Sharks never cease to captivate our imaginations. Strange, scary, beautiful, powerful, unique, special ... the long list of descriptors would dwarf a whale shark! Sharks have had hundreds of millions of years to evolve and dominate the sea as perfectly honed predators. The more we study them, the more surprises they reveal. Here are just a few fascinating facts about sharks around the world.
Hammerhead sharks have a 360-degree field of vision
Hammerheads are a standout species thanks to their curiously shaped heads. Scientists have been curious about the hammerheads' head shape — and its purpose — for a long time.
Because their eyes are placed on the very tip of the elongated head, they have particularly excellent binocular vision, a 2009 study showed. Most sharks have eyes placed on the sides of their heads, rather than in the front, which means they don’t have very good stereo vision. Hammerheads, on the other hand, get a 360-degree view of the world. Ed Yong reports on Science Blogs about the work of Michelle McComb from Florida Atlantic University and her research team:
[H]ammerhead eyes, though far apart, have the greatest overlap in their fields of view. The winghead shark has a 48-degree arc in front of it that’s covered by both eyes, which must give it exceptional depth perception. By comparison, the scalloped hammerhead has a binocular overlap of 34 degrees, the bonnethead has a much smaller one of 13 degrees, and the lemon and blacknose sharks have the smallest of all with 10 and 11 degrees respectively. And that’s if the sharks swim straight ahead with their heads completely still. A hammerhead can improve its stereoscopic vision even further by rotating its eyes and sweeping its head from side to side. McComb measured these movements too by filming the sharks swimming around their tanks. Taking these movements into account, she found that the binocular overlaps of the scalloped hammerhead and bonnethead increase to a substantial 69 and 52 degrees respectively, still outclassing the 44 and 48 degree arcs of the pointy-headed species. The hammerhead species even have visual fields that overlap behind them, giving them a full 360-degree view of the world.
The only place a hammerhead has a blind spot is directly above and below its head. The substantially improved binocular vision helps explain why these sharks evolved with such a unique profile.
Cookiecutter sharks steal circular chunks of flesh from living prey
These sharks grow to less than two feet in length, yet have the largest teeth-relative-to-body size of any shark species. Why? Because they grab a bite on the go.
Cookiecutter sharks specialize in taking circular chunks out of living prey. In a way, it’s a smart strategy. They get a mouth full of food, and their prey lives on to become yet another meal in the future. It’s a win-win — albeit a painful win for the victim.
The shark accomplishes the feat through a highly specialized mouth. It swims up to a victim and latches on like a sucker, with its suctorial lips forming a tight seal. Then its huge bottom teeth sink into the flesh while it twists its body to make a circular cut. Once a chunk of flesh is removed, the prey can escape.
Cookie cutter sharks aren’t picky eaters and will take a bite out of pretty much anything swimming in the sea. Everything from tuna to whales, seals and other shark species bear the circular tell-tale scars of cookiecutter shark bites. There has even been a documented attack on a human, when long-distance swimmer Mike Spalding had a plug of flesh bitten from his calf during a nighttime swim in Hawaii.
Shark embryos in eggs can sense when danger is approaching
The most vulnerable time for a baby shark is probably when it's stuck inside an egg case without any ability to escape danger. Indeed, even embryos seem to know they’re in a dangerous situation locked inside a leathery pouch for any predator to feast upon. So they've come up with a survival strategy.
As shark embryos grow, the seal on the egg case starts to open, and at this point, predators can sense these developing fish through the electric fields emitted by their movement. But the embryos can also sense the movement of the approaching predator. When they do, the embryos freeze, and even stop breathing, in an effort to “hide” from predators and avoid detection.
Researchers tested this by mimicking the electric field of predators, recording the embryos ceasing movement until the danger passed.
"And if the embryos continued to be exposed to these electric fields when they needed to start breathing again, they did so at a reduced rate of movement of their gills, suggesting they were ‘hiding’ from the perceived predator," writes BBC.
Scientists are using this knowledge as a lead for developing better shark repellants, noting that the embryos are less wary if the electric field never varies.
Tiger shark embryos eat one another in the womb
For sand tiger sharks, life is never easy, not even in the womb. The females of this species have two uteri, and produce two pups at the end of each breeding season. But they start out the season with perhaps a dozen embryos. What happens?
The first tiny shark embryo to hatch will grow faster than its siblings, and when it reaches about 100mm in size, it will begin to kill and eat its siblings. Once all the sibling embryos are consumed, the baby sand shark will start dining on its mother’s unfertilized eggs.
The strategy of ravenously feasting on current and future generations of siblings pays off by the time birth rolls around.
“By the time those two surviving babies are finally ready to be introduced into the big, bright world, all of the pre-birth inner feasting has paid off. They emerge from their mother measuring in at about 95 to 125 centimeters long, or a bit longer than a baseball bat, meaning fewer predators can pick them off than if they had shared food with siblings and were smaller,” writes Smithsonian Magazine.
From the moment these sharks hatch from a fertilized egg in the womb, the race is on to become the biggest the fastest. And you thought baby sharks in egg cases had it tough!
The Greenland shark is the slowest moving fish ever recorded
While the Greenland shark can rival the whale shark for size, with a maximum size of around 24 feet long and an average size of 8-14 feet, it beats the whale shark (and every other fish) in another record: the slowest.
It’s no surprise, really, since these ectothermic animals primarily live in frigid waters. In a recent study, Greenland sharks were found to cruse at around 0.8 mph. That’s less than one third the average speed at which a human walks. When they turn on the speed, they max out at around 1.7 mph. In other words, you could probably walk at about half your normal speed and still outpace a Greenland shark.
If they’re so slow, how is it that they manage to catch and eat seals, a prey item often found in their stomachs? Scientists think they use their great sense of smell to locate sleeping seals and make an ambush attack on the unsuspecting mammals.
The rare megamouth shark feeds on krill
Little is known about this rarely seen species. (Photo: FLMNH Ichtyology/Wikipedia)
With a name like megamouth shark, you’d think this species would be the stuff of nightmares. And maybe it is — but only the nightmares of krill.
This large shark cruises through schools of krill, capturing food with its mega-sized mouth. It's one of three large filter-feeding sharks, including the basking shark and the much more famous whale shark.
This rarely spotted species is still a mystery to science. The first of its kind was only documented by humans in 1976. Luckily, a tiny piece of information about the life of the megamouth was put in place in 1990. Scientists caught a megamouth in a net and radio tagged it before releasing it. They tracked the shark for two days and discovered that it participates in vertical migration.
During the day, the shark hung out at depths of 450-500 feet. At night, it migrated up to around 40-45 feet below the surface. The migration follows the movement of its food source, such as krill, which also makes a daily vertical migration. Megamouth sharks caught since the first sighting have had species of krill and other tiny prey in their stomachs.
And how does it feed? Well, scientists aren't exactly sure because no one has witnessed a megamouth shark actively feeding. But researchers can take an educated guess. George Burgess, director of FLMNH's Florida Program for Shark Research, states in an interview with Scientific American:
What we can infer from the morphology of the mouth is that it is huge and the big fish has the ability to protrude its jaws upward and outward. Hence, of course, the name, "megamouth". What undoubtedly it does, it protrudes its jaws and expands the buccal cavity (the mouth) and inhales the krill and then closes the mouth, forcing those krill down the tube to the stomach.
There have been 41 megamouths caught since that first 1976 specimen, and with each encounter, we learn just a little bit more about this strange species.
Great white sharks can go weeks without eating
One shark species famous for its eating habits is the great white. This powerful predator is perfectly evolved to hunt down large prey. However, the great white can also go a long time between meals.
“A great white shark can go without eating for 3 months, and some sharks can live for a year without eating, by surviving on the oil stored in their livers,” note the biologists at Adventure Aquarium.
This is particularly useful for migration. For instance, females that feed off the coast of California will head out to an area known as White Shark Cafe, an area midway between Hawaii and California, during breeding season. Having plenty of oil stored in their liver helps them make this long journey through areas of ocean where little food can be found.
However, claims that great whites regularly go weeks on end without eating is a bit of a stretch. Indeed, a recent study from the University of Tasmania showed great whites eat three or four times more than previously thought to sustain the high levels of energy they expend during hunting.
The study tagged 12 great whites feeding on seal pups off South Australia. “The white sharks at a seal colony where we worked, they’re working pretty hard … they’re coming up to some pretty high speeds to catch the seals. Their metabolic rate or the engine that runs them is much faster than what we had assumed. These animals are probably going to be feeding you know every few days, rather than multiple weeks,” senior research scientist Jayson Semmens told ABC Television.
This new understanding of their activity level helps us better understand their crucial role in marine ecosystems, as the sharks are helping balance out larger populations of animals than previously suspected.
Some shark species return to their birthplace to reproduce
Sharks have a long memory, and where some shark species choose to give birth is proof they can hold on to information starting at a very young age.
A long-term study published in 2013 showed that at least some species of shark will return to where they were born to give birth, something called natal philopatry. It's the same behavior seen in many other species, such as sea turtles that return to their birth beach to lay eggs, or albatross that return sometimes to within feet of where they were born to build nests for their own chicks.
The study tagged 2,000 baby sharks starting in 1995 and followed them for two decades.
“At least six females born in the 1993-1997 cohorts returned to give birth 14-17 years later, providing the first direct evidence of natal philopatry in the chondrichthyans. Long-term fidelity to specific nursery sites coupled with natal philopatry highlights the merits of emerging spatial and local conservation efforts for these threatened predators,” write the study authors.
For lemon sharks, this is particularly important information as they use mangrove forests as nurseries.
According to Arkive:
The pups have a very slow growth rate and remain within nursery grounds for a considerable length of time, where they are less vulnerable to predation by larger sharks (2). The mangroves that the young frequently inhabit are highly productive waters, creating a marvellous site for feeding, but also an area of very low oxygen content. Luckily, the lemon shark has numerous adaptations that enhance oxygen uptake, such as blood with an unusually high affinity for oxygen, and thus the pups can lie feeding in the rich waters, protected from any large potential predators by the mangrove’s tangled roots.
Preserving mangrove habitat not only is key to protecting the future of this shark species, but of countless other species that need mangroves for protection, including humans.
Understanding more about sharks continuously reveals more about their crucial role in marine ecosystems, which also affect our own survival as a species. Studying sharks not only reveals more of these strange facts, but also reveals more about our reliance on them to keep our oceans in balance. Reversing the trend toward extinction of these ancient creatures has never been more important.