What are Tiger Sharks?
Galeocerdo cuvier
Though the tiger shark is colloquially known as the waste bucket of the sea, this species of requiem shark is the largest predatory shark after the great white. The tiger shark grows to a length of over 5 metres and is the largest member of the Carcharhinidae family, which includes the other well known species such as the blue shark (Prionace glauca), the lemon shark (Negaprion brevirostris) and the bull shark (Carcharias leucas). The tiger shark gets its name from the distinctive black spots and vertical bars which run along the length of their body. Often found close to the coast and in tropical and sub-tropical waters, tiger sharks are a nomadic species that feed on a wide variety of prey including turtles, rays, other sharks, fish, sea birds, dolphins, crustaceans and carrion. Tiger sharks are a large and dangerous predator and therefore require significant resources to enable comprehensive study. Due to this reason, peer reviewed publications on its basic biology and ecology are lacking.
Figure 1 Distribution Figure 2 - Tiger shark (CC Albert Kok)
Kingdom: Animalia Phylum: Chordata Class: Chondrichthyes -Sub Class: Elasmobranchii Order: Carcharhiniformes Family: Sphyrnidae Genus: Sphyrna Species: mokarran
Taxonomy and Anatomy
The shark was first described by Peron and Lessueur (1822) and was given the name Squalus cuvier(1). It was later renamed by Muller and Henle (1837) to Galeocerdo tigrinus before being finally re-named Galeocerdo cuvier1. The genus, Galeocerdo is derived from the Greek, galeos, "shark" and the latin, cerdus, "hard hair of pigs".
Tiger sharks have a green, grey or black dorsal surface (top) and a yellow-white ventral side (underbelly). The easily recognisable dark spots and stripes of the tiger shark are most distinctive in the juveniles and range from blue to light green. The juvenile colourings are thought to act as camouflage in coastal waters as they resemble the shadows of waves in the water. Scientists have suggested this camouflage makes them undetectable from the surface, especially over seagrass beds where they will often hunt air-breathing prey such as sea birds and turtles(2). The spots and stripes fade into pale bars as the sharks mature, when they are no longer exposed to predation by other sharks and are less reliant on surface prey.
The large eyes of the tiger sharks have a reflective layer behind the retina called the tapetum lucidum. This reflects light back through the eye, allowing light sensitive cells to collect light and enhance the shark's vision in low light conditions.
The wide and robust head of the tiger shark is wedge shaped and allows the shark to turn quickly. The tiger shark's dentition is very distinctive compared to most sharks, possessing teeth with similar shapes in their upper and lower jaws. The teeth on the upper jaw are large with backwards curved cusps and finely serrated edges which have evolved to not only cut their prey but to saw through it also. This tooth type is specialised in cutting and rasping through sea turtle shells(3). Tiger shark tooth
The jaws are divided into 24 rows of identical teeth that decrease in measurement as they move towards the corners of the mouth. In contrast with other sharks, the shape of the jaw is square rather than round and gives the tiger shark skull its distinctive wedge shaped look.
The serrated edges are highly effective at cutting (CC Stephan Kuhn)
Size, Age & Growth
The tiger shark is one of the largest sharks, growing to a maximum of 7.5 metres (27ft)(4) and weighing over 800 kgs(5). There is very little known about how old tiger sharks can grow and this is one aspect of tiger shark biology that scientists are currently studying. Once fully grown, tiger sharks no longer have any natural predators due to their size and the speed at which they can swim, which has been estimated at 32kmh (20 mph) or short bursts of even higher speeds(6).
Range & Habitat
Tiger sharks are a resilient species and able to tolerate a wide variety of habitats. They are most commonly found close to the coastline in tropical and sub tropical waters worldwide (Figure 1), however they have also been recorded in warmer temperate waters(7). On-going studies have suggested that tiger sharks undergo seasonal migrations, traveling into temperate waters during warmer months and then returning to the tropics for winter. Their behaviour is primarily nomadic, meaning they tend to move widely, undergoing large voyages between large island chains to areas where prey is more highly available.
Even though they are found to make these voyages, tiger sharks are not considered a truly oceanic species(8). Due to their large range in diet and a preference for shallow water habitats, tiger sharks regularly come into contact with human populations in the tropics. Tiger sharks are curious to humans in the water and can be aggressive so must be considered with a great deal of respect.
Even though scientists have a basic knowledge of tiger shark behaviour and habitat type, there is still a lot to learn about specific behaviours such as their movements and the factors that determine them. Preliminary research suggests that differences in both behaviour and general biology do exist between tiger sharks located in different regions of the world.
Diet
The diet of the tiger shark has been more thoroughly researched than many other sharks, due to the broad range of species this opportunistic hunter preys upon. As the shark ages, its diet changes (called ontogenetic diet shift) and scientists are still working to better quantify this change with shark size. Young sharks (
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