Shark
Introduction
The shark belongs to the family of fish whose skeletons are composed of cartilage. The earliest shark appeared 450 million years ago, and they have evolved into 470 species (1). Their size ranges from one of the smallest, the spined pygmy shark who is only seven to eight inches long, to the whale shark who is about 50ft in length (2). Sharks breathe through their gill slits, which are located on each side of their body. Sharks lose teeth regularly, and they will grow new teeth for replacement (2).
Sharks can be found in all kinds of seas because they have adapted to different aquatic environments and temperatures. It is uncommon for sharks to live in freshwater, but there are some exceptions. For instance, the bull shark, it can live in both freshwater and seawater (3). It takes sharks twelve to fifteen years to reach their reproductive age. Generally, sharks live for twenty to thirty years, and some species like the whale shark can live up to over 100 years. It is hard to estimate the population of shark, but research indicates that the number of sharks has been declined due to human activities (2).
The shark is a carnivore
Sharks only eat meats. They usually eat fish, marine mammals (see lions, seals), other sharks and etc (3).
The shark belongs to the family of fish whose skeletons are composed of cartilage. The earliest shark appeared 450 million years ago, and they have evolved into 470 species (1). Their size ranges from one of the smallest, the spined pygmy shark who is only seven to eight inches long, to the whale shark who is about 50ft in length (2). Sharks breathe through their gill slits, which are located on each side of their body. Sharks lose teeth regularly, and they will grow new teeth for replacement (2).
Sharks can be found in all kinds of seas because they have adapted to different aquatic environments and temperatures. It is uncommon for sharks to live in freshwater, but there are some exceptions. For instance, the bull shark, it can live in both freshwater and seawater (3). It takes sharks twelve to fifteen years to reach their reproductive age. Generally, sharks live for twenty to thirty years, and some species like the whale shark can live up to over 100 years. It is hard to estimate the population of shark, but research indicates that the number of sharks has been declined due to human activities (2).
The shark is a carnivore
Sharks only eat meats. They usually eat fish, marine mammals (see lions, seals), other sharks and etc (3).
Shark Digestive System
The digestive system of shark has two openings: mouth and the anus, which make the alimentary canal like a one-way tube. Both chemical and mechanical processes are used by sharks to break down food for nutrients. The digestive tract can be divided into four parts: headgut (the oral cavity), foregut (esophagus and stomach), midgut (duodenum and ileum) and hindgut (rectum and cloaca). Generally, carnivores such as sharks have shorter intestines compared to herbivores. Instead, sharks have spiral valves, which increase the surface area of the intestine.
Anatomy (4&5)
Mouth-Since sharks swallow food as a whole or as large pieces, very little physical digestion happens in the mouth. Therefore, there is no report on secretion of digestive juices in the mouth.
Esophagus-Connects the oral cavity and pharynx with the stomach. It has "finger-like extensions", which prevents food from going out of the mouth. There are striated muscles, so esophagus can stretch when swallowing.
Stomach- Food is stored in stomach and digestion starts here. It secrets strong acids and enzymes (gastric acids and pepsinogen), which helps to digest what is eaten and turns food into liquid mush.
Pyloric Valve- The valve between stomach and the small intestine. Mush goes into intestine through this valve. The valve is small in size, so only liquid mush can enter the intestine.
Small Intestine- It consists of duodenum and valvular intestine (ileum). Duodenum is a short portion of the small intestine, which connects the stomach with the small intestine. Valvular intestine is the larger portion of small intestine. Compared to herbivores, the intestine is relatively short because meat is easier to digest than plants. There is "spiral valve" in the intestine, which is "screw-like and symmetrical shaped". The spiral valve adds surface area to enhance digestion and absorption.
Colon- Continuation of the small intestine.
Rectum and Cloaca-Collects waste products from colon and leads to excretion.
The digestive system of shark has two openings: mouth and the anus, which make the alimentary canal like a one-way tube. Both chemical and mechanical processes are used by sharks to break down food for nutrients. The digestive tract can be divided into four parts: headgut (the oral cavity), foregut (esophagus and stomach), midgut (duodenum and ileum) and hindgut (rectum and cloaca). Generally, carnivores such as sharks have shorter intestines compared to herbivores. Instead, sharks have spiral valves, which increase the surface area of the intestine.
Anatomy (4&5)
Mouth-Since sharks swallow food as a whole or as large pieces, very little physical digestion happens in the mouth. Therefore, there is no report on secretion of digestive juices in the mouth.
Esophagus-Connects the oral cavity and pharynx with the stomach. It has "finger-like extensions", which prevents food from going out of the mouth. There are striated muscles, so esophagus can stretch when swallowing.
Stomach- Food is stored in stomach and digestion starts here. It secrets strong acids and enzymes (gastric acids and pepsinogen), which helps to digest what is eaten and turns food into liquid mush.
Pyloric Valve- The valve between stomach and the small intestine. Mush goes into intestine through this valve. The valve is small in size, so only liquid mush can enter the intestine.
Small Intestine- It consists of duodenum and valvular intestine (ileum). Duodenum is a short portion of the small intestine, which connects the stomach with the small intestine. Valvular intestine is the larger portion of small intestine. Compared to herbivores, the intestine is relatively short because meat is easier to digest than plants. There is "spiral valve" in the intestine, which is "screw-like and symmetrical shaped". The spiral valve adds surface area to enhance digestion and absorption.
Colon- Continuation of the small intestine.
Rectum and Cloaca-Collects waste products from colon and leads to excretion.
Physiology (5)
Rectal gland (See 1.1)-A special feature of sharks. It is a finger like structure, which "opens into the intestine-rectum posterior to the spiral fold". It regulates shark's salt balance by excreting NaCl in areas where concentration of salt is higher than that of the shark's body fluids.
Control of juice secretion in stomach-The secretion is stimulated by cholinergic drugs and histamine. The secretion is inhibited by the stimulation of sympathetic nervous system and adrenergic drugs due to the reduced blood flow.
The function of exocrine pancreas-When food goes into the intestine, the exocrine pancreas secrets enzymes and releases them into duodenum. Some examples of digestive enzymes include, peptidases like trypsinogen, elastase and etc. The secretion of pancreatic juice is stimulated by HCl.
Rectal gland (See 1.1)-A special feature of sharks. It is a finger like structure, which "opens into the intestine-rectum posterior to the spiral fold". It regulates shark's salt balance by excreting NaCl in areas where concentration of salt is higher than that of the shark's body fluids.
Control of juice secretion in stomach-The secretion is stimulated by cholinergic drugs and histamine. The secretion is inhibited by the stimulation of sympathetic nervous system and adrenergic drugs due to the reduced blood flow.
The function of exocrine pancreas-When food goes into the intestine, the exocrine pancreas secrets enzymes and releases them into duodenum. Some examples of digestive enzymes include, peptidases like trypsinogen, elastase and etc. The secretion of pancreatic juice is stimulated by HCl.
References:
1.http://www.sharksavers.org/en/education/biology/450-million-years-of-sharks1/
2. http://www.defenders.org/sharks/basic-facts
3.http://www.enchantedlearning.com/subjects/sharks/anatomy/Diet.shtml
4.http://www.pc.maricopa.edu/Biology/ppepe/BIO145/lab04_2.html
5. Hamlett, William C. Sharks, Skates, and Rays: The Biology of Elasmobranch Fishes. Baltimore, MD: John Hopkins University Press. 1999. Web
1.http://www.sharksavers.org/en/education/biology/450-million-years-of-sharks1/
2. http://www.defenders.org/sharks/basic-facts
3.http://www.enchantedlearning.com/subjects/sharks/anatomy/Diet.shtml
4.http://www.pc.maricopa.edu/Biology/ppepe/BIO145/lab04_2.html
5. Hamlett, William C. Sharks, Skates, and Rays: The Biology of Elasmobranch Fishes. Baltimore, MD: John Hopkins University Press. 1999. Web