Dogfish Shark (Chordata)
By: Gabrielle Crosby


smooth dogfish shark
smooth dogfish shark


Table of Contents1. Classification/ Diagnostic Characteristics
2. Relationship To Humans3.Habitat and Niche4. Predator Avoidance5. Nutrient Acquisition6. Reproduction and Life Cycle7. Growth and Development8. Integument9. Movement10. Sensing the Environment11. Gas Exchange12. Waste Removal13. Environmental Physiology14. Internal Circulations15. Chemical Control16. Review Questions


Classification/ Diagnostic Characteristics

The Dogfish shark in of the phylum Chordata meaning it has a notochord. It is a jawed vertebrae in the class chondrichthyes; meaning it has a flexible skeleton made of cartilage and paired fins. Some major features of vertebra are their anterior skulls protecting large brains, well developed circulatory systems driven by ventral heart contractions, and internal organs suspended in coelom. Present in embryos, the notochord is a longitudinal, [[#|flexible rod]] between the digestive tube and and nerve cord. The notochord serves the purpose of supporting the body of the Dogfish shark as it is made of large, fluid [[#|filled]] cells surrounded by stiff, fibrous tissue. Typically, the adult shark develops a jointed skeleton and very little of the notochord remains. In addition to the notochord as a pertinent characteristic, these sharks also have a hollow dorsal nerve chord, pharyngeal slits and muscular, postnatal tails. The nerve chord of the embryonic shark develops into a tube behind the notochord that functions as the highway for the central nervous system. On either side of the pharynx, a portion of the digestive tube that sits just behind the mouth, are pharyngeal slits; acting as water passages that connects the sharks interior and and exterior without passing the water through the digestive system, these slits aid in gas exchange, jaw support, and hearing. Sharks also have tails that extend past the anus, contain many layers of muscle tissue, and aide propulsion.
The dogfish shark may be identified by its two dorsal fins which are in line with each other on the top of the shark, its two pectoral fins towards the front. a single pelvic fin towards the bottom rear, and a caudal fin at the end of the tail.


external image fig008.jpg


Relationship To Humans

They are commonly found in aquariums. Humans and dogfish sharks share the traits of a jaw and backbone, and it is likely that humans diverged from sharks soon after the evolution of these traits. Also, they are used, in primarily Britain, in its iconic dish of fish and chips. This hunting creates fear that they may become endangered species.Spiny dogfish are common in the Mid-Atlantic region and are considered [[#|pests]] by commerial and sportfishermen because it has no great value on the local market and doesn't put up much of a fight when hooked.



Habitat and Niche

A marine animal, it cam be found in many saltwater biomes around the world. When not eating or swimming, it can be found resting on the sea floor burred under a thin layer of sand. Their location varies widely from costal to oceanic waters and from cool temperate to deep tropical waters. They can be found at extreme depths (up to thousands of meters).



Predator Avoidance
A dogfish shark who was unable to avoid a predator, in this case a sea lion.
A dogfish shark who was unable to avoid a predator, in this case a sea lion.

Sharks have exceptional senses, which aid in the avoidance of predators. This animal has extremely sensitive hearing that can identify much lower sounds than human ears can detect, and can hear more than 700 feet away. Scientists think hearing is typically the first sense sharks use in detecting their prey and avoiding their predators. Sharks also have a remarkable sense of smell that helps them track a trail of microscopic particles of protein or blood, and help them identify a predator that hunts them. Dogfish shark have very sharp teeth that are very effective against predators and maintaining its position as top dog in the food chain. Predators of dogfish shark include humans of course, larger sharks, elephant seals, and Giant Pacific Octopus. The shark also has sharp spines in front of the dorsal fins that can be used defensively; it curls the body, exposing the spine and thrusting at the attacker. Dogfish Sharks use their spines as a defense mechanism. They curl their spines around in a bow and strike the enemy. The spines are also thought to be somewhat poisonous.





Nutrient Acquisition

Preying upon fish and sometime shark, it is a carnivore. Food is taken in through the mouth, travels down the esophagus into the stomach, which partially break it down, and it is further processed in finger-like pouches called pyloric ceca. These pouches secrete [[#|digestive enzymes]] and absorb nutrients from food. The liver and pancreas also secrete digestive enzymes to help breakdown food as it passes through the digestive track. The intestines are the final [[#|step]] of this system that breaks down and absorbs nutrients before the undigested leftovers are excreted through the anus. Thought sharks have relatively short intestines, they have ilium in the intestines lower region, which have spiral valves to increase surface area. The sharks jaw helps for grasping, subduing, and swallowing large prey.



Reproduction and Life Cycle


Life Cycle of a Dogfish Shark
Life Cycle of a Dogfish Shark


Dogfish sharks usually have a life span of 25-100 years and reproduce through the method of Aplacental Viviparity. This means that the babies hatch from eggs, but they hatch and develop inside the female's body. About 2-11 pups are in each litter and the pups are between 8-12 inches. Male dogfish reach maturity at 11 years while female dogfish reach maturity at 19-20 years.











Growth and Development

These sharks sexually reproduce. Eggs are fertilized internally and embryos obtain nourishment from yolk while developing inside female making shark viviparous The babies are 'born alive'. Characteristically of chordates, during early development, the mouth forms at one end of the embryo opposit to the blastopore, which becomes anus.
Female dogfish sharks are ovoviviparous live-bearers, which means they retain their eggs inside the body until the young hatch, which are then born "alive." They produce large, yolky eggs called ova. These ova erupt from the ovarian surface when they are ripe,and travel tailward down the mullerian ducts until they reach a swelling called the nidamental gland. In the nidamental gland, the eggs are fertilized by sperm stored from an earlier copulation. At the site of fertilization there develops a plate of cells termed a blastodisc, which will eventually develop into an individual dogfish. The blastodisc folds up on itself, from back to front, to form what will develop into the cartilaginous backbone and associated structures. Continuing to develop from tail to head, the backbone differentiates into a skull at the front and adds an arch above that protects the spinal cord and another arch below that protects the body's main blood vessel, the dorsal aorta. As all this is going on, the embryo develops a stalk separating it from the yolk sac and a network of blood vessels to transport yolk that fuels its development. As the embryo develops, it continues to add complexity and becomes a fetus, using up its yolk supply as it grows. Development lasts for about 12 to 24 months (depending on water temperature) until the fully-formed pups are ready to be born alive. This method provides the young with protection from predators during their earliest developmental stages.
Like most sharks, Dogfish sharks reproduce and grow relatively slowly. Sharks have relatively few (zero to around 100) offspring each year, and the mother invests much energy in each to increase the chance that it will survive. Some female sharks put so much energy into a litter that they must take two years to recover their strength before breeding again! Although young sharks are born relatively large and able to take care of themselves, they grow slowly, sometimes only a few centimeters a year. It may take 15-20 years for an individual to reach sexual maturity. Such low reproductive rates and slow growth combine to make sharks highly vulnerable to over-fishing.




Integument

Tooth-like scales, called denticles, cover the exterior of the shark making it as rough as sandpaper but still flexible allowing protection from its predators. The scales are also used for increasing movement efficiency as they help to reduce friction while swimming.

Extreme close-up of great white shark skin denticles.
Extreme close-up of great white shark skin denticles.



Movement

In addition to the muscular post anal tail, the shark moves by way of lateral undulation, alternating contracting muscles on either side of the backbone. This creates an S shaped wave that moves down the fish body to help propulsion. Paired fins act as stabilizers and rudders to help as well as providing a speed boost.. A streamline body decreases friction. Because the body tissue of a shark s denser than that of the water it swims in, sinking is a potential issue, but these fish contain an organ beneath the backbone called the swim bladder, which adjusts for buoyancy, allowing the shark to float at any depth without expending much energy. Sharks also have denticle scales that are rough to the tough, featuring numerous grooves that are able to reduce drag. This is because the grooves facilitate smooth flow of water over the scales, rather than causing drag. This provides a movement advantage to sharks.




external image shark_skin.png
Sensing the Environment

Fish have well developed nervous systems with a large brain as the focal point. Connected to two lobes of the cerebrum, olfactory bulbs in the interior part of the brain are used for sense of smell. In fish the cerebellum is primarily used for sense of smell. The cerebellum coordinated body movement. They have well developed eye, comparable to those of a human, that are capable of color vision. Specialized cells called chemoreceptors that detect particular ions and molecules enhance the sense of taste and smell. The lateral line system, which are sensitive receptors that detect currents and vibrations in the water help fish to sense the motion of prey. Sharks have also developed the ability to detect low electric currents.










Gas Exchange

The gill slits at the sides of their head is the site of gas exchange. The oxygenated water flows over the gill filaments after passing through the mouth and pharynx. Gas exchange occurs and the deoxygenated water leaves through the gill slits. The gills are made of feathery, threadlike filaments on either side of the pharynx, oxygen rich water is pumped over gills and the water diffuses into deoxygenated blood vessels.
Sharks in are capable of delivering oxygen more efficiently to their muscles because of their blood composition. Unlike other types of bony fish, sharks have fewer but larger red blood cells and its hemoglobin has twice the affinity for oxygen. Like many other chordates, they also maximize gas exchange through counter-current exchange.





Waste Removal

The digestive system of the dogfish shark is extremely similar to that of the human. It consists of the oral cavity, pharynx, esophagus, stomach, intestines, and anus. Accessory organs include pancreas, gallbladder, and liver. The end portion of the large intestine is followed by a muscular rectum hidden under the pelvic girdle. The anus is controlled by sphincter muscles at the base of the tale. The kidneys filter waste from the blood stream. Nitrogenous waste is converted to ammonia and diffuses through the gills.





Environmental Physiology (temperature, water, salt regulation)

Creature that live in water of high salinity face the issue of water loss through osmosis, where water diffuses down its concentration gradient from a place of lower to higher concentration for other substances in the water. Sharks can convert nitrogenous waste to urea and trimethylamine, large amounts of which are kept in the blood system so the bodies osmolarity is close enough to that of the surrounding salt that water does not diffuse out. Excess salt is eliminated by a rectal gland.





Internal Circulation
This Picture can help illustrate the counter-current exchange.
This Picture can help illustrate the counter-current exchange.

This image shows the internal circulatory system of the dogfish shark.
This image shows the internal circulatory system of the dogfish shark.

Sharks have single looped closed circulation systems. Blood is pumped by the heart to the gills onto the rest of the body and back to the heart, which has four parts: the sinus venosus, a thin walled sac that collects blood from ventricles before sending them to the atrium; the atrium, a large, muscular 1 way chamber that sends blood to the ventricles; the ventricle, a think chamber that pumps blood to bulbs; and the bulbous arterious, which is connected to the aorta and sends blood to the gills. The single circuitry lowers the pressure and speed of blood traveling through the body and limits the capacity of the circulatory system. The shark has internal gills, meaning they are protectively encased with a body cavity. Blood enter the body by means of counter-current exchange, which increases the gradient, making diffusion more efficient. Water flows unidirectional from the mouth perpendicular to the blood vessels which flow in opposite directions depending on whether they are carrying oxygen rich blood to the body or deoxygenated blood from the heart. Oxygen diffuses first down its gradient from water to deoxygenated vessel then to other vessels. Gills have hundreds of filaments with thousands of folds called lamellae, where the gas exchange actually occurs. Sharks swim constantly to ventilate gills.













Chemical Control (i.e. endocrine system)

Dogfish Shark contain an adrenal gland that is located above the kidney, whose functions is to release adrenalin or epinephrines. Like humans, sharks have ovaries or tests (depending on sex) which secret sex hormones.



Review Questions

1. What are the finger-like pouches that help digest food within the shark's body called?
2. The body tissue of a dog fish shark is denser than that of the water it resides in. What mechanisms prevent it from sinking and allow it to move through the water?
3. Where are the digestive enzymes secreted in order to breakdown food?
4. In the reproduction cycle, the pup swims away and basically lives on it's own without any help from the mother shark right after the egg is hatched. Are these sharks adaptively born to recognize their surroundings and know how to obtain food and live?
5. Compare and contrast the nervous system capabilities between the dogfish shark and human.



Citations
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http://rjd.miami.edu/assets/pdfs/learning-tools/high-school/MODULE%202%20Ichthyology%20-%20SECTION%205%20Shark%20Anatomy%20and%20Dogfish%20Dissection.pdf

http://rjd.miami.edu/assets/pdfs/learning-tools/high-school/MODULE%202%20Ichthyology%20-%20SECTION%205%20Shark%20Anatomy%20and%20Dogfish%20Dissection.pdf