Shark Summer

Sharks: An Overview

Sharks play a critical role in keeping our marine ecosystems healthy. Find out why these ocean predators need our support.

© Paul Selvaggio

A sand tiger shark silhouetted in light.

Sharks are magnificent wild creatures. Found in all parts of the world’s ocean, sharks fire our imaginations. With 410 known species, sharks can be found in a wide range of sizes. Some are powerful and awe inspiring. Others blend in with the world around them, living a quiet and still life.

The smallest, at only seven inches, is the dwarf lantern shark that could fit in the palm of an adult’s hand. The largest is the whale shark, measuring larger than a school bus at lengths of about 40 feet. Most sharks are less than three feet long as adults. Those larger than six feet are known as the great sharks.

Sharks and their relatives the rays, skates, and chimeras make up the major group of fishes in the class Chondrichthyes. These animals have a skeleton made of cartilage rather than the hard bone that is found in most other vertebrates. The advantage of a cartilaginous skeleton is that it is less dense than bone, making it easier to swim. Sharks and rays belong in a subclass known as Elasmobranchii. A few shark species have a lot of calcification on their cartilage that makes their skeletons harder. But other than those exceptions, the teeth and scales of elasmobranchs are the only hard parts of their bodies.

In general, rays are flat-bodied sharks. The gill slits are on the underside of the body, rather than on the sides of the head. Most rays swim by flapping or undulating their pectoral fins, rather than using the tail. But again, there are always exceptions to the rule. Guitarfishes, electric rays, and sawfish all have their gill slits on the underside of their bodies, but they swim using their tails, like sharks.

The Aquarium features eight species of rays. In various exhibits, guests can find a bowmouth guitarfish, a reticulated ray, bat rays, cownose stingrays, round rays, Southern rays, a sawfish, shovelnose guitarfish, a diamond ray, and mangrove rays. Although not true rays, we also have California skates.

© Chris Lowe

Magnified shark scales.

Scales and Teeth

Sharks’ scales are different from those of other fishes. Their placoid scales are smaller than most other bony fishes’ scales. When viewed under a microscope, the scales actually look like miniature teeth. If rubbed from back to front, these scales can cause abrasions. In the past, the rough shark skin itself was used in many ways, including as sandpaper and a material (shagreen) for upholstery and bookbinding.

Each placoid scale is covered in dentine, a calcified tissue that is also a component of human teeth. The various species of sharks have different types of these “dermal denticles,” another name for placoid scales. They improve water flow over the shark’s body, making them more efficient swimmers, in much the same way as the dimples on a golf ball help it to fly farther. Swimmers in the 2004 Olympics in Athens were the first to don swimsuits that had been designed based on the science of dermal denticles. The shark-inspired suits cut drag through the water by up to four percent—enough to mean the difference between winning and losing a close race.

All sharks and rays are carnivores. Some sharks are top predators and play a crucial role in keeping the natural balance between the different creatures that live in marine ecosystems. Yet not all are top predators. In fact, some of the largest sharks are filter feeders, sifting plankton out of the water. Others nudge around in the sand looking for shellfish and worms. A wide variety of teeth reflects the different foods they eat, which range from seals and sea lions to snails and crabs to plankton.

There are a variety of shark teeth shapes. The goblin shark has long, thin, impaling teeth. Serrated teeth can be found on the great white and tiger shark, both of which eat large leathery-skinned prey. The gummy shark’s teeth are designed to crush rather than bite its prey. Because sharks and rays have no hard bony jaw that provides sockets for their teeth, sharks and rays continually replace worn out or missing teeth throughout their lives. It is thought that some sharks may go through up to 30,000 teeth during their lives.

Breathing and Buoyancy

Most bony fishes have an air bladder that can be inflated with air or other gases from their blood to adjust buoyancy. Sharks do not have an air-filled bladder. Instead, they have a large oily liver that helps provide buoyancy, as oil is less dense than water. Blue sharks have livers that account for 20 percent of their body weight. Deep-sea sharks have enough liver oil to attain neutral buoyancy. Other sharks and rays simply spend most of their time resting on the ocean floor. Sandtiger sharks are unique in that they will go to the water surface and gulp air to help make them neutrally buoyant. Their stomachs become flotation chambers!

To absorb enough oxygen, sharks and rays need to maintain a good flow of water over their gills. Some of the larger and highly active sharks, such as the mackerel and requiem sharks, rely on ram ventilation to breathe. This means they use movement alone to force the water over their gills. But most sharks do not need to move constantly to breathe. Most species can pump water over their gills while lying still. Typically, water is taken in through the mouth, flows over the internal openings over the gills (that act like our lungs), and leaves through the five to seven slits on each side of the head called gill slits. Some sharks and all rays pump their breathing water in through a large opening called a spiracle, located just behind the eye, and then expel the water out of their gill slits.

In sharks and rays, the nostrils are not used for breathing but for smelling. Often described as “swimming noses,” sharks have an excellent sense of smell and can detect very faint traces of blood and other substances in water that help them to find prey. Smell may also play an important role in socializing, finding a mate, and navigation.

Shark Sense

Sharks have a “sixth” sense not found in most other animals—the ability to detect minute electric fields given off by other animals. Sharks use this ability to locate prey that might even be buried under the sand. Amorous male stingrays use this sensory ability to detect coy females. Animals like a pelagic stingray use these organs to sense squid gathered in a cluster to mate in the dark at night.

This electrosense comes from an organ called the “ampullae of Lorenzini,” and it can detect electrical signals up to four inches away. Concentrated on the head and jaw, these structures consist of small pouches beneath the skin that are lined with specialized hair cells. They are connected to an opening on the skin by a tube filled with conductive jelly.

Sharks also have five other very good senses. They can hear up to one mile away (sound travels better through water than through air). Sharks can smell up to 1,000 feet away. Using their fins, teeth, and skin, sharks can sense through touch. They have excellent vision, with the ability to see up to 70 feet away. Using an organ called the lateral line, sharks can feel movement or the vibration of water up to 300 feet away.

While sharks are so perfectly designed for their environment, they cannot compete with humans and the impacts we have on the ocean. Find out how we use sharks and what we can do to reduce our demand on these important ocean animals.