Sharks have a reputation for being bloodthirsty, all-powerful monsters lording over the ocean, but many of the hundreds of described species are actually quite small, retiring and inoffensive. Almost all sharks face the prospect of predation at some point in their lives: Even apex predators such as white and tiger sharks must deal with larger members of their own kind. From active self-defense to evasion, sharks pursue a variety of strategies to stay out of harm's way.
Once they reach adult proportions, larger sharks deter most predators simply by being too big, strong and fierce to tangle with. Aside from orcas and, especially, bigger sharks, few marine creatures are capable of realistically tackling medium- to large-sized species such as most hammerheads (Sphyrnidae), requiem sharks (Carcharhinidae) and mackerel sharks (Lamnidae). The huge filter-feeding sharks -- megamouths, basking sharks and whale sharks -- are not formidably toothy, but they're so massive that only pods of orcas threaten them.
Sharks will turn the powerful, tooth-filled jaws they use to subdue prey upon an attacker. The gray reef shark performs elaborate threat displays to ward off potential predators. When it feels threatened, this bold, mid-sized member of the requiem shark family will hunch its back, raise its snout, droop its pectoral fins and swim with amplified movements. If the antagonist -- say, a scuba diver -- doesn't heed the warning, the shark may deliver a swift bite or two before taking flight.
Aside from razor-sharp teeth and generally tough, abrasive hides, some sharks have specialized physical features that act as defensive armor. Certain species, such as the horn shark, have dorsal spines to discourage predation. The small, bottom-dwelling swell shark displays one of the more distinctive anti-predator adaptations: Exposed to a threat, it will gulp in water -- or air, if it's been removed from the sea -- to double its normal size. This transformation is particularly effective if the fish has retreated to a rocky nook, from which a predator would be hard-pressed to dislodge a fully swollen shark.
Sharks may also hide from predators by camouflaging themselves against the seafloor or reefs. The flattened wobbegong of tropical waters is a standout example in terms of cryptic coloration, although its disguise is as much to support its ambush hunting as for lying low. Mangrove swamps and seagrass beds function as important nurseries for many shark species, as hiding places are plentiful and large predators less common. In Bimini in the Bahamas, for example, young lemon sharks spend most of their first few years of life frequenting the tangled shelter of the island's coastal mangroves.
Sharks can also take active flight from potential predators. Some species are among the fastest fish in the ocean: The shortfin mako, likely the fleetest of all, can zip along at 50 kilometers per hour (31 miles per hour). In 1998, researchers documented an attack on sevengill sharks -- a large, mainly deep-water species -- by four orcas off the Patagonian coast. Evidence suggested some of the sharks attempted to escape the whales by stranding themselves -- a rather extreme form of evasion.
Shaw, Ethan. "How Do Sharks Protect Themselves?" Sciencing, https://sciencing.com/sharks-protect-themselves-4567351.html. 24 April 2017.