Coral Reefs: Diversity on Display
One of the largest and most notorious of sharks, the Tiger Shark (Galeocerdo cuvier) is better known by reputation than it is in Real Life. This species is much feared as a man-eater and is responsible for much of sharks’ collective reputation as “swimming garbage cans”.
Just the Facts:
Habitat: Intertidal, Estuaries, Sandy Plains, Rocky Reefs, Coral Reefs, Open Ocean, Deep Sea
Depth: surface-1000 ft (305 m)
Distribution: Arctic, Central Pacific, South Pacific?, Tropical Eastern Pacific, Chilean, Western North Atlantic, Caribbean, Amazonian, Argentinean, Eastern North Atlantic/Mediterranean, West African, Southern African, Central South Indian, Madagascaran, Arabian, Indian, South East Asian, Western Australian, Southeast Australian/New Zealand, Northern Australian, Japanese
Just about every ‘weird’ thing ever pulled from the stomachs of sharks came from Tiger Sharks. Among the indigestible items that have been removed at one time or another from the stomachs of Tiger Sharks are a roll of tar paper, a tom-tom, a chicken coop, and unexploded munitions. Among such an impressive miscellany, human remains hardly seem ‘exotic’. Clearly, the Tiger Shark is gastronomically adventurous, willing to eat virtually anything it finds in its environment.
Despite the frequency and enthusiasm with which they are recounted, inedible junk and human remains are extremely rare among Tiger Shark stomach contents and cannot be considered a normal component of this species’ diet. What a Tiger Shark consumes is largely a function of what food is available in the waters in which it finds itself. Discarded rubbish, remains of domestic animals and humans are occasionally found among this species’ stomach contents near centers of human habitation and are rare or unheard of far from such influence. For the most part, Tiger Sharks prey on whatever marine species are locally abundant. Normal Tiger Shark prey includes a wide variety of teleost fishes, rays, other sharks, marine turtles, sea snakes, seabirds, seals, dolphins, spiny lobsters, crabs, horseshoe crabs, octopuses, squids, marine snails, and jellyfishes. As such, the normal diet of the Tiger Shark is more varied than that of any other shark.
A recent study explored dietary changes in the Tiger Shark associated with growth. Stomach contents from 217 Tiger Sharks caught during Hawaiian ‘shark control’ programs were divided into nine prey categories and were analyzed in terms of percent occurrence by three shark size classes. Of sharks in the smallest size class, less than 6.5 feet (2 metres) in length, nearly 90% of stomachs contained teleost fishes. Of sharks in the middle size class, between 6.5 and 10 feet (2 and 3 metres) in length, nearly 80% of stomachs contained teleosts and 35% contained crustaceans, the second-most abundant prey category for this size class. Of sharks in the largest size class, greater than 10 feet (3 metres) in length, 40% of stomachs contained teleosts, anther 40% contained elasmobranchs (sharks and/or rays), and about 30% contained crustaceans, the third-most abundant prey category in this size class. Birds were found in about 20% of stomachs of the two smaller size classes but were found in about 25% of stomachs from the largest size class. Sea turtles were not found among stomach contents of the smallest size class, were found in fewer than 10% of stomachs from the middle size class and 15% of stomachs from the largest size class. Similarly, remains of marine mammals were not found among stomach contents from the smallest Tiger Shark size class, were found in fewer than 5% of the stomachs from the middle size class, and were found in fewer than 10% of stomachs from the largest size class. The incidence of inedible items increased steadily with size class. Similar patterns were found in Tiger Sharks from other regions.
From these data, the researchers concluded that the Tiger Shark is an opportunistic feeder that undergoes a dietary shift as it grows. While I agree that Tiger Sharks are opportunists, I think it is misleading to characterize their growth-associated changes in feeding habits as a ‘dietary shift’. Rather, I prefer to think of this as ‘dietary expansion’. At every Tiger Shark size class, fishes are the predominant prey type. This is supplemented with a variety of other prey categories. As these sharks grow, the diversity and dimensions of their prey likewise increases. This growth-associated dietary expansion is most likely associated with sheer increase in size and strength, expanded range and variety of habitats exploited, as well as increased hunting skill. Tiger Sharks less than 6.5 feet (2 metres) in length simply lack the body mass and jaw power to overpower and shear through the tough carapaces of sea turtles. In consequence, this prey category does not occur in the smallest size class of Tiger Shark. Tagging and telemetry studies have shown that young Tiger Sharks remain close to the shallow nursery area where they were born but larger individuals may undertake long-distance, open ocean voyages of at least 1,870 nautical miles (3,460 kilometres). It stands to reason that a large shark with a range of thousands of miles (kilometres) would encounter a greater diversity of prey than one restricted to a small, relatively uniform coastal habitat. Further, seasonal observations of Tiger Sharks feeding upon fledgling seabirds off oceanic islands indicate increased prey capture success with practice, which is strongly suggestive of learning. It follows, then, that predatory skills are honed over time and that a younger, less experienced Tiger Shark would probably be less competent under a narrower range of tactical situations than would an older, more experienced shark. Therefore, Tiger Sharks are opportunistic predators whose diet expands as they increase in size, range, and experience.
The Tiger Shark owes its ability to exploit such a broad prey spectrum to its large, powerful jaws and unique, cock’s comb-shaped teeth. The mouth of this species is proportionately wider and shorter than that of a typical whaler shark of the genus Carcharhinus. The short, heavily calcified jaws of the Tiger Shark, combined with thick adductor (closing) muscles, affords this species a very powerful bite. But perhaps the most significant factor contributing to the Tiger Shark’s euryphagous habits is its dentition. Its teeth are asymmetric, heavily constructed, and similar in both jaws. Each tooth blade is serrated and features a large, primary cusp inclined toward the nearest jaw corner followed by a series of secondary cusplets. It is a design that is astonishingly beautiful from an engineering standpoint: the primary cusp serves to puncture the defenses of tough-skinned or hard-shelled prey while the secondary cusplets tear through skin, shell, bone, and flesh. This tooth design enables large Tiger Sharks to puncture and rip even the tough shells of marine turtles. The bony carapace and plastron of a sea turtle are ample protection against the dentition of most sharks, but the puncture-and-rip dentition of the Tiger Shark is ideally suited to expose the meat within with the brutal efficiency of a tin-opener. When a Tiger Shark’s head is shaken violently from side-to-side, the cutting efficiency of its rip-saw teeth is maximized and no sea turtle stands a chance. Thus, the Tiger Shark consumes marine turtles far more frequently than other large sharks because only it combines great mass with large, heavily fortified jaws and efficient puncture-and-rip teeth.
The predatory abilities of the Tiger Shark are also enhanced by varying its behavior to accommodate that of its prey. Combined fishing, shark meshing, and tracking data indicate that, in most localities, this species spends its daylight hours offshore in deeper water and enters shallow, coastal waters at night. But this pattern does not hold true in the Leeward Island Chain of Hawaii. On these Islands occur the only known breeding colonies of the endangered Hawaiian Monk Seal (Monachus schauinslandi). These seals measure 8 to 9 feet (2.4 to 2.8 metres) in length and about 375 pounds (170 kilograms) in mass. That’s a lot of protein and fat, so it’s not surprising that large Tiger Sharks prey on Hawaiian Monk Seals when they can. In response, the seals have developed anti-predator strategies, including remaining near shore during daylight hours, ducking into favored underwater caves where exhaled air (which is oxygen-reduced but still breathable) is trapped to avoid a potential predator, and foraging in deeper waters under cover of darkness. Telemetry studies indicate that in the leeward Hawaiian Islands, Tiger Sharks reverse their usual day-night pattern, remaining inshore during the day and moving offshore at night. This localized reversal may serve to increase Tiger Sharks’ likelihood of intercepting a Hawaiian Monk Seal and possibly catching it off-guard or when its vision is severely limited by darkness. However they manage to catch Hawaiian Monk Seals, Tiger Sharks certainly do prey upon them regularly enough to cause some concern for the continued survival of these endangered marine mammals.
Adult Tiger Sharks are huge and powerful animals that have little to fear from any creature in the sea. The same cannot be said of juveniles of this species. Juvenile Tiger Sharks are exceedingly slender and delicately built, with over-sized fins and featuring an elongate upper caudal lobe with a low thrust angle. This tail structure condemns new-born Tiger Sharks to an eel-like wriggle, an inefficient swimming style that renders them extremely susceptible to predators. To counteract this vulnerability, Tiger Sharks have rather large litters — usually 35 to 55 pups, but occasionally as many as 82 — and the newborns grow very quickly. In fact, Tiger Shark pups typically double their length at birth within their first year. But Tiger Sharks do not outgrow their ‘awkward stage’ until they reach a length of about 8 feet (2.4 metres) at about four years of age.
A fascinating study compared growth rates of Tiger Sharks from off Virginia and from the northwestern Gulf of Mexico. In both locations, males mature at a length of about 10.2 feet (3.1 metres) and females mature at a length of about 10.5 feet (3.35 metres). Off Virginia, both male and female Tiger Sharks mature at about 10 years of age, but in the northwestern Gulf of Mexico, male Tiger Sharks mature in only 7 years and females in only 8. Therefore, Tiger Sharks in the northwestern Gulf of Mexico grow significantly faster and reach maturity at a much earlier age.
Why? According to the researchers, unlike those from off the U.S. Atlantic seaboard, Tiger Sharks from the northwestern Gulf do not seem to undergo a long-distance northerly migration during their first two years of life. This may enable Tiger Sharks from the Gulf to conserve their energy stores toward rapid linear growth. This rapid growth may, in turn, enable Gulf of Mexico Tiger Sharks to outgrow their ‘awkward stage’ sooner — thereby reducing their period of vulnerability to such predators as the Bull Shark (Carcharhinus leucas) which is relatively common in the Gulf and seems to have a dietary preference for feeding on the young of other shark species.
Unfortunately, rapid linear growth during their earliest years does nothing to discourage the Tiger Shark’s single most dangerous predator. If anything, the large size and fearsome reputation of this species actively encourages sport anglers to answer its ‘challenge’ via hook and line. In truth, the Tiger Shark makes no such challenges. It is simply — or perhaps not-so-simply — a large shark doing what it can to earn a living in the sea. If its broad diet should occasionally expand to include a large, terrestrial primate, we should perhaps resist the urge to take such accidents so personally. If we were not paying attention while invading the Tiger Shark’s dining room, why should we insist that the shark accept all the blame?
Odd Objects Swallowed by Tiger Sharks
A veritable “swimming garbage can”, the Tiger Shark (Galeocerdo cuvier) is notorious for swallowing strange objects. An incredible variety of objects – some of which are edible, others of which are not – have been found inside Tiger Shark stomachs. Despite the fact that the tropical seas it typically inhabits are rather nutrient-poor, this species grows to impressive size – a length of at least 18 feet (5.5 metres) and a mass of more than 2,000 pounds (900 kilograms). Perhaps as an adaptation to its nutrient-poor habitat, the Tiger Shark seems willing to extend its diet to just about anything it can swallow. As though to protect it from its own gastronomic adventurousness, the Tiger Shark has a capacious stomach with a muscular wall that is proportionately some three times thicker than that of any other carcharhinid. Below is a list of some of the more unusual to downright bizarre objects that have been recorded from the stomachs of Tiger Sharks.
roll of tar paper
roll of chicken wire
bag of potatoes
sac of coal
aluminum soft-drink cans
nuts and bolts
2-pound (1 kilogram) coil of copper wire
pair of shoes
bag of money
pieces of coal
mass of tangled hair
brass casing from an 18-pound (8 kilogram) shell
unopened tin of salmon
2-pound (1 kilogram) can of peas
oral contraceptive dispenser
oceanographic drift marker
conch shells and opercula
head and forequarters of a crocodile
spaniel with collar
hind legs of sheep
cattle bones and hooves