Sorry Kid, You're On Your Own
There are four different ways that sharks reproduce: Viviparous, oviparous, ovoviviparous, and asexually. The differences depend on the species and sometimes, environmental pressures.
Viviparity is when an egg develops and hatches inside of the womb. After hatching, the pups live on a placenta until being born alive. They will have an umbilical cord located between the pectoral fins (a belly button!). Some viviparous shark species include:
Oviparity is when eggs are fertilized in the womb and then a shark lays the eggs encased in a leathery pouch, called a “mermaid’s purse.” The eggs are left unprotected and are oftentimes eaten by predators before they can hatch. The eggs hatch with no parental care after the eggs are laid.
Oviparous shark species include:
- Zebra Sharks
Besides being fun to say, ovoviviparous sharks have eggs that hatch inside of the womb and the shark has a live birth. This might sound just like the viviparous pups above, but there is no placenta involved. Pups live on a yolk sac until they are ready to hatch. The first shark to hatch from its egg will then consume the other shark fetuses and their yolk sacs. Obviously, ovoviviparous give birth to small litters.
Asexual reproduction is when a female shark gives birth without any contact with a male. Although very rare, there have been two documented cases of asexual reproduction in sharks. Female sharks may asexually reproduce when they are in captivity or when there are no available male sharks in the wild.
You’re Getting Sleepy…
Did you know that when you flip a shark upside down they go into a trance-like state called tonic immobility? This is a reflex called tonic immobility. It causes a temporary state of inactivity in an animal–similar to hypnosis. This is the reason scientists often turn over a sawfish when they’re working with them in the water. Check out this video of NOAA scientists using tonic immobility to implant a tagging device!
Tonic immobility can be observed in a wide range of animals from insects and crustaceans to mammals, birds, reptiles, amphibians, and fish. Specifically, it can be brought on by stroking a particular area of a lobster's shell or focusing a hen's attention on a line on the ground.
By the Teeth of their Skin
A shark's skin is covered in tiny flat diamond-shaped scales called dermal denticles. Like the name denticles suggests, these scales are similar to human teeth. Like running your hand over a velvet fabric, these scales make sharks feel very smooth from head to tail. But when you run your hand in the other direction, the denticles feel very rough and sandpapery from tail to head.
Dermal denticles reduce friction and drag in the water allowing for more lift and helping the shark to glide quickly and smoothly. On shortfin makos, the dermal denticles can be raised to increase the amount of water flowing behind the denticles, “pushing” them forward.
Dermal denticles have long inspired engineers trying to reduce drag in the air and water. They've been studied and applied to designs of everything from swimsuits to airplane wings!
Senses and Sensibilities
Sharks are well known for their sensory capabilities. It was once believed that sharks had 6 senses. But now we can identify 8 distinct shark senses. They are hearing, smell, vision, touch, taste, distant touch (vibration), electrosense, magnetoreception.
Sharks primary sensory structures are the lateral line, pit organs, and ampullae of Lorenzini. Here’s how they work:
Vibration/Pressure: The shark’s lateral line is made up of a row of small pores that run all the way from the snout to the tail. It allows them to feel pressure changes in the water and vibrations of nearby prey and predators.
Electrosense: The pit organs on the shark’s skin can pick up the weak, low-frequency electrical fields given off by prey. This allows sharks to hunt at night or find hiding prey. Hammerhead sharks can find prey that’s completely buried under the sand, making them experts at hunting stingrays.
Magnetoreception: The Ampullae of Lorenzini are like little semiconductors connected to snout pores that help to give sharks the ability to detect and orient to the Earth’s magnetic field.
A Tail of Two Sharks
There are hundreds of different shark species, each with its own special body characteristics that help them survive. For example, a shark’s tail–or caudal fin–can be an incredible propeller or a tool to smack some prey.
Some sharks have homocercal tails, where the upper and lower tail lobes are the same size, or symmetrical. A symmetrical tail can propel a shark through the water at high speeds. A great example of this tail is the shortfin mako whose latin name, Isurus oxyrinchus, means “same tail, pointy snout.” The shortfin mako can have swim bursts up to 90 miles per hour!
Other sharks have heterocercal tails, which means that the top and bottom parts (the lobes) of the fin are asymmetrical. The bottom of the tail moves first and the top trails behind and bends at the tip. This lifts the body and allows the shark to make quick turns. The most dramatic example of a heterocercal tail is the thresher shark. The upper lobe of the tail is as long as the rest of its body! The thresher shark uses that long tail to slap and stun fish when hunting.
Check out a video of a thresher whipping its tail at prey.
You Can Teach an Old Shark New Tricks
Did you know that sharks can be trained in the same way we train dogs and other animals? Sharks are highly intelligent and some species are capable complex behaviors.
In the 1950s, "Shark Lady” and ocean science pioneer, Dr. Eugenie Clark, discovered that sharks can recognize colors, shapes, learn to perform behaviors. By teaching lemon sharks that pressing an underwater target would earn food rewards, Clark demonstrated that sharks can be trained.
Today, many aquariums and research facilities use Dr. Clark’s target training and feeding method to attract sharks to a designated feeding area. The feeding method is a helpful way to ensure each shark gets the right amount and type of food, which is especially important when feeding a mix of shark species in one pool.
See a trained target feeding in action at New England Aquarium!
Shark Tagging: More than Just Tracking
Some shark species move and migrate large distances in the ocean, from nearshore to offshore, from the sea surface to the sea floor, and even across international boundaries. Tagging sharks makes it possible to track them. Advanced technology also allows scientists to receive data about the ocean environments they visit and the way the shark's body moves. The type of tag determines the type of information scientists can record.
A pinger tag is inserted into a shark. When the shark swims near a listening station on the sea floor the listening station hears the “ping” and records the time, date, and individual identification of the shark.
Fin-Mounted Smart Position and Satellite Tags
These types of tags are placed on a shark's fin. When the shark comes to the surface, the tag is exposed to air and a signal is sent to the satellite, providing information on the shark's location.
Popoff Satellite Archival Tags
These types of tags are placed at the base of a shark’s fin to record information about the ocean environment (such as temperature, pressure, salinity, and light levels) and location of the shark. The tag is programmed to pop off the shark after a specified amount of time, float to the surface, and transmit the information to the satellite.
Fin-mounted technology, such as accelerometers and cameras, are used to collect data about the movement of a shark’s body. They record video of a shark’s behavior and the ocean environment. Accelerometers use the same technology as cellphones to determine the pitch, roll, yaw, and speed of the animal as it swims. Fin-mounted cameras have helped researchers make discoveries about coral reefs, seagrass beds, and white shark feeding behavior.
Spaghetti, Dart, or Roto Tags
These simple numbered tags, attached to the shark with identification information, do not contain any technology. Rather, they allow scientists to collect basic information such as growth rates of the shark when the shark is recaptured, sometimes years later. The information collected with these tags has been critical for shark science and management for decades.