Sharks are ancient fish that have evolved over millions of years to occupy diverse ecological niches across all the world’s oceans. Few marine creatures prompt as much public debate regarding their diet. Their varied size, environment, and physiology mean that a single label cannot accurately describe the dietary strategy of all 500-plus shark species. The answer to how they eat is not a simple choice between two extremes but rather an intricate blend of behaviors driven by necessity and opportunity.
Defining Feeding Roles in the Ocean
Scientific classification provides a clear framework for understanding marine diets. A predator is an organism that actively hunts, captures, kills, and consumes other living animals for sustenance. In contrast, a scavenger feeds primarily on carrion, which is the dead and decaying organic matter left behind by other events. For example, invertebrates like crabs and whelks are well-known oceanic scavengers. These two roles are not mutually exclusive, and many species fall under the category of an opportunistic feeder, adjusting its diet based on the most readily available food source in its immediate environment.
Sharks as Active Predators
The primary ecological identity for the majority of shark species is that of an active predator, utilizing highly specialized senses and techniques to capture living prey. Apex species like the Great White, Mako, and Hammerhead Sharks actively target agile marine mammals and large fish. Their hunting prowess is supported by an extraordinary sensory organ known as the Ampullae of Lorenzini. These tiny, jelly-filled canals terminate in pores around the shark’s snout and can detect incredibly weak bioelectric fields.
The electrical signals generated by muscle contractions and gill movements of a nearby fish are easily sensed by the Ampullae. This electroreception allows a shark to locate prey even when it is completely hidden from view, such as a flatfish buried under the sand or a seal swimming in murky water. The organs are so sensitive they can register fields as small as five billionths of a volt per centimeter.
The Role of Opportunistic Scavenging
While most sharks are built to hunt, many also function as opportunistic scavengers, securing nourishment with less effort when the chance arises. The Tiger Shark, famous for its wide diet, is a prime example of this flexible feeding strategy. These sharks are sometimes called the “garbage cans of the sea” because they readily consume carrion, including the massive carcasses of whales or dolphins that sink to the seabed. Scavenging a large whale carcass provides a huge, energy-rich meal that requires minimal effort compared to an active hunt. This behavior is also pronounced in deep-sea species such as the Greenland Shark, which exhibits an extremely sluggish metabolism.
Due to their slow movement and cold habitat, an average Greenland Shark only requires between 61 and 193 grams of food daily to maintain its body weight. This low energy requirement means they are highly adapted to survive on sporadic, large scavenging meals or by ambushing slow or injured prey.