Carcharhinidae, Sphyrnidae, Triakidae, Squalidae, Rhincodontidae and Ginglymostomatidae - sharks
Sharks, as well as rays, are known as "cartilaginous" fishes, as opposed to true "bony" fishes because their skeleton is made out of cartilage, a soft somewhat elastic tissue, rather than bone. Cartilaginous fishes also differ by having five to seven gill slits whereas bony fishes have only one gill opening usually covered by a bony gill plate.
Sharks have elongate, cylindrical bodies. Their gill openings and pectoral fins are on the sides of their body. Most have several rows of sharp, pointed teeth. When a tooth breaks off, it is replaced by the tooth behind it. The skin is covered by tooth-like dermal denticles which give it a sandpapery feel. There are usually two dorsal fins present and most have an anal fin. Their tails are well-developed with the upper lobe usually longer than the lower lobe. All sharks are marine, but some will enter freshwater and may be found miles upstream in rivers and bayous.
Unlike most fishes, all sharks reproduce by internal fertilization (sperm are deposited inside the female's uterus). Most bear their young live and fully developed, termed viviparity or viviparous reproduction. The bullhead sharks (Heterodontidae) and cat sharks (Scyliorhinidae) lay egg cases containing one or more embryos on the ocean floor. This is termed oviparity (oviparous reproduction). When the eggs hatch, the young sharks are fully developed.
Carcharhinidae, the requiem sharks, contain the largest number of sharks. They have five gill slits, per side, the last slit over or behind the origin of the pectoral fins. Upper teeth are blade-like with no more than one cusp. Eyes have nictitating membranes within the lower lid. Two dorsal fins are present, the first being anterior to the origin of the pelvic fin. The caudal fin is asymmetrical. The longer upper lobe has a well-developed subterminal notch. A groove called a precaudal pit is located dorsally on the caudal peduncle. All carcharhinids are born fully developed. They are voracious predators and feed on sharks, rays, fishes, turtles, marine mammals, sea birds, invertebrates such as squids, octopuses, lobsters, and crabs, and occasionally garbage and debris. These sharks are responsible for a large proportion of shark attacks on humans.
Sphyrnidae, the hammerhead sharks, are distinguished from other sharks by their much dorsoventrally flattened head, resembling a hammer or shovel, with eyes on its outer edges. There are five gill slits per side. The upper teeth are blade-like with a single cusp. Eyes have nictitating membranes within the lower lid. They have two dorsal fins, the first fin being high and pointed and anterior to the origin of the pelvic fin. The caudal fin is asymmetrical with a subterminal notch on the upper lobe. A caudal pit is present on the caudal peduncle. Differentiation between species is can be determined by the shape of the head. The hammerhead shape of their head, called a cephalofoil, is thought to increase sensory reception by spreading their electroreceptor pores over a wider area. It also places their nostrils farther apart which may increase their ability to detect chemicals and other "scents" in the area. Like the requiem sharks, these sharks bear their young live.
Triakidae, the houndsharks, are small elongated sharks with horizontally compressed oval eyes. They have five gill slits per side, the last gill slit being posterior to the pectoral fin origin. Teeth are small, blunt, used for crushing and are arranged in a mosaic-like pattern. Eyes have nictitating membranes within the lower lid. They possess anterior nasal flaps. They have two dorsal fins, the first fin being anterior to the origin of the pelvic fin. The origin of the second dorsal fin is anterior to the origin of the anal fin. The caudal fin is asymmetrical with a subterminal notch in the upper lobe. The lower lobe is very small. There is no precaudal pit. They are live bearers and eat mainly invertebrates and bony fishes. They are small to moderate sized sharks and are not considered dangerous to humans.
Squalidae, the dogfish sharks, look very similar the houndsharks, in that they have small elongated bodies and horizontally compressed oval eyes. They have five gill slits, all anterior to the pectoral fin. They possess two dorsal fins, each preceded by a thick, smooth spine. They lack an anal fin and their eyes have no nictitating membranes. The caudal peduncle has lateral keels (ridges) and usually there is a caudal pit present. The caudal fin is asymmetrical and lacks a subterminal notch on the upper lobe. The dorsal spines are covered with venom which can be mildly irritating to humans. Because of their small size though, the dogfishes are not considered dangerous to humans. Sharks in the families Triakidae (houndsharks) and Scyliorhinidae (cat sharks) are sometimes referred to as "dogfishes". Triakidae and Squalidae sharks bear their young live while Scyliorhinidae sharks lay egg cases.
Rhincodontidae contains only one species, the whale shark. This is the largest living fish in the oceans, attaining a size up to 20 m (65.5 ft). It can be recognized by its size, blunt square-shaped snout, and patterning (spots and lines, almost checkerboard-like) on its body. It has a flattened head, nearly terminal mouth with eyes behind the mouth, and three longitudinal body ridges on each side. Like most other sharks, it has two dorsal fins and a heterocercal tail (no subterminal notch on upper lobe). Unlike most other sharks, the whale shark has extremely small teeth and feeds by filtering plankton and other small organisms (fishes and invertebrates) through its gill apparatus modified into a sieve. It commonly feeds at or near the surface and is often seen basking at the surface which makes them vulnerable to ship strikes.
Ginglymostomatidae, the nurse sharks, have two large dorsal fins set close together and far back on the body. The second one is only slightly smaller than the first. The head is broad, flat and blunt with small eyes. Their mouths are set far forward of the eyes and are connected to the nostrils by a groove. Each nostril has a long, fleshy barbel which is used to search for prey in the sediments. Their tails have a strong upper notch and no lower lobe. The nurse sharks are bottom dwelling (benthic) and nocturnal. They like to hide in caves and crevices during the day, coming out to feed at night. There are 3 species in this family but only 1 occurs in the Gulf of Mexico and the Atlantic.