Sharks: Species, Biology, Behavior, and Conservation Status

Sharks are marine fish with cartilaginous skeletons, five to seven gill slits on the sides of the head, and a prominent dorsal fin on the back (the fin that is seen above the water surface when a shark is swimming near the surface). There are more than 460 species of sharks classified into several orders within the class Chondrichthyes in the phylum Chordata. Sharks are related to rays, skates, and sawfish. Most sharks are fierce, aggressive apex predators of fish and other marine animals; they are at the top of the marine food chain. Although they have a reputation for attacking humans, such attacks are relatively rare. There were six human deaths in ninety-eight attacks worldwide in 2015, and human attacks are carried out by only about thirty species. Fossil evidence indicates that the earliest-known sharks lived approximately 400 million years ago. Most species of sharks live for two or three decades, though some of the larger-sized species may live more than a century.

DISTRIBUTION AND PHYSICAL CHARACTERISTICS. Sharks live throughout the oceans of the world. Most species live in temperate or tropical seas, but some species, such as sleeper sharks, live in arctic waters. A few species, such as bull sharks, sometimes venture into the freshwater of rivers or the brackish waters of estuaries. Sharks, depending on the species, may be found in shallow coastal habitats, in deep-water sea floor habitats, or in the open ocean.

Most sharks are rather large, though the smallest sharks, the spiny pygmy sharks, grow to a length of only about 7 inches (18 centimeters). The largest living shark is the whale shark, which can reach a length of more than 50 feet (15 meters). The skin of sharks is covered with tough toothlike dermal denticles (also called placoid scales), which make for a hard, protective covering. Sharks are well-known for their jaws and their multiple rows of sharp teeth, which are replaced by new teeth as they fall out. Some sharks may have tens of thousands of replacement teeth during their lifetime Unlike bony fish, sharks have skeletons made of cartilage, which is more flexible and lighter than bone.

BEHAVIOR AND REPRODUCTION. The torpedo-like shape of sharks’ bodies enhances their fluid dynamics and allows for smooth travel through the water. Their backward-pointing dermal denticles further reduce turbulence as they swim. Sharks typically swim at speeds of about 3 miles (5 kilometers) per hour, though some kinds, mainly the mako sharks, can reach speeds of about 30 miles (48 kilometers) per hour. As a shark swims, respiration occurs as seawater passes into the mouth and out through the gill slits. The gills extract oxygen from the water and move it into blood vessels. When a shark is resting on the sea floor, a pumping mechanism keeps the water flowing over the gills in some species. Species that lack such a pumping mechanism must swim continuously to breathe.

Sharks can use their keen sense of smell to detect even tiny amounts of blood in the water, allowing them to easily find wounded prey many miles away. Sharks also use their lateral line systems to locate prey. A lateral line is a row of pores along the side of the shark’s body; the pores contain sensitive hairlike projections that can detect vibrations and pressure changes in the water. Most sharks eat fish, squid, seals, crustaceans, mollusks, and other marine animals, hunting mostly at dusk and night-time. A few species, such as the whale and basking sharks, consume plankton through filter-feeding mechanisms, similar to the feeding methods of baleen whales. Sharks of many species are solitary hunters, socializing only for reproductive purposes. Some sharks, such as hammerhead sharks, regularly socialize and hunt in schools, or packs, of various sizes.

Sharks generally reach sexual maturity slowly, with smaller species reaching it in three or four years and larger species in fifteen or more years. Sharks typically reproduce sexually through internal fertilization. The male inserts his sperm into the female’s cloaca with a specialized organ, called a clasper, on one of his pelvic fins. Rarely, the female of certain species can give birth asexually, with no sexual contact from a male. In most species, the female shark gives birth to one or two pups at a time. Most female sharks are ovoviviparous, with fertilized eggs hatching in the mother’s uterus and the young sharks nourished by egg yolk and uterine fluids in the mother’s body until they are born live, fully functional. Some sharks are viviparous, with the young developing sharks nourished by a placenta-like organ in the uterus until they are born live. A few kinds of sharks are oviparous, with fertilized eggs being laid in the water inside yolk-containing egg cases.

THREATS AND CONSERVATION. Tens of millions of sharks are killed by commercial and recreational fishermen every year. Sharks—especially their fins—are highly valued for food in certain nations, including Japan, Australia, India, and Iceland. Shark habitats are frequently disrupted as a result of ocean pollution, coastal development, and overharvesting of their prey species. Consequently, many species of sharks are classified as threatened or endangered. According to the Red List maintained by the Union for the Conservation of Nature (IUCN), seventy-four species are considered threatened, including eleven that are listed as critically endangered.

Many countries have passed legislation to protect sharks. For example, Mexico banned shark finning in 2007, and the state of California banned the possession, trade, importing, and distribution of shark fins in 2011. Shark fisheries are managed in the United States by the National Marine Fisheries Service. Five shark species have international protection through the Convention on International Trade in Endangered Species (CITES)—the oceanic whitetip shark, the porbeagle shark, and three species of hammerheads.

FURTHER READING: Defenders of Wildlife. 2016a. “Basic Facts About Sharks.” https://www.defenders.org/sharks/basic -facts. Accessed December 6, 2016.

Defenders of Wildlife. 2016b. “Success Stories.” https://www.defenders.org/sharks/success-stories. Accessed December 6, 2016.

Defenders of Wildlife. 2016c. “Threats to Sharks.” https://www.defenders.org/sharks/threats. Accessed December 6, 2016.

Goyenechea, Alejandra. 2013 “Victory for Sharks and Mantas!” http://www.defendersblog.org /2013/03/victory-for-sharks-and-mantas/. Accessed December 6, 2016.

Klimley, A. Peter. 2013. The Biology of Sharks and Rays. Chicago: University of Chicago Press.

Long, John A. 1996. The Rise of Fishes: 500 Million Years of Evolution. Baltimore, MD: The Johns Hopkins University Press.

Ridgley, Heidi. 2016. “Shark (Fact) Attack.” http://www.defendersblog.org/2016/06/real-facts-shark -attacks. Accessed December 6, 2016.

SeaWorld Parks & Entertainment. 2016. “Behavior.” https://seaworld.org/en/animal-info/animal -infobooks/sharks-and-rays/behavior. Accessed December 6, 2016.

Shark Savers. 2016. “Shark Biology Primer.” http://www.sharksavers.org/en/education/biology/ shark-biology. Accessed December 6, 2016.