only a few ounces. A similar fish, the duck-billed eel shown in the lower color insert on page C-6, is a resident of continental slopes of the Indo-Pacific seas. This long, flattened fish is gray with black edges and a gray-black snout.
The oarfish, family Regalecidae, the longest bony fish in the ocean, also occupies the midwater region. The oarfish can grow to 50 feet (15.2 m) in length and can weigh as much as 100 pounds (45.4 kg). Its pelvic fin is one extremely long ray. To catch tiny crustaceans in the water, oarfish take water into their small, toothless mouths, then strain it over gill rakers, tiny projections that prevent prey from escaping through the gills. Fish that are carnivores typically have short, widely spaced gill rakers. In fish that eat plankton, the structures are long, thin, and much more numerous.
heads. The water passes over their gills and exits through the gill slits on the sides of the head. Most species of sharks can pump water over their gills by opening and clos- ing their mouths. Some sharks, the “ram ventilators,” must swim continuously to move water over their gills. Oxygen in water is picked up by tiny blood vessels in the gills, then carried to the heart, a small two-chambered, S-shaped tube. From there, oxygenated blood is pumped to the rest of the body.
Sharks fertilize their eggs internally. Males transfer sperm to females using modified pelvic fins. Some species are oviparous, which means the female lays fertilized eggs. Shark eggs may be deposited in lagoons or shallow reef water, where they incubate for six to 15 months. Many of the eggs’ cases
are equipped with hairy or leathery tendrils that help hold them to rocks or plants. Other species are viviparous, so the embryos develop inside the mother and are born alive. Several species are ovoviviparous, which means that the embryo develops inside an egg within the female’s body. The egg hatches inside the mother, the hatch- ling eats the yolk and any unfertilized eggs, then is born alive.
Shark populations are relatively small compared to other kinds of fish. One rea- son is because shark reproduction rates are low. Unlike fish and many of the inverte- brates, a female shark produces only a few offspring each year. In addition, the gesta- tion period, time when the embryo devel- ops inside the mother, of viviparous species is long.
One member of the oarfish family the king of the herrings (Regalecus glesne), lives in depths of up to 3,280.8 feet (1,000 m). The head and body of this fish are silver with blue streaks, and the dorsal fin is crimson in color. Favorite foods of the king of the herrings include small fish, crustaceans, and squid. Sharks can also be found in the mesopelagic zone. Cat sharks, family Scyliorhinidae, have flat heads and long, cat- like eyes that glow when light hits them. Because their eyes are extremely sensitive to light, cat sharks are successful pred- ators in nearly dark conditions. The file-tail cat shark (Parmaturus xaniurus) is a relatively small shark that ranges between 12 to 40 inches (30 to 100 cm) in length. This shark gets its name from the file-like projections that stick out of its body. The file-tail cat shark prowls the ocean bottom search- ing for crustaceans and fish to eat.
Another type of shark, the mackerel or white shark of the family Lamnidae, inhabits the midwater regions. Members of this family have crescent-shaped tails with the lower lobe slightly shorter than the upper one. The most widely recog- nized species in family Lamnidae is the great white shark (Carcharodon carcharias). The dorsal surface of the great white shark is dark while the belly is white, and adult sharks grow to lengths of 21.3 feet (6.5 m) and have large, serrated teeth that are triangular in shape. These sharks are very aggressive feeders and will eat other sharks, fish, seals, and sea turtles. Even though great white sharks swim as deep as 4,200 feet (1,280 m), they occasionally come to the surface, and they are notable as the only species that ever raises their heads out of water, possibly to get a better view of their prey.
Ranging from the continental slope to depths of 6,561.7 feet (2,000 m), another species of shark found in the mid- ocean region is the six-gilled shark (Hexanchus griseus) of the family Hexanchidae. Most sharks have five gill slits rather than the six found in all members of this family, thus the ori- gin of the common name. Growing to a length of 18 feet (5.5 m), this is one of the largest carnivorous sharks. Six-gilled sharks, found all over the world, are very slow swimmers, but they have the unusual ability to change colors to blend in with their surroundings. During the day these sharks stay in
deep water but move to shallower regions at night to feed on cephalopods, fish, crustaceans, and marine mammals.
A relative of the sharks, the ratfish of the family Chimaeridae, is also associated with the continental slope and deep waters. The ratfish, known to some as the shortnose chi- maera, feeds on mollusks, crustaceans, worms, echinoderms, and small fish. One representative of this family, the spotted ratfish (Hydrolagus colliei), has smooth skin, large green eyes, a spotted brown body, and a pointed tail. Females, which are larger than males, can grow up to 3.3 feet (1 m) in length. Spotted ratfish prefer cold waters near the bottom of the ocean. A dorsal spine and a strong, chisel-like plate of teeth in the fish’s jaws can cause painful wounds.