Hammerhead sharks are strange-looking sharks. They look like someone grabbed the eye sockets of their skulls and stretched their heads sideways, while the rest of their bodies look like normal sharks. You may ask - what are the benefits of a hammer-shaped head? How did the hammerhead shark become like this in the first place? I am a scientist who has studied sharks for nearly 30 years. The answers to some of these questions surprised even me.
Hammerhead sharks have extraordinary vision. Their eyes are uniquely positioned on the broad side of their hammer-shaped heads, allowing them to see almost 360 degrees. This position allows them to see above and below easily, giving them a significant advantage in spotting prey and predators.
The benefits of growing a hammer head
Scientists believe that sharks with hammer-shaped heads have three major advantages.
The first is vision. If the eyes are facing in two opposite directions, the field of view will undoubtedly be wider. Each eye sees a different part of the world, allowing for a better perception of things around you. But there are also disadvantages. It will be difficult to visually distinguish the distance of things.
To make up for this shortcoming, hammerhead sharks have a special sensory organ called the "ampullae of Lorenzini" scattered at the bottom of the "hammer." These pore-like organs can detect electrical currents.
These pores act like metal detectors, sensing and locating prey buried beneath the sand on the ocean floor. Common sharks also have these sense organs, but hammerhead sharks have more. The further apart these sense organs are in a hammerhead shark's extended head, the more accurately they can locate food.
Finally, scientists believe the hammerhead helps the shark turn faster while swimming. If you've ever walked with an umbrella in a strong wind or flown on an airplane, you know how helpful a large surface can be to help gain momentum during movement. If you were a hammerhead shark and your dinner was swimming by quickly, you could turn around and catch it faster than other fish.
Hammerhead Shark Family Tree
It would be great if scientists like me could trace the development of hammerhead sharks through fossils. Unfortunately, hammerhead shark fossils consist almost exclusively of their teeth. This is because sharks have no bones in their bodies. Instead, they are made of cartilage, which is also what ears and noses are made of. Cartilage breaks down faster than teeth or bones, so it rarely evolves into fossils. And the tooth fossils don't tell much about the evolution of the hammerhead shark's skull.
Today, there are nine different species of hammerhead sharks swimming in the ocean. They vary in body size and head shape. Some hammerhead sharks have very wide heads relative to their bodies. These include the T-shaped hammerhead shark (E. blochii), the great hammerhead shark (S. mokarran), the smooth hammerhead shark (S. zygaena), the scalloped hammerhead shark (S. lewini), and the Carolina hammerhead shark (S. gilberti).
Other sharks have smaller hammerheads relative to their bodies, including the narrow-headed hammerhead shark (S. tiburo), the short-nose hammerhead shark (S. media), the small-eyed hammerhead shark (S. tudes), and the long-nose hammerhead shark (S. corona).
Scientists have long believed that the original hammerhead sharks did not have very large hammers, but over time some hammerheads slowly evolved larger hammers. We think the different hammerhead sharks living today are snapshots of different times in evolution - the small hammerhead shark is the oldest species on the family tree, while the giant hammerhead shark is the newest species.
Since we had no fossils to look at, scientists like me explored this idea using DNA. DNA is the genetic material found in cells and carries information about an organism's appearance and function. It can also be used to study relationships between living things.
Hammerhead sharks are excellent navigators. Researchers believe the shape of hammerhead sharks' heads enhances their ability to detect Earth's magnetic field, helping them navigate the ocean. This sense of direction is crucial as they migrate long distances in search of food and breeding grounds.
We extracted DNA from eight of the nine species of hammerhead sharks and used it to study their relationships. The results were completely different from what we expected. Older species have proportionally larger hammerheads, while younger species have smaller hammerheads.
When scientists think about evolution, we usually think of organisms changing a little at a time, slowly fine-tuning themselves to better suit their environment. This process is called natural selection. But that's not always the case, as the evolution of hammerhead sharks illustrates.
Sometimes an animal is born with a genetic defect that is useful to its survival. As long as the abnormality is viable and the animals are able to mate, the trait can be inherited. We think this is exactly the case with hammerhead sharks.
The earliest diverging species of hammerhead shark is the T-shaped hammerhead shark (E. blochii), which has the widest head. Over time, natural selection actually reduced the size of the hammerhead. The newest species of hammerhead shark is the bonnethead shark (S. tiburo), which has the smallest hammerhead.
Author: Gavin Naylor, director of the Florida Shark Research Program at the University of Florida.
Compiled source: ScitechDaily