Sharks: Quite Possibly Our Planet’s Most Resilient Vertebrate
Sharks are some of the most feared animals in existence today. But let’s take a look at the numbers: swimming at the beach gives you exactly a one in 11.5 million risk of being eaten by a shark. Sharks, on the other hand, are killed by humans to the tune of about 100 million each year. Whether accidentally snagged as bycatch in the fishing nets of commercial boats or intentionally sought-after for their dorsal fins to make shark fin soup, they’re being killed at an alarming rate – although you wouldn’t necessarily know it based on their reputation. Sharks may not be the kind of creature you want to cuddle up with, but these cartilaginous fish are extraordinary, fascinating creatures. To shift the bias around sharks, we decided to take a deep dive into what makes them so effortlessly cool and wondrous.
Against The Odds
450 million years ago, the Earth was a very different place to the world we inhabit today. An Ice Age was coming to a close, causing sea levels to rise. The seven continents were all connected, forming one giant landmass. Life above the water line was virtually nonexistent. Under the sea, the first coral reefs emerged and invertebrates like mollusks and crustaceans prevailed. The evolution of aquatic vertebrates also began during this period – and included a group of fish we call sharks.
If the evolution of sharks tells us anything, it’s that these beasts are true survivors. Since their early days, sharks have lived through 5 mass extinction events(!). What’s made them so resilient? It’s hard to know for sure, but one likely reason is that many species of sharks tend to hang out in the deep sea, meaning above-surface happenings (like giant meteorites hitting the Earth’s surface) aren’t as disruptive. Sharks are also dietary generalists: the calories they consume come from a wide range of sources. In desperate times, chances of survival increase for those able to eat whatever crosses their path. It is in the very nature of many shark species to do just that.
This isn’t to say that all sharks have weathered these last few hundred million years. On the contrary, many species have died out and new ones have evolved over the Earth’s cycles of growth and extinction. A period known as the Carboniferous Period some 359 million years ago is said to have been the Golden Age of Sharks. About 3/4 of all the Earth’s species had recently died out – including many fish – so sharks were free to dominate.
And dominate they did. Uninhibited by predators, a number of strange new shark species evolved: the stethacanthus with its unusual fin sticking out of its back; the helicoprion with its spiral saw-like lower jaw; and the falcatus with a spike-like protrusion reaching over its head, to name just a few.
A few hundred million years – and another wave of extinction – later, the largest shark ever known to exist evolved. It is called otodus megalodon and it is thought to have measured between 50 and 65 feet in length. This enormous predator used its jawline of 276 teeth to feast on a wide range of aquatic animals, from dolphins to humpback whales. Here’s another piece of trivia about the mighty megalodon: it is estimated that their jaw bite had a force of between 108,000 and 182,000 Newtons. Compared to a human’s 1,300-Newton bite and a great white shark’s 18,000-Newton bite, that’s some serious chow power.
Sharks’ Dental Situation
Based on their diets of just about anything, it will probably come as no surprise that sharks go through teeth rather rapidly. We’re talking at a cadence of about one to two weeks for every new set of teeth. It’s not unusual for a single shark to go through 40,000 teeth in their lifetime. This means plenty of teeth trickle down to the ocean floor, and often end up along beaches or riverbeds, ripe for the picking. As teeth are the only part of sharks that can be fossilized and preserved over time (the rest is soft cartilage), these dental fixtures offer a wealth of information about both past and present species, like when and where they live(d) and what they eat.
Is The Skin of A Shark Bulletproof?
Whether or not you’ve actually considered this question, you may like to know that the answer is yes, in the case of a whale shark, their skin is essentially bulletproof. With a 6-inch layer protecting them from predators, the epidermis of these spotted carpet sharks has made it tricky for scientists to get a blood sample (on top of all the other challenges that come along with pricking the skin of a shark to draw blood). Whale shark’s skin is in fact the thickest, toughest skin of any animal on the planet.
From the 55-foot whale shark to the tiny 8-inch dwarf lantern shark, sharks are a pretty diverse group. While they’re generally solitary ocean dwellers who only contact each other during mating season, there are species such as the lemon shark who are quite social and live together in groups. Although for a shark, being social is more a matter of expressing themselves physically with movement: they are completely silent creatures with no audible abilities.
Sharks also reproduce in different ways, some by laying eggs and others by bearing live young. For the sharks that grow in utero, their hunting practice gets an early start. In certain shark species like the tiger shark, the strongest sibling actually eats the other embryos, followed by their mother’s unfertilized eggs. Sharks have two uteri, which means there are usually two “pups,” or baby sharks, born in a litter – the triumphant one from each uterus. No one said being a shark was easy.
A shark’s lifespan is another factor that varies greatly. On average, a shark in the wild lives to be about 25-30 years old. However, there are many exceptions. Take, for instance, the Greenland shark. It spends its life in the frigid waters of the Arctic, and it’s not unusual to find them swimming at depths as low as 7,200 feet. They are the slowest-moving shark around (traveling up to 1.8 miles per hour) and they also grow less than half an inch per year – but have been found to reach lengths of 20 feet. How is this possible? It’s because they can live to be at least 250 years old, and potentially up to the ripe old age of 500(!), making the Greenland shark not only the longest living shark, but also the longest living vertebrate on the planet. Apparently slow and steady is the name of the game.