It would take the rest of this website to list all the species of animal, plant and the inanimate objects that sharks have been documented to consume. Sufficed to say, elasmobranchs feed on bony fishes, annelids (worms), crustaceans (crabs, lobsters, shrimps, copepods, etc.), molluscs (clams, mussels, cuttlefish, octopuses etc.), reptiles, mammals (marine and terrestrial), birds and other elasmobranchs. Algae and seagrass have been found in the stomachs of some sharks, although it is not clear whether it was ingested intentionally of during the consumption of another desired prey item.
Popular literature is rife with stories of sharks having consumed some bizarre items. Pieces of wood and plastic, cow and sheep bones, licence plates, sacking, jars, sheets of rubber and even bizarre items like suits of armour have apparently turned up in shark stomachs. Many of these items are purported to have been found in the stomach of one species in particular, that affectionately named a “garbage can with fins” by certain popular science authors: the tiger shark (Galeocerdo cuvier). Invariably, several authors point to human remains having been found in the stomachs of some sharks, but it must be emphasised that there is no evidence supporting the idea that sharks actively predate humans. Testimony for this can be seen along the world’s beaches every day, when thousands of people share the waters with sharks, with attacks remaining rare.
Sharks and rays tend to prey primarily on fish and crustaceans, although several species, including the tiger, great white, porbeagle and possibly the thresher shark (Alopias vulpinus), are known to consume seabirds. Indeed, tiger sharks are famed for regularly turning up at the Hawaiian Islands each year to feed on fledging black-footed albatross (Dimedea nigripes). The albatross chicks hatch out on the small island of Gardener Pinnacles, about 1,600km (1,000 miles) off shore from the main island of Hawaii, and take to the skies for the first time about three months later. Being inexperienced fliers, the chicks often crash-land into the water, where they frequently fall victim to the sharks. There are also reports in the literature of the thresher shark using its highly elongated upper caudal lobe to “thwack” seabirds resting on the surface.
Perhaps the world’s most infamous shark species, the great white (Carcharodon carcharias), is famed for feeding on pinnipeds (seals and sealions) and dolphins. It has become an almost dogmatic prophecy in the popular shark literature that as the white shark grows, it changes its diet from fish to mammals. This is, however, an erroneous interpretation of the fact that as sharks grow larger they can tackle larger prey; throughout its entire life cycle, Carcharodon feeds primarily on fish, supplementing its diet with marine mammals when it grows large enough. Interestingly, research published in 2017 by Georgia French and coworkers suggests that the change in dentition one thought to cause the “switch” to marine mammals happens only in males. Conversely, tiger sharks do appear to switch their diet from fish as juveniles to mammals and turtles as adults.
Studies on captive elasmobranchs have found that many of the reef sharks consume between 0.5 and 1.5% of their bodyweight per day, with juvenile animals consuming more than adults. The percentage is higher for the mackerel sharks (i.e. great white, mako and porbeagle), which appear to need just over 3% of their body weight per day. It should be noted that these values are averaged over time; a given shark need not necessarily feed every day. Indeed, a group of researchers working on great whites off California in the late 1970s calculated that 30kg (65 lbs) of whale blubber could sustain a 5m (15ft) shark for as long as six weeks.
The ability of lamnid sharks to fast for long periods is impressive because many are warm-bodied, meaning they’re able to maintain parts of their body (usually the eyes, brain and stomach) a few degrees warmer than the surrounding water. It is important to make clear that this is not the same as the “warm-bloodedness” observed in mammals. These sharks do not maintain a stable internal temperature and if the water temperature decreases, so does their core temperature. Instead, fluctuations in their core temperature closely follow the water temperature, albeit that there’s a thermal lag (i.e. if the water temperature falls by three degrees, then an hour or so later the shark’s core temperature will drop by three degrees).
Studies on the white shark have found that muscle temperature may be 3-5C (5-9F) higher than ambient, whilst stomach temperatures may be up to 14C (25F) above that of the surrounding water. Similarly, shortfin mako sharks (Isurus oxyrinchus) have been found to maintain body temperatures as high as 10C (18F) above ambient, although despite possessing the physiological “equipment” to raise its body temperature the longfin mako (Isurus paucus) doesn’t appear to do so. The salmon shark (Lamna ditropis), a relative of the porbeagle, may have core temperatures approaching 14C above ambient.
The method by which sharks achieve this ‘warm-bodiedness’ is via an arrangement of blood vessels called a rete mirable (“wonderful net”), or more commonly, a counter-current heat exchange system. Basically, blood warmed by the metabolic activity of the shark’s muscles is passed very close to cool (freshly-oxygenated) blood coming to the organs from the gills. The respective blood vessels split into numerous smaller capillaries, which intertwine with each other and the heat from the blood heading towards the gills into that coming in from the gills. If this didn’t happen, the heat generated during metabolism would be lost to the water over the gills, as it is in most other fish. By maintaining a higher internal temperature, the sharks have increased muscle power and more rapid digestion; warm-bodiedness also permits them to tolerate greater extremes of temperature, although offloading of heat may cause problems in tropical regions.
The feeding mechanics of elasmobranchs have been well studied. In sharks, the bite sequence begins with a lifting of the snout, followed by a lowering of the mandibular cartilage (bottom jaw) and a protrusion of the platoquadrate cartilage (upper jaw) to expose the teeth. The mandibular cartilage then drops further to expose the bottom teeth. After the bite, the snout drops and the jaws return to their normal position. Until this mechanism was fully understood, it was considered that sharks had to roll onto their backs in order to bite, but the lifting of the snout and “swinging out” of the jaws means that rolling is not required. In the smaller sharks, there is often a suction phase to the bite, involving an expansion of the pharynx (mouth and throat).
The bite sequence in rays is similar to that of sharks although, according to studies on the cow-nosed ray (Rhinoptera bonasus) by Philip Motta and Desiree Sasko at the University of Florida, the bite sequence begins with closure of the spiracles (openings on the head above the eye that lead to the gills). With the spiracles closed the lower jaw drops and the upper jaw protrudes, the lower jaw then raises to pin the prey against the platoquadrate and the jaws are brought back into their resting position in the mouth; the spiracles are then re-opened. A significant difference in the way the rays feed when compared with their selachian (shark) kin is in their teeth. Many species of batoid feed on shellfish and, as such, have flat slab-like teeth that are used for crushing, rather than biting. Although some sharks also possess these molar forms (e.g. horn sharks), many have the typical awl-like or stereotypical triangular teeth frequently associated with Jaws image.
Should an elasmobranch ingest something unpalatable, several observations from the wild suggest that they are capable of regurgitating it. Moreover, some species have actually been observed to regurgitate their stomachs, wash them out and then re-swallow them. In a paper published in the journal Nature in April 2000, David Sims and colleague Paul Andrews found that this stomach eversion could be induced in thornback rays (Raja clavata) with an injection of the vomit-inducing chemical veratrine hydrochloride. The authors report that the rays vomited up, rinsed and then swallowed their stomach again; the process of everting and re-swallowing the stomach took about 4.5 seconds. The specific mechanism by which elasmobranchs are able to evert their stomachs is still unknown, although Sims and Andrews suggest that it probably involves the fish relaxing its stomach muscles and rapidly increasing the pressure in its abdominal cavity. Alternatively, peristaltic (i.e. a sort of Mexican-wave of muscle movement) motion along the gastric tract may be involved. Whatever the specific cause, stomach eversion has been observed in several species of elasmobranch, and in some instances seems to be associated with a stressful situation.