Negaprion brevirostris (Poey, 1868)
Lemon shark, Requiem shark, Shark
The lemon shark has been the subject of one of the most long-term and intensive studies on a shark species. Much of what is known about this shark is due to the work of Dr Samuel Gruber and his colleagues. This predator is most active at dawn and dusk, and occurs singly or in loose aggregations of up to 20 individuals. It feeds primarily on fishes, including sea catfishes, mullet, stingrays and eagle rays, but also on crustaceans and molluscs. During the day they often lie quietly on the seabed, apparently resting, but in reality this behaviour uses up more energy than when swimming, due to the extra effort required to pump water over the gills (2). Therefore, they may be lying motionless waiting for wrasses or other small reef fishes to clean them of any parasites (3). The lemon shark is viviparous; the embryos develop inside the mother and receive nutrients via a yolk sac placenta. After a gestation period of 10 to 12 months, pregnant females enter shallow nursery areas in spring and summer to give birth to litters of 4 to 17 pups. The pups have a very slow growth rate and remain within nursery grounds for a considerable length of time, where they are less vulnerable to predation by larger sharks (2). The mangroves that the young frequently inhabit are highly productive waters, creating a marvellous site for feeding, but also an area of very low oxygen content. Luckily, the lemon shark has numerous adaptations that enhance oxygen uptake, such as blood with an unusually high affinity for oxygen, and thus the pups can lie feeding in the rich waters, protected from any large potential predators by the mangrove's tangled roots (2) (6). As they grow, their range expands dramatically, from six to eight kilometres up to around 300 kilometres. Maturity is reached at about six and a half years of age, and it is believed that the lemon shark lives for up to 27 years (2).