By Todd Sigley, Campaign Mentor
When you think of sharks, you probably picture a powerful great white chomping a seal, or a oceanic whitetip chasing down speedy tuna. These images represent much of the popular perception of sharks. The most visible sharks are the large predators, capable of taking big and fast animals as prey. Like the predatory lions, tigers, and bears (“Oh my!”) of terrestrial habitats, the big predators receive most of the attention paid to sharks. However, these species are just a small portion of the impressive diversity of the sharks. Sharks have been evolving and diversifying for hundreds of millions of years, and this has led to a stunning array of adaptations. Let’s take a look at some other, lesser-known sharks. Here we will examine shark species that have very different modes of life, and occupy unique ecological niches not commonly associated with “typical” sharks.
1. Our first stop is the deep sea, where dwells the frilled shark (Chlamydosechalus anguineus), a bizarrely shaped creature with little resemblance to what we picture as a normal shark. Its body is far more elongated and flexible than most other shark species, leading some to speculate that it is responsible for some supposed sightings of mythical sea serpents. Its eponymous frilly gill slits are unusual among sharks, most species of which are highly streamlined for fast and efficient swimming. By contrast the frilled shark lives in deep water and tends to be less active. It feeds mostly on cephalopods, which it catches with a mouthful of needle-like teeth. Seldom observed, the frilled shark is thought to attack rather like a snake, curling its body in wait, then rapidly striking to seize its prey. These adaptations to its deep sea habitat make the frilled shark seem unusual to us, but it is quietly successful in its dark world on the seafloor.
2. Next we’ll meet an open ocean dwelling shark with strange habits. The cookie cutter shark (Isistius brasiliensis) is a little-known but fascinating species. Its name comes from its unique feeding strategy. Its teeth have developed into a pair of saw-like ridges in the upper and lower jaws, which allow it to bite circular chunks out of the skin of much larger animals. Its telltale cookie cutter scars have been found on a variety of marine mammals, fishes, and other sharks. Its coloration helps it escape notice by would-be predators, and it has a series of photophores (light-producing cells) on its underside which make it difficult to see from below. The three closely related members of the genus Isistius are the only known parasitic sharks, truly a unique adaptation for members of a group that are almost all predatory.
3. Wobbegongs (family Orectolobidae) are a group of strange looking sharks. These sharks spend much of their time resting on the seafloor in warm, tropical waters. Their relatively lethargic lifestyle is unusual in sharks, most of which must swim actively to ventilate their gills. The wobbegongs and some other species of bottom-dwelling sharks have evolved the ability to force water across the gills while stationary, eliminating the need to be continuously swimming. Wobbegongs are also unique in their feeding strategy. Their cryptic coloration and plant-like tassels allow them to remain unseen by prey animals, which they can rapidly engulf in one bite. Check out this video of a diver feeding a big fish to a hungry wobbegong near Australia. The wobbegongs’ camouflage, feeding strategy, and anatomical adaptations make them very well suited for life on the bottom.
4. The spined pygmy shark (Squaliolus laticaudus) is among the smallest species of shark with a maximum length of only 11 inches. Small size confers several advantages, especially in light of the species’ behavior. It spends much of the day at depths over 500 meters, where very little sunlight reaches. It migrates vertically at night to follow its prey (mostly small fish and invertebrates), which in turn are migrating to feed on plankton under cover of darkness. At the end of the night it returns to the dark and relative safety of the depths. It has large eyes that help it sense its dark environment. Like the cookie cutter shark, the spined pygmy shark has a series of photophores on its ventral side, hypothesized to provide camouflage through counter-illumination. This species is proof that you don’t need to be big and vicious to be a successful predator.
5. Our last shark is perhaps one of the oddest animals in the oceans. The sixgill sawshark (Pliotrema warreni) is one of several species of sawshark (in the order Pristiophoriformes; not to be confused with sawfishes, which look similar but are evolutionarily distinct), distinguished by the presence of a long sawlike projection at the front of the head. The saw is an elongation of the snout, and has a series of teeth along each side and a pair of sensory barbels about halfway along it. The barbels are used in conjunction with the ampullae of Lorenzini (specialized organs which detect tiny electrical impulses produced by nerves and muscles in all animals) to sense prey animals. When it finds likely prey, it uses its saw like a sword to slash and disable the unfortunate fish or invertebrate before eating it whole. This bizarre adaptation showcases the almost limitless possibilities of evolution, and shows us that some predators have more precise methods of predation.
This is just a tiny fraction of the diversity of sharks, but you probably won’t see any of them on TV this Shark Week. These and many other less publicized shark species are in danger of harm at the hands of humans, by fishing, habitat destruction, prey competition, and climate change. When programs like Shark Week emphasize only the largest and most charismatic animals, they do a disservice to the animals, the environment, and the audience. We would benefit from a more balanced approach to Shark Week, one that emphasizes science and conservation in place of cheap thrills and hyperbole.