Shark+External

water from these pores that continue down to the posterior of the body. These pores are very small and cupped shaped. This was even one of their adaptations throughout the millions of years they have been alive. Some research even shows that the pores, in some areas, may be taste buds. Some of the pores are lateral canals but they are located on the lateral area of the body. This is part of the shark’s electro sensory system, since it is incorporated with the perception of the animal and electric forces.

Observe that there are three main parts to the spiny dogfish. To begin, there is the head region. It begins at the anterior region of the snout, and then follows down on the dorsal side, and ends at the lateral gill slits. The last gill slits are called the caudal gill slits and are numbered from one to five, one being the most cranial and five is the most caudal gill slit. The second main region of the spiny dog fish is the trunk. This is from the gills slits just explained to the cloacal opening, which is present in both the male and female. If it is a male the opening can be viewed just below the claspers. The last region of the shark is called the tail. This is from the caudal dorsal fin to the Heterocercal tail. Notice from the explanation of the three main parts of the body that the gills slits are on the lateral area of the shark. The purpose of the gill slits are for respiration and also circulation. The main reason for gills are for oxygen to enter the body and the blood. The skin also does allow for respiration, however, the gills do most of the work. When the gill slits open, the pharyngeal chamber give out the water, which allows the shark to breath. A very important fin is the large fin found at the end of the tail. It is broken down into two different parts. The epichordal lobe is the larger part of the tail located on the dorsal side of the shark, where as the hypochordal lobe is the smaller part located on the ventral side. Note how the epichordal lobe is extended from the body of the shark forming heterocercal-type tail configuration which aids in the sharks movement. Other fins that help in the locomotion of the shark are the lateral, paired fins. There are two sets of paired fins: the pectoral fins, which are located just behind the gill slits, and the pelvic fins which are found beside the cloacal aperture. Both sets of fins act as stabilizers and are used for steering when the shark is in motion.

Observe that the shark has two lateral eyes. They are located on either side of the head to prevent their eyes from going cross-eyed and jeopardizing their already poor eyesight. Also, please identify the spiracles and spiracle valves found caudal to the eyes. Their purpose is for allowing the shark to receive extra water to the pharynx, especially when the mouth is buried in the mud at the ocean floor. After observing the ventral side of the shark, start to me around the teeth and view the crescent shaped mouth. At the corner of each side of the mouth locate the small flaps known as the labial pockets. These are cartilage folds that serve the purpose of lips in that they hold food in the mouth while the shark is chewing. Inside the mouth you will find several tiny teeth attached to the jaw. Although they are small the teeth are quite sharp so they can efficiently break down their food, like hermit crabs and mollusks, easier. After locating these parts of the shark, move on to observe the skin. Once the overall external anatomy of the shark is done notice that the shark skin consists of two different parts, the outer epidermis and the inner dermis. The epidermis of the shark is structured in several layers of live cells. Cells in the top layer of skin are constantly being lost for various reasons so the cells in the deepest layer (stratum germinativum) have to go through the process of mitosis quite frequently in order to replace them. The dermis is the layer of skin under the epidermis and consists of connective tissue cells as well as the cells that depict the color of the shark’s skin, chromatophores. Now you should draw your attention to the texture of the surface of the skin. If you stoke the dorsal side towards the tail, it feels smooth; however, if you stroke the dorsal side towards the head you will realize the skin is rough, almost like sandpaper. This is because the shark has teeth like structures, known as placoid scales, all along their back. To see these placoids, cut off a piece of skin and observe it from under the magnification of a dissecting microscope. Not only are they similar shape to the teeth, they also have dentine and enamel layers on the outside; although, they do differ in that the placoids are filled with pulp. In sharks and rays there is a specific molecule that is found internally in the body. This is a molecule known as trimethylamine oxide and it has a specific task for the shark. Sharks and rays live in the water during their life. Both the environment the sharks live in and their bodies contain salt. From the concept of osmosis, certain substances and concentrations would like to cross over gradients to reduce concentrations. TMAO is involved with the protein metabolism and keeps the shark’s water in their body. This allows for less posibility of dehydration in the shark. TMAO is a way to break down nitrogen and it is a method to hold waste and urea, so that the sea is less salty than the internal of the shark. This way there is no loss of important nutrients or water from osmosis. TMAO is specifically used for sharks, and the spiny dog fish would likely not survive without this molecule helping their metabolism.

http://www.zoology.ubc.ca/courses/bio204/dogfish_external_anatomy.htm http://www.mbayaq.org/PDF_files/activities/seasearchers/aquarium_ss_sharksskatesrays.pdf [|__http://graysreef.noaa.gov/tw/sharks.html__]

Ada, Kortnie and Shawn


 * __Day 1: External Anatomy of Shark__**

The main goal of the first lab day is to basically acquaint you with the external anatomy of the spiny dogfish. It is necessary to understand how the external structure is characterized by identifying and observing certain features. This will allow you to have a greater knowledge of how the features operate and complete their specific task(s), which in the end contributes to the proper functioning of the entire organism.

To begin with, set up your dissecting station with all the necessary materials and place your shark on its ventral surface. Don’t be alarmed by the foul stench. It is actually created by TMAO (Trimethylamine //n//-oxide) which is a compound of tissues found in marine animals and during spoilage begins to give off this “fishy smell”. At first glance of the shark one may notice two things: the general shape of the specimen and the skin color distribution. The fusiform shape of this specimen also applies to all other shark types. It is like so to basically aid in the locomotion of the shark. The two different skin pigmentations is also a common characteristic. It is presented in such a manner to provide camouflage by allowing the shark to blend in with the dark depths of the water when viewing it dorsally, and allowing it to blend with the sunlight surface of the ocean when peering ventrally. One last aspect that you may catch is the fact that their body can then be sectioned off into three main parts: the head, the trunk, and the tail. After making these very broad observations you may now begin to look at individual specific structures. Start off at the very anterior of the shark while keeping its ventral surface facing downwards. This first part to be labeled should be the rostrum. The rostrum is the snout of the shark and is curved like so to allow for superior locomotion, and may we not forget that sharks are famously known for this attribute. Moving along posteriorly you will find the eyes (which are located at a more lateral angle). Sharks generally have a lousy sight but that doesn’t set them back any since the rest of their senses are outstandingly high, especially their sense of smell. Next to come you will find the spiracles which are an extra entry by which water can come through and reach the pharynx allowing for more water to be taken in and go through the respiratory cycle. Close by will also be the five gill slits and their valve flaps. The gills are also more laterally positioned and they are the most important members of the respiratory cycle. What happens is that water is taken in either by the mouth or spiracles and stays within the mouth cavity; oxygen is then extracted from the water which is then forced out through the gills. Before leaving the head, turn your specimen to a lateral position. Located between the spiracle and the first gill slit is a section of skin that has a couple of darker patches creating a wave-like boarder between light and dark skin. These spots contain specialized pores called the Ampullae of Lorenzini. They serve a purpose of detecting temperature fluctuations and weak electrical fields by acting like sensors. To clearly define which pores they are, squeeze the rostrum causing mucous droplets to ooze out of the tiny pores. Moving on to the trunk of the body there are four different fins (two of them are actually more on the ventral side, however they are still very visible) to observe. Beginning with the most cranial fin is the pectoral fin. There is actually a pair of them and can be observed with a greater perspective on the ventral surface since it is where they are actually situated. The pectoral fins along with the pelvic fins act as the steering devices while there is motion and they also provide stability for the organism. Moving along in with your original path you will come across the cranial dorsal fin and directly posterior to it, the caudal dorsal fin. Just like all other fins they stabilize and steer, however these ones carry a very special feature. They are both equipped with spines which come out of the anterior end of each fin. These spines serve a purpose of enhancing locomotion by slicing through water, and more importantly by providing protection. These specialized spines contain venom which if injected with can possibly cause death for humans. In between these two dorsal fins you will notice the pelvic fins coming outwards from the ventral area. Once again you will need to observe this pair of fins by flipping over the shark. Perhaps someone by now has maybe noticed that there is a faint yet distinct line light in colour running along the side of the dogfish. This is known as the lateral line. It contains a tiny canal where one would find numerous sensors that are able to detect low frequency disturbances in the water. Last to be observed on the dorsal side would be the tail which is at the very posterior of the shark. This unequally divided tail is composed of two lobes: the bigger one being the epichordal lobe and the smaller is the hypochordal lobe. The tail only has one function and it is for locomotion. Now that you have successfully viewed the dorsal area, you may now turn over your shark exposing it ventrally. Once again begin at the posterior end and work your way down. South of the rostrum, the first feature you will look at are the external nares. As you can see there are two holes. The distal one is called the incurrent aperture and it is the entry by which water can enter and flow through the internally located olfactory sac. Division by the transverse flap creates the exit way called the excurrent aperture. Moving along you will come across the mouth. Interestingly sharks, just like humans, also have lips but of course theirs are much less noticeable. They are only found at the corners of the mouth and appear to be two simple folds interiorly lined with thin cartilage. When you open the mouth to observe the inside, watch out for the shark’s sharp teeth. Take a closer look and notice that there are numerous rows of them. It is like so because sharks are constantly damaging and loosing teeth so it is quite economic to have an extra set ready to take over immediately. Also make note of their structure and appearance since near the end of today’s dissection they will be compared to the denticles (a.k.a. the placoid scales) of the spiny dogfish. As you travel caudally, find the pelvic fins and focus your attention to that area. If you pull these two fins apart there will be three (at the most) parts exposed for you to label. Starting at the anteriror, which will be right where the two fins attach, you will see a flap of skin called to cloacal aperture. It is a vent that opens to the cloaca found inside which is the chamber where fecal material is stored. Just under the cloacal aperture, depending on the gender of your shark, you will find either the urinary papilla or the urogenital papilla. Both exert the task of releasing urinary products from the kidneys; however the male’s papilla also receives sperm cells from the testes. The third trait to investigate will only be present on males, therefore it creates an accurate way of determining the sex of your dogfish. Sticking out from the ends of the pelvic fins you will find two long, extended organs which are used during mating as a means to eject sperm into the females genital duct. The external anatomy of the spiny dogfish has now been carefully inspected, however there is one last observation yet to be made. If you place your dogfish on its ventral surface you will be able to run your hand up and down the dorsal skin of the specimen. Does the texture feel familiar? Maybe so since you have more than likely once felt sandpaper. Shark skin actually used to be manufactured into sandpaper until animal rights put a halt to the production. To view the components of the rigid skin, cut a piece of 3cm x 3cm skin, but be careful not to remove too much of the underlying musculature. Using a petri dish, place the skin under a dissecting microscope and set the magnification at 4X. As you peer closely you may notice tiny teeth-like structures. These spikes are the denticles which create the sensation of sandpaper on the shark’s surface. If you remembered from before it was these denticles which are to be compared with the shark’s teeth. From researching one may come up with the fact that a shark tooth consists of an outer layer of enamel, a layer of dentine, and a central pulp cavity. Surprisingly these are the exact three components which create the denticles. A shark’s tooth is composed like so because of its rugged eating manners. Sharks do not chew their food but rather swallow it, so they need hard, sharp teeth to tear the meat into bite sized pieces. Therefore, it may seem quite strange to someone that the denticles are perfectly modeled right after the shark’s most vicious asset, but perhaps the answer lies in the evolutionary history. Millions of years ago, the first noted existing sharks had an interesting attribute. As a means of powerful protection these ancient sharks were covered with huge platelets constructed like their own sharp teeth. It is assumed that back then they must have been more commonly attacked by other predators. As the years gone by, sharks have successfully found a way to “beat the system” and become almost predator-free. Hence, there is no need for such extravagant protection so as sharks have evolved, these plates have diminished in size and have become the denticles they are today.

Day 1 of this lab has now come to an end so it is important to leave off with a few tips on how one should deal with completing a day of the shark lab. First of all pay attention to your lab manual! If you don’t read it carefully before starting the dissection it’ll be like walking blindfolded. Your manual has been created to help with the dissection and give straightforward directions so you won’t become confused. Familiarize yourself with the vocabulary. This will provide assistance when making incisions and locating certain parts. Make sure to communicate with your lab group. Know exactly who is doing what and when you will all be meeting up to work on the lab. This will also lead to preventing procrastination, which is probably the worst thing one could do when completing dissection labs. Don’t be afraid to get messy. You learn most when actually doing the dissecting since it will allow you to quickly become acquainted with the surrounding structures, nonetheless watch out for getting overly excited and inattentively cut out a little too much than needed. Last and most important of all, don’t be afraid to ask questions or for help when it is truly needed. The instructor is there to aid you, nevertheless make sure not to become too dependent on this assistance. In the end you will find that such awareness comes naturally anyways so don’t be frightened and have fun with the lab.