The teeth are sharp and pointed. There are several rows of flattened teeth lying behind the upright set ready to replace them when worn out or lost. The nares or external nostrils are located on the underside ventral surface of the rostrum anterior to the jaws. A nasal flap separates the incurrent from the excurrent opening. Water passes into and out of the olfactory sac, permitting the shark to detect the odors of the water. The patches of pores on the head in the areas of the eyes, snout, and nostrils are the openings of the ampullae of Lorenzini.
These sense organs are sensitive to changes in temperature, water pressure, electrical fields, and salinity. Labeled Snout Examine the photographs of the male shark's pelvic region by clicking the blue lettered links in the column to the right. Males have stout, grooved copulatory organs called claspers on the inner side of their pelvic fins.
Fertilization in the dogfish shark is internal. During copulation, one of the claspers is inserted into the oviduct orifice of the female. The sperm proceed from the cloaca of the male along the groove on the dorsal surface of the clasper into the female. Labeled Male Pelvis Examine the photographs of the female shark's pelvic region by clicking the blue lettered links in the column to the right. The cloacal opening located on the ventral surface between the pelvic fins.
It receives the products of the intestine, the urinary and the genital ducts. The name cloaca, meaning sewer, seems quite appropriate. Labeled Female Pelvis Examine the photographs of the skinned shark by clicking the blue lettered links in the column to the right. Depending on the species, there may be either one or two dorsal fins. The first dorsal fin is the most prominent as it is situated on the top of the animal, behind the head. This is frequently the fin that is seen sticking out of the water.
The dorsal fins keep the shark :upright:, i. If there is a second dorsal fin, it is much smaller and positioned further down the torso, in line with the pelvic fin. Some sharks present with spines on their dorsal fin s.
Not all shark species present with an anal fin. Essentially the structure consists of a layer of parallel, plate-like cells filled with silver guanine crystals.
The crystals reflect light that has already passed through the retina and redirects it back to restimulate the retina as it passes out through the eye. This effectively boosts the visual signal, especially in low light levels giving sharks high visual acuity. Another modification found in some sharks is the presence of a nictitating membrane. This structure is a denticle covered membrane that protects the eye.
It closes when the shark passes close to a objects and also during biting or feeding. Figure 1 : The nictitating membrane can be seen on this example of a blue shark. Figure 2 : Here is another example of the nictitating membrane in use as a blue shark feeds on some bait cod. You can see that the eye is completely covered by the membrane as the shark bites.
All sharks have five to seven pairs of gills on the side of the head. Gas exchange occurs at the gills and oxygenated water must always be flowing over the gill filaments for respiration to occur. Water enters through the mouth of the animal, into the pharynx, over the gills and exits through the gill slits. Respiratory gas exchange takes place on the surface of the gill filaments as the water passes over and out the gills. The spiracle is a vestigial first gill slit.
It appears as an opening behind the eye, as in the spiny dogfish photo below. It is absent or reduced in many sharks, especially the fast swimming sharks and is usually larger and present in sedentary or bottom dwelling sharks. The spiracle in sharks is used to provide oxygenated blood directly to the eye and brain through a separate blood vessel. In the rays, the spiracle is much larger and more developed and is used to actively pump water over the gills to allow the ray to breathe while buried in the sand.
Shark teeth are not lodged permanently within the jaw, but are attached to a membrane known as a tooth bed. The tooth bed membrane is similar to a conveyor belt, moving the rows of teeth forward as the shark grows, thus replacing the older teeth in front that have become damaged, fallen out or worn down.
It is not uncommon for shark teeth to be found lodged in large prey such as whale carcasses or loose on the ocean floor. The shape, number and appearance of shark teeth varies considerably among shark species, and can be one of the most important features for species identification.
However, tooth appearance can also differ between the upper and lower jaw, and from front to back, within any given shark.
The blue shark is a good example of how teeth can differ between the upper and lower jaws. The upper teeth left are triangular and curved with serrated edges and overlapping bases. The lower teeth right are more straight and slender with finely serrated edges. The teeth of the porbeagle and mako are alike in both the upper and lower jaws. The porbeagle left has smooth edged teeth with lateral denticles while the mako right has more slender teeth without lateral denticles.
The ampullae of Lorenzini are small vesicles and pores that form part of an extensive subcutaneous sensory network system. These vesicles and pores are found around the head of the shark and are visible to the naked eye. They appear as dark spots in the photo of a porbeagle shark head below. 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. This asymmetrical profile was common in many ancient fishes, but sharks are the only group to maintain the shape over a period of million years.
Scientists still debate the function of this unique shape, and it may serve a different purpose from shark species to shark species when combined with other physical characteristics. However, some research suggests the shape allows for easy, horizontal cruising. In this illustration, four sharks from the IndoPacific region are placed side by side for comparison.
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