slits evolved over time to function for gas exchange, mandible support, hearing, and other functions.
  • The postanal tail, containing skeletal muscles and elements, allows aquatic chordates to propel themselves forward and provide a mechanism for locomotion.
A Salp Chain. Salps are examples of free-floating tunicates.)
A Salp Chain. Salps are examples of free-floating tunicates.)

Tunicates are commonly called sea squirts and also known as ascidians. Each tunicate resembles a bulging sac in the shape of a U. They are sessile marine creatures, signifying that they are permanently fixed to their respective surfaces, i.e. rocks, docks, and boats. Tunicates that are not attached to some substrate are known as Thaliaceans, gelatinous animals that use their siphons to jet-propel themselves through the water. Each tunicate is enveloped in an outer layer made out of celluloselike carbohydrate. Tunicates also display pharyngeal slits that function in the suspension feeding of the organism. However, there is no trace of a notochord, a nerve cord, or a tail, which stand as distinguishing characteristics of a chordate. Although all chordate traits are clearly observed on the tunicate larvae, most traits save the pharyngeal traits are unseen on the adult tunicates.

Lancelets, on the other hand, were named for their sharp weapon shape and their structure more closely identifies with the structure of an ideal chordate. This is due to the fact that the notochord, nerve cord, gill slits, and postanal tail all remain in the adult stage of the lancelet. Lancelets display tentacles and can grow up to approximately 5 cm in length and live in the sandy bottoms of coastal regions. Their numbers are relatively small but they live in certain places at huge densities, over 5,000 lancelets per square meter!

lancelet.gif
Basic structure of a lancelet

Basic structure of a tunicate
Basic structure of a tunicate



Sensing the Environment

Though sessile on a stationary surface, tunicates contain notable sensory organs. These include eyespots to detect light as well as otoliths, or structures composed of calcium carbonate that help tunicates orient to the pull of gravity.Otoliths are also used by maring biologists to determine the age of an organism. The Otolith itself forms chemically different rings each year, like the rings on a tree, and interpretations of these rings can determine an organism's age. Otoliths are also found in fish.

As for lancelets, they are found buried in the sand with only their mouths protruding from the surface. Therefore, the tentacles on the surface of the organism's mouth act as sensory devices for "feeling out" the environment.



Locomotion

Chordates are capable of movement (by using their muscles) at different stages in life. Tunicate larvae swim to a surface, which they latch onto using their heads and trigger metamorphosis. During this process, the aforementioned chordate characteristics slowly disappear and fade away. During adult stage, the organisms are sessile and are therefore attached onto one surface.
In contrast, lancelets are forced to swim from one location to another. Their swimming mechanism lies in the arrangement of the lancelet's muscles along the sides of the notochord, which are organized like rows of V-shaped chevrons (<<<<). When these muscles contract, the movements flex the notochord and produce side-to-side vibrations that push the body forward. The process of locomotion by a lancelet displays another similarity between lancelets and vertebrates: the segmentation of the muscles. The muscles develop from somites, or blocks of mesoderm, that are arranged along the sides of an embryonic notochord.



Food Intake and Digestion

A tunicate inhales seawater through an incurrent siphon located at the top of its bulbous structure. The seawater passes through the pharyngeal slits and enter a chamber called the atrium. A mucous net covering the pharyngeal slits filters food from the seawater, and the food travels via cilia into the intestine.
The lancelet undergoes a similar digestion process. Through ciliary pumping, water is drawn into the lancelet's mouth, where it is filtered by oral cirri, small projections that surround the mouth, and travels to the pharyngeal slits, where a mucous net traps food particles. The food particles are transferred to the digestive tube while the water exits through the slits. A distinct characteristic of the digestive processes of both organisms is that both are suspension feeders, or organisms that eat small particles that are suspended in water.
Other digestive features include the hepatic caecum, a pouch that secretes digestive enzymes, and the iliocolnic ring, a specialized part of the intestine where actual digestion takes place. Urochordates are the only animal able to create cellulose.


Respiration

Besides aiding in suspension feeding and digestion, the pharyngeal slits and gill slits play a minimal role in respiration for both the tunicate and the lancelet. Even though they carry blood vessels and veins, lancelets do not have blood vessels nor do they have respiratory pigments. However, gas exchange mainly occurs on the external surface of the organism's body, i.e. on the cilia covering the organism's gills.


Waste Removal

For urochordates, excess seawater which does not contain nutrients anymore is sent through the pharyngeal slits into the anus. The anus empties into the excurrent siphon, where the water is expelled.
The cephalochordate's excretory system, specifically the lancelet's, contains not kidneys but segmentally arranged nephridia. Nephridia are organs that have a similar fuction to kidneys and are only found in invertebrates. There are two types of nephridia: metanephridia (found in cephalochoradtes) and protonephridia (found in the Phylums Platyhelminthes and Rotifera). From the nephridia, water exits via the atriopore.


Circulation

Tunicates have an open circulatory system that contains no vessels. Instead, blood flows through spaces and channels within the tissue, as well as channels that pass through the gills. Arteries, veins, and capillaries are absent from the circulatory system; however, in place of a heart, a tube and its walls contract to force blood through it.

Lancelet.JPG
A lancelet's head as viewed under a microscope

As lancelets are much more closely related to modern-day chordates that tunicates are, lancelets share similar characteristics of a closed circulatory system. Instead of a heart, lancelets contain large blood vessels that expand and contract to move oxygenated blood to all parts of the body. The veins of the lancelet transfer blood to the gills, where the blood is replenished with fresh oxygen. The urochordata have main ventral and paired dorsal aorta. Their blood lacks hemoglobin and contains no color.


Osmotic Balance

Tunicates and lancelets are both organisms that live in the ocean, a.k.a. in a saltwater, not freshwater, environment. The high concentration of salt in the water leads to huge water loss by osmosis, making the ocean a strongly dehydrating environment for marine organisms. Tunicates and lancelets regulate the concentrations of specific solutes and flow of seawater via the pharyngeal slits and in doing so regulate their internal compositions.


Temperature Balance

Tunicates generally live in the pelagic stratus of the ocean while lancelets burrow into loose gravel and sand in shallow, marine environments. However, both organisms are custom to migrating and relocating within their respective environments. There is no certain mechanism in the tunicate or the lancelet that regulates temperature.


Self Protection

When disturbed or molested, tunicates shoot a jet of water out from the excurrent siphon, hence the common name "sea squirts." In order to protect the vulnerable creature, a tunic, which is transparent or translucent and varies in texture from gelatinous to leathery, surrounds the soft tunicate body. The body wall of an adult tunicate secretes a tunic made of cellulose. Also, when adults, tunicates can develop a thick covering, called a tunic, for protection.


Reproduction

Tunicates are hermaphroditic and the reproductive organs/gonads can form almost anywhere on the organism. Gametes are released from the organism's gonoducts, which in turn empty into the atrium chamber near the anus. The eggs are kept inside the tunicates' bodies until they hatch, while sperm is released into the water where it fertilizes other individuals' eggs. The gametes then develop into non-feeding tadpoles. These tadpole larvae each contain a notochord, a dorsal, hollow nerve cord, and non-functional gill slits. Soon after, the larvae exit the parent's excurrent siphon and attach themselves to a surface. Upon sticking to a surface, a drastic metamorphosis occurs as epidermal cells crush the notochord, nerve cord, and other sensory structures into a mass of tissue. The mouth and anus open as the viscera and dorsal ganglion develops. Little squirts start to feed a few days after settlement.
Unlike tunicates, sexes are clearly distinguished in the lancelets. After the organism reaches maturity, sperm and eggs are released simultaneously in the water where fertilization occurs, a process also known as external fertilization. There is otherwise no parental care involved in the development of the lancelet.

Although ascidians are generally very small (less than 1 mm to 60 cm), some reproduce asexually to form colonies (which can be several meters wide) from a single individual.


Here is an video of an amphioxus (cephalochordate) embryonic development. Notice how the development is almost identical to that of any other chordate group in regards to gastrulation and formation of the germ layers. The notochord is also somewhat developed by the end of the video..


Fun Facts

1. One of the most interesting physical changes that occur within the tunicate is the digestion of its own cerebral ganglion, which is the equivalent of the human brain.
2. Tunicates are the only animals capable of making cellulose.
3. On top of being called sea squirts, tunicates (Urochordates) are also nicknamed sea porks! Not that they taste like pork or anything...
4. Lancelets (Cephalochordates) are commercially harvested in Asia for human and animal consumption.
5.Tunicates contain a host of potentially useful chemical compounds including:Didemnins-, effective against various types of cancer as antivirals and immunosuppressants, Aplidine- effective against various types of cancer.


Review Questions:
1. How big of a role do the gills play in respiration for this phyla?
2. What are the four main characteristics that distinguish chordate subphyla?
3. How do tunicates and lancelets regulate the concentrations of specific solutes and flow of seawater?
4. What types of circulatory systems do tunicates and lancelets have?
5. If lancelets do not have a heart, how does blood, which transports oxygen, travel through the lancelet's body?
6. What distinct/specialized parts do tunicates have that allow them to sense their environment?

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