9 Surprising Facts About Sharks
Far from mindless killing machines, as they’re so often portrayed, these members of the class Chondrichthyes, or cartilaginous fish, are fascinating age-old survivors with a critical role in the ocean ecosystem.
1. The Shark Immune System Could Help Humans.
Sharks are among the oldest animals with a modern immune system, one similar to ours but with a sophisticated twist that may prove beneficial to humans. Shark blood contains large quantities of urea, which protects them from dehydrating in their salt water habitat. Urea can also destabilize sensitive protein molecules such as antibodies, though, and similar levels would destroy those in humans. Sharks have an additional salt bridge between structurally important amino acid chains and a particularly large non-polar nucleus of the Immunoglobulin fold in their antibodies—a complicated way of saying they have special adaptations to handle all that urea. Researchers now have integrated these adaptations into human antibodies, resulting in increased stability that could lead to improved therapy and diagnosis for human diseases.
2. Great Whites Live Much Longer than We Previously Thought.
Estimating the age of a great white shark presents a challenge—and no, getting close enough to one to ask isn’t the problem. Scientists determine the age of bony fish by analyzing mineralized tissues—ear bones, vertebrae, and fin rays—that have annual rings much the same as trees. Sharks have skeletons made of cartilage, not bone, except for their vertebrae. And while vertebrae do contain layers of tissue laid down sequentially over time, the bands can be less distinct and don't necessarily equate to annual growth. Using this method had previously yielded top ages for great whites of 23 years. When researchers used radiocarbon to analyze collagen in the vertebrae, they estimated the largest male was 73 years old, making great whites among the longest-lived cartilaginous fishes. Guess they need that unlimited supply of teeth.
3. Some sharks return to their birthplace to reproduce.
Sea turtles are famous for returning to the beaches where they hatched to lay their own eggs, many miles and decades later. Scientists call that natal philopatry and long-term fidelity to parturition sites. Turns out, some sharks have that, too.
A 19-year study that began in 1995 and involved the capture, tagging, and release of more than 2000 baby sharks revealed that female lemon sharks returned to where they were born to give birth, up to 15 years later. The discovery means that preserving local nursery habitats could protect future generations of sharks.
4. Oils stored in the liver fuel a great white’s long migration.
Great white sharks make non-stop trips of more than 2500 miles across the Pacific Ocean, crossing large swaths of open water with little if anything for them to eat. A study reveals that fuel for the journey comes from fat stored in the sharks’ livers, which account for up to a quarter of their body weight. It’s an approach similar to how hibernating bears bulk up and migrating whales pack on the blubber. Oils stored in the liver also provide the sharks with increased buoyancy.
Scientists used data records from white sharks in the eastern Pacific, equipped with electronic tags that record location, depth, and water temperature, to identify periods of drift diving—when marine animals descend passively and allow momentum to carry them forward like underwater hang gliders. The researchers estimated the amount of oil in an animal’s liver by measuring the rate at which it sank during drift dives; less oil to provide buoyancy meant a quicker descent while more oil equaled a slower one. Consistent decreases in buoyancy during migration indicated gradual but steady depletion of oil in the liver, meaning the sharks depended on that stored energy for their journey.
5. Shark embryos can detect danger.
Shark embryos inside an external egg case can detect the presence of predators and freeze, Bambi-style, to avoid being detected themselves. Adult sharks detect electric fields emitted by potential prey, and their embryos employ similar receptors to detect potential predators. When researchers created electric fields mimicking a predator, brown-banded bamboo shark embryos grew more still by reducing respiratory gill movements. Knowing about this behavior could help humans develop more effective ways of repelling grown-up sharks.
6. Humans and sharks share a common ancestor and similar genes.
Jawed vertebrates on earth—including sharks and humans—have a common ancestor, most likely Acanthodes bronni. Using more than 100 body characteristics, researchers compared resemblance among the earliest jawed fishes and found that Acanthodians as a whole clustered with ancient sharks. Perhaps it won’t surprise you that our relationship didn’t last long; the descendants of this shark-like fish from the Paleozoic era split more than 420 million years ago into early sharks and the first bony fishes, with humans eventually evolving from the latter. We remain connected, though, as recent large-scale analysis of the genes of great white sharks revealed that the proportion of its genes associated with metabolism and its heart RNA molecules were more similar to those of humans than of zebrafish, part of the bony fish line.
7. The shark family is large and diverse.
There are nearly 500 species of sharks. This large extended family includes the 6 inch dwarf lantern shark and the 40 foot whale shark, the round and flattened angel shark and the gape-mouthed basking shark. There are families of gulper sharks, lantern sharks, sleeper sharks and dogfish sharks; angel, bullhead, and carpet sharks. There are even zebra, crocodile, mackerel, hound, weasel, and cat sharks (something of a theme there). The family includes rays and skates as well. And as-yet undiscovered species likely lurk in the abyss, where as recently as 1976 we discovered Megamouth, a deep-water shark reaching lengths of 16 feet with a short snout and large mouth sporting 50 rows of teeth. And you thought your relatives were strange.
8. Shark skin reduces drag—and provides thrust.
Sharks are legendary for their efficiency at moving through the water, thanks to a streamlined body and tiny denticles, or tooth-like scales, on their skin that reduce drag. Sharkskin has inspired the design of suits worn by human swimmers and other marvels of engineering. It turns out that as a shark’s body flexes when it swims, the denticles alter the structure of water flow—technically they ‘promote enhanced leading-edge suction’—which may actually provide thrust in addition to reducing drag. Advantage: shark.
9. Sharks and human hunter-gatherers share the same foraging pattern.
Sharks, bees, and many other animals follow a pattern known as the Levy walk when they forage. This pattern of movement is similar to the mathematical ratio phi, which has been found to describe proportions in plants and animals throughout nature. A recent study confirmed that hunter-gatherer tribes of humans also follow the pattern, showing yet again that we have more in common with sharks than you might think.
BONUS RAY FACT: Devil Rays dive deeper than a mile.
Devil rays grow up to 13 feet across, travel across large areas of the ocean, and are often spotted in warm, shallow waters. But scientists recently discovered that these rays dive deeper than a mile. They tracked 15 animals in the central North Atlantic using pop-up satellite archival transmitting tags, which stay on the animals for up to 9 months, recording water temperature, depth, and light levels. The tags eventually pop off, float to the surface, and beam their data via satellite to waiting computers on shore. These 15 tags revealed that their bearers routinely descended almost 1.24 miles deep, traveling at speeds up to 13.4 miles per hour, and remaining there for two or three hours. That’s a hell of a dive.
All images courtesy of iStock.