The ampullae detect weak magnetic fields produced by other fishes, at least over short ranges. This enables the shark to locate prey that are buried in the sand, or orient to nearby movement. Recent research suggests that the ampullae may also allow the shark to detect changes in water temperature. Each ampulla is a bundle of sensory cells that are enervated by several nerve fibers. These fibers are enclosed in a gel-filled tubule which has a direct opening to the surface through a pore.
The gel a glyco-protein based substance has electrical properties similar to a semiconductor, allowing temperature changes to be translated into electrical information that the shark can use to help detect temperature gradients. Figure 1 : The tiny pores of the lateral line system of a porbeagle shark run the length of the body from head to tail.
The lateral line, together with the ampullae of Lorenzini comprise the electrosensory component of the sharks sensory system. The lateral line allows the shark to orient to particle movement or sound. It consists of structures called neuromasts which are located in canals that lie just below the surface of the skin or the scales.
Similar to the ampullae of Lorenzini there are pores that open to the outside and movement caused by prey can be detcted by the neuromasts. Shark skin feels like sandpaper because it has small rough placoid scales also known as dermal denticles. As a result, it is often dried and used as a leather product or sandpaper. Placoid scales consist of a basal bony plate buried within the skin and a raised portion that is exposed. Dermal denticles are homologous in structure to teeth, and are what gives the skin a rough feeling.
Dermal denticles, as seen in this image taken from the dorsal fin of a porbeagle shark, are small tooth-like structures on the skin which form a protective barrier and aid in swimming. Magnified images of porbeagle and spiny dogfish dermal denticles taken by scanning electron microscope.
Seen in this picture of a blue shark are the liver, stomach and intestine. The oviduct, part of the female reproductive tract is also visible. Upon incision of the belly from the pelvic fins to the pectoral fins the first organ encountered is the liver. The liver of sharks occupies most of the body cavity.
It serves two functions within the shark. The first is as an energy store since all fatty reserves are stored here. The second function of the liver is to serve as a hydrostatic organ. Oils that are lighter than water are stored in the liver. This decreases the density of the body providing buoyancy to counteract the sinking tendency of sharks.
Aside from the liver, the stomach can be seen within the body cavity. Often found within the stomach are the contents of the sharks last meal. The stomach itself terminates at a constriction known as the pylorus , which leads to the duodenum and then to the spiral valve intestine. The spiral valve intestine is an internally coiled organ that increases the surface area across which nutrients can be absorbed.
The spiral valve intestine empties into the rectum and anus which in turn empties into the cloaca. The claoca is the chamber where the digestive, urinary and genital tracts all open to the outside.
Some of the organs mentioned can be seen in this photograph of a mature male porbeagle shark. Also seen here is the epididymis , part of the male reproductive tract.
Also easily found within the body cavity is the pancreas. The pancreas is a digestive gland with two pink lobes. Secretions pass from this organ to the duodenum from the ventral lobe through a small duct. There are two other organs that are visible but do not belong to the digestive system. The first is the spleen , which is a dark organ near the stomach that belongs to the lymphatic system. The second is the rectal gland , a small organ that opens by a duct into the rectum.
It acts as a salt gland, removing excess sodium chloride salt from the blood. The secretion is a colourlesss solution of salt that is twice the concentration found within the blood plasma and higher than that of the surrounding saltwater. Upon removal of the digestive organs the reproductive organs can be viewed.
For details about the reproductive anatomy of sharks visit the Shark Reproduction page within this site. Figure 2 : Photo showing the epididymis, epigonal organ, testes, duodenum, stomach, pylorus, spiral valve intestine, rectal gland, rectum, and anus of an adult male Porbeagle shark. Figure 3 : Photo showing the rectal gland and lower intestine of a porbeagle shark. Fins The fins of sharks are used for stabilizing, steering, lift and propulsion. Some different species of shark also have a singular anal fin.
Understanding how to recognize each type of fin and how they help a shark navigate through the water is crucial to understanding the biology of sharks. The first dorsal fin is the fin that Hollywood movies always portray as cutting through the surface of the ocean like a knife. The second dorsal fin is much smaller than the first dorsal fin. The second dorsal fin is located on the back of the shark midway between the first dorsal fin and the tail.
The second dorsal fin, much like the first dorsal fin, helps to stabilize the shark in the water. Pectoral fins are often thought of as the wings of a shark. They are the pair of fins located on either side of the body along the pectoral muscle line.
The pectoral fins control the direction the shark swims and help maintain balance in the water. Pectoral fins help the shark make turns, swim up or down, and roll its body. Depending on the species, the pectoral fins are also usually the largest fins on a shark. Ventral fins are usually located along the side of the body between the pectoral fins and the tail and are smaller than the pectoral fins. Ventral fins, like pectoral fins, help sharks turn, go up and down in the water, and roll.
Anal fins are not present on all shark species. Most shark fins are virtually unrecognizable by species. Once it is in the market or in the bowl, most consumers will not know where the fin came from, or if it was harvested legally or illegally. Any shark is fair game, but some species are more prized than others. The large fins of Whale Sharks, Basking Sharks are coveted for decoration at restaurants.
These species are among the most threatened. Pelagic species such as Oceanic White tip and Silky sharks are common in the high-end trade. Illegal fisheries such as those that target the Galapagos, Cocos Island reserve and other remote islands capture reef sharks and hammerhead sharks.
The Blue shark is among the most common traded with an estimated 20 million killed for their fins annually. By nature, sharks are difficult to study and good fisheries data are hard to obtain. The practice of finning, which is mostly an unreported practice is robbing scientists of population and capture data.
Many pelagic shark species are widespread and do not school. Many larger sharks travel vast distances alone. Most large sharks have late onset of fertility decades give birth to few young and have long gestation periods, making them very vulnerable to overfishing. Therefore, it is very difficult to arrive at a sustainable number. This is why most commercial shark fisheries collapse economically. Until that is achieved and it can be enforced, then the source of fins must stop and fins made illegal.
Only the sawfishes Sawfishes Pristidae spp. Oceanic whitetip shark Carcharhinus longimanus Porbeagle shark Lamna nasus Scalloped hammerhead shark Sphyrna lewini Smooth hammerhead shark Sphyrna zygaena Great hammerhead shark Sphyrna mokarran Manta rays Manta spp. In Silky shark Carcharhinus falciformis, Thresher sharks Alopias spp. Great white sharks are protected under California and Federal Law from fishing commercially or recreationally. Even,fins from these protected sharks have been identified in the shark fin trade through DNA analysis.
These laws regulate the trade and transport of listed species across country lines by countries that recognize the treaty. Nearly twice as many nations had teams in the World Cup. In investigators performed DNA testing on wholesale shark fin distributors, discovering the fins of the few protected species in Hong Kong, Singapore and here in the US.
In Shark Stewards, working with at the California Academy of Sciences sequenced 17 species of sharks from shark fins bought in San Francisco Chinatown, including threatened species like thresher sharks, and endangered hammerhead sharks. Once the fin is dried and treated, the species is nearly unidentifiable.
The protections in place are not strong enough, or broad enough to protect sharks. Shark fins are used to make shark fin soup, a delicacy once prepared exclusively for the Chinese emperors and nobility.
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