Shark+intro

The **taxonomy** of the spiny dog fish: Kingdom: Anamalia Category: Fish Phylum and subphylum: Chordata, Vertebrate Class and Subclass: Chondrichthyes, Elasmobranchiomorphi Order: Squaliformes Family: Squalidae Genus: Squalus Species**:** Acanthias

__History of the class Chondrichthyes__

Class Chondrichthyes, one of the most unique classes in the taxonomic system, presents a similar anatomical structure to that of humans. For this reason, this dihssection is based on the spiny dogfish shark; the most abundant species of shark in the world. The specific characteristic of class Chondrichthyes that sets them apart from the other classes is their unique skeletal system, comprised totally of cartilage rather than bone. The majority of species in this class have bodies covered in dermal denticles to enhance their locomotive efficiency in water. They also have a two-chambered heart, and a unique digestive system. Their brain relative to their body size compares closely to that of mammals. These jawed fish with paired fins are most often found in coastal habitats near the bottom of the ocean floor. Class Chondrichthyes is comprised of four main groups: rays, skates, chimaeras and sharks. Rays and skates are similar in structure; both flat-bodied. They survey the ocean floor for their daily meals. Skates have dark coloured upper surfaces and lighter coloured lower surfaces. Chimaeras have smooth skin and lack scales unlike the other members of this class. They resemble fish and grow up to a length of two meters and range from black to brownish grey in color. They also have a venomous spine, located anterior to their dorsal fin. Class Chondrichthyes contains mainly sharks, and some rays in that grouping. There are two sub classes which include Holocephalii and Elasmobranchii (with 600 species in it). Holocephalii subclass has species in the group from 180 million years ago, so one can conclude some of the species in the class Chondrichthyes have been alive for a long time (IV century BC). The class dates back 450 million years ago, prior to the age of dinosaurs. Fossils have helped scientists provide educated assumptions that early species of class Chondrichthyes had bones as opposed to cartilage, which is present in the modern species of this class. Early species had bony plates in their mouths that later developed into teeth. It is also believed that dermal denticles, like large bony plates of armour, migrated into the mouth to form these teeth. There is around 465 shark species and there has never been a decrease in the spiny dogfish. This species is not threatened for extinction since there is such a wide range of living area for the spiny dog fish, and the other species. Sharks, the focus of our research, are the largest fish and grow up to 15 meters in length. They have no bones, and have leathery skin covered in tooth like denticles. They also have a closed circulatory system. In order to continue breathing, they must continually move to flush out of their gill slits through which they breathe. Sensory organs are important to this class and they have many ways of protecting themselves. Sharks noses are very good at smelling, so they may use this for finding food. Sharks, in particular, can sense vibrations in the water, therefore if there is any movement under water they are likely aware of it. The spiny dog fish even have little hair like structures, called lateral line canals that allow for the feel of motion in the water and can pick up changes in water pressure. This helps for preying on other fish. There is no research shown that there have been distinctive changes in the classes’ sensory perception. Scientists have found that the vision, smell, pressure, taste and sound have never been bad. The fact that most of the species in this class are not threatened for being extinct demonstrates that their senses have been reliable for protecting and finding their own food. Sharks usually have 5 to 7 gill slits and jaws that are not fused to the braincase. Their tales are asymmetrical, meaning the upper lobe is larger than the lower lobe. Sharks also lack a swim bladder, but have a large oily liver that aids them with buoyancy. The circulatory system of a shark comprises of a two-chambered heart, containing one atria and one ventricle. The heart is S-shaped and located at the head region. The blood is circulated throughout the shark’s body by veins and arteries. Blood is pumped from the ventral aorta to the capillaries in the gills to be oxygenated, and flows through the body by way of paired dorsal aorta. The blood then flows back to the heart through veins. A feature of the class Chondrichthyes is that it has slowly changed its outward appearance, as well as many other characteristics, such as the hard, teeth-like plates that used to be placed on the shark’s back and body. These migrated to the mouth area to assist in feeding. The three large triangular teeth, deltodus angularis, on the back of the ancestral sharks have over the years slowly gotten smaller in size, eventually pairing off and migrating to the mouth. However, not all of the teeth moved to the mouth, which is why the sharks still have rough skin when you stroke them from tail to head. On the other hand, other research shows that the migration of the teeth occurred in opposite sequence to the order previously explained. It says that the teeth started originally in the mouth and moved out of the mouth to form the dermal denticles, which make their skin feel like sandpaper, but there is not strong enough evidence to prove either situation is more correct over the other. Either way this change caused a change in the musculoskeletal configuration of the shark and the class Chondrichthyes. The musculoskeletal area in the shark was a cause for many changes, the main one being the physical change of the upper jaw, protruding out further than normal. Because the teeth have moved, either from the back to the mouth or vice versa, it has caused the shark to adapt to different types of fish for its diet. Since they used to eat small animals such as molluscs, as they evolved they increased in size. These small crustaceans which they ate required the use of different muscles in the mouth than the ones used to digest larger crustaceans and fish which they started eating once their teeth changed. After an extended period of consuming crabs and fish, the muscles in the mouth gradually conformed to the new diet by making it use a lesser amount of energy to digest them. This conformation resulted in the protrusion of the upper jaw. The spiny dog fish feeds on typical species that share the same environment as itself. It is known that they eat octopus, cod, herring, ctenophores and a lot of squid. As the spiny dog fish mature, there seems to be less consumption of the ctenophores than when they were younger. The diets do change from male to female, yet there is not a huge difference between their eating habits, but other species in the class could have because of their protruding jaw. Through the years, with the changes of the eating habits of the class Chondrichthyes, there have also been internal changes. This is related to the amount of food consumption, the surface area of internal systems, and the bladder. There have been internal changes of the class Chondrichthyes that involve the digestive tract, and also the excretory system. They usually swallow food whole or in large pieces. The stomach, which is U-shaped, is where most of the digestion takes place due to its acidity and the enzymes it contains. Indigestible matter, such as bones and non-nutritional matter are vomited later. The nutrient absorption occurs in the intestines. The intestines are not very long, but they have folds and spiral valves to increase the surface area for optimal absorption of nutrients. This, in turn, helped the sharks rise to the top of the food chain in the water environment. Some water bound vertebrates evolved swim bladders, yet some of them turned into lungs. The spiny dog fish, for example, has gills that allow the blood to be oxygenated, therefore they do not have lungs. Their ancestors had more complex lungs and their use of gills where not as important. The swimming bladder allows for pressure to be equalized, and to allow the animal to float. Gas is put into the blood and either causes the animal to float or sink. The gills allow for a large surface area, therefore allowing for gas exchange and relate to the swim bladder. Certain animals have had to adapt due to the fact that they only do have lungs. Whether it is related to other changes in the body, it has been found that their external body has been adapted (laterally compressed) so that it can move easier. The movement would be in the water, and with the lateral compression it can push the water away faster, therefore increasing the possible speed from before these changes occurred. There have been specific external adaptations to the spiny dog fish. This includes the colour of the skin. It’s the dorsal darker area of the skin that helps with protection from larger predators and hiding from its prey. They are able to hide, in a way, at the bottom of their environment. The ventral side of the skin, on the other hand, is lighter. The colour of their skin has been adapted over millions of years for the species’ own protection. Overall, there have been many adaptations, externally and internally of the class Chondrichthyes. Sharks can reproduce in a number of ways: oviparity, viviparity, and ovoviviparity. Oviparity is the process of reproduction whereby the embryo is protected by a leathery egg case. Viviparity is the reproductive cycle whereby young are born alive and fully functional. Ovoviviparity is the cycle in which the embryo is nourished by the yolk of their egg and the eggs hatch within the oviduct. The young continue to be nourished by the yolk and oviduct fluids after hatching and can often be functional before being born. Depending of the species of shark, the style of reproduction varies. There are more than 350 different species of sharks belonging to Class Chondrichthyes. The lab will be primarily based on the most abundant of these species, the spiny dogfish shark. This shark measures approximately 75 cm to 105 cm and weighs in at 9 kg. This species of shark has an estimated life span of 25 to 30 years and migrates seasonally usually around the north-eastern coast of North America.

http://www.woodbridge.tased.edu.au/mdc/Species%20Register/class_chondrichthyes.htm http://www.ucmp.berkeley.edu/vertebrates/basalfish/chondrintro.html http://en.wikipedia.org/wiki/Chondrichthyes