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Not all sharks have the streamlined Carcharhynidae body shape for speed and power. Some are eel-like, such as the Frilled Shark (Chlamydoselachus anguineas); some have stocky bodies, like the Port Jackson Shark (Heterodontus portusjacksoni); and some have ray-like flattened bodies, and does the Angel Shark (Squatina squatina).
In most sharks the body tapers in front of the tail, forming the caudle peduncle. Here, faster sharks have two keels to reduce water resistance. Most sharks have two triangular dorsal fins, the first fin larger than the second, and the others have one, above the anal fin. The front edge of the dorsal fin sometimes is strengthened by a spine; in a few cases the dorsal fin is little more than a spine. The pectoral fins are broad-based and located just behind or partially below the gill slits. They cannot be folded back but are out and erect at all times. Their design allows for decreased pressure above the fin, lifting the shark's body effortlessly.
Sharks with paired pelvic fins have them at the midpoint of the underside. In males, they are modified into penis-like intromittent structures called claspers. These fins have hook-like denticles, presumably to hold the female during sperm transferrance. anal fins may be absent, and size varies by species, but they are used to prevent tolling. Anal fins, or the lack thereof, are very important in classification. The caudal fin is heterocercal; the upper lobe is larger than the lower. The vertebral column runs into the upper lobe, making it asymmetrical in and out. The lower is stiffened by cartilaginous radials articulating with the underside of the spine.
The shape of the tail reveals how a shark lives. Lunate tails are fast swimmers. These sharks have a narrow caudal peduncle with a small pit in the back in front of the upper lobe; along with lateral keels to increase effeciency. Sharks with a greatly enlarged upper caudal lobe (like Threshers) use their tails to catch prey. A greatly reduced caudal fin makes a rather sluggish and clumsy swimmer, like Dogfishes.
Instead of scales, sharks have very small thorn-like structures, or denticles. A basal plate is bedded in the dermis and a distal sharp process points backwards. This shape varies by species, therefore it is used in identification. Most denticles are microscopically small, but some sharks have very few, very large ones.
