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Darkfield Digital Image Gallery
Dogfish Shark Placoid Scales
Dogfish sharks are small sharks belonging to one of three families: the dogfish shark family, Squalidae; the requiem shark family, Carcharhinidae; and the cat shark family, Scyliorhinidae. The best-known species are the spiny dogfish of the dogfish shark family, and the smooth dogfish of the requiem shark family.
Dogfish sharks make up one subfamily of the dogfish shark family, and are characterized by a hard spine at the base of each of the two dorsal fins. Other species of the dogfish shark family that are called dogfish include the Atlantic black dogfish and the green dogfish, found in deep waters of the Gulf of Mexico. The green dogfish, along with several other members of the dogfish shark family, is luminescent.
Scales are small plates that form part of the skin of certain animals. They provide protection from the environment and from predators. Sharks have placoid scales, bony, spiny projections with an enamel-like covering. These scales have the same structure as their teeth, and are also referred to as dermal denticles ( dermal=skin, denticle=teeth). These denticles are slanted toward the tail of the shark and help direct the flow of water around the shark's body, reducing friction so it can swim with less effort. Stroking a shark from head to tail, its skin feels smooth to the touch, but stroking it from tail to head, the skin feels rough as sandpaper. Shark's skin has, in fact, been used as sandpaper in some countries for many centuries. A shark can also inflict wounds on potential prey by breaking the creature's skin with its scales. Like teeth, the shape of the scales varies among shark species and can be used to identify them.
Contributing Authors
Cynthia D. Kelly, Thomas J. Fellers and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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