Lemon Shark

Description

The shark's yellow colouring serves as a perfect camouflage when swimming over the sandy seafloor in its coastal habitat. The lemon shark commonly attains a length of 2.4 to 3.1 m and a weight up to 90 kg by adulthood, although sexual maturity is attained at 2.24 m in males and 2.4 m in females. The maximum recorded length and weight is 3.43 m and 183.7 kg, respectively. It has a flattened head with a short, broad snout, and the second dorsal fin is almost as large as the first. Lemon sharks have electroreceptors concentrated in their heads, called the ampullae of Lorenzini. These receptors detect electrical pulses emitted by potential prey and allow these nocturnal feeders to sense their prey in the dark.

Distribution

Habitat selection

Information about activity patterns and the use of space is important in understanding a species’ behavioral ecology. Animals often make decisions about habitat use by evaluating their environment’s abiotic conditions that serve as valuable indicators of good foraging sites or predator-safe locations. Lemon sharks select habitats in warm and shallow water with a rocky or sandy bottom.

The environmental temperature influences an individual’s body temperature, which ultimately affects physiological processes such as growth and metabolism. Lemon sharks, therefore, select warm-water habitats to maintain optimal metabolic levels. They are believed to avoid areas with thick seagrasses because they make finding prey more difficult. Lemon sharks tend to live in or near shallow-water mangroves, which are often the nursery areas of several species of fish. One theory is that lemon sharks select mangrove habitats due to the abundance of prey that resides there, while another theory posits that mangroves provide a safe haven from adult lemon sharks that occasionally feed on juvenile sharks and are unable to enter the shallow waters. Ontogenetic niche shifts, or changes in an animal’s niche breadth or position, to deeper waters are known to occur in relation to a lemon shark’s size. These changes occur due to the dramatic decrease in the risk of predation as body size increases. Habitat selection clearly depends on a variety of biological and environmental variables.

The mangrove areas that lemon sharks inhabit are often referred to as their nursery sites. A nursery site is best defined as the most common area sharks are encountered, the location sharks tend to remain at after birth or frequently return to, and the habitat used by shark groups repeatedly for several years. The nursery ground concept has been known and studied for at least a century. In addition, fossil evidence from 320 million years ago suggests the use of shallow, coastal areas as pupping grounds is primitive.

Feeding behavior

Lemon sharks feed at night and are mainly piscivorous; however, they have been known to feed on crustaceans and benthic organisms. Intraspecific predation, or cannibalism, of juvenile lemon sharks by larger conspecifics has also been documented. Rather than feeding randomly, lemon sharks display a high degree of preference for certain species and size of prey when environmental conditions are favorable. They also tend to prefer a prey when it is more abundant and available. Lemon sharks feed selectively on species that are slower and more easily captured by using a stalking technique. For example, parrotfish and mojarras are common prey in the Bahamas because they use camouflage rather than an escape response and are vulnerable due to their stationary foraging behavior. Lemon sharks feed on prey that are intermediate in size compared to other available prey. This tendency can be explained by the tradeoff between the probability of capture and the profitability when it comes to prey size. The general trend in the foraging behavior of lemon sharks conforms to the optimal foraging theory, which suggests a positive relationship between prey selectivity and availability.

Rather than rolling on their sides to rip off chunks of prey, lemon sharks approach their victim with speed only to brake suddenly using their pectoral fins upon contact. The animal then jabs forward multiple times until it has a good grasp of its prey in its jaw and proceeds to shake its head from side to side until it tears off a chunk of flesh. A feeding frenzy, or large swarm of other sharks, then forms as the individuals sense the blood and bodily fluids released from the prey. Sounds of struggling prey also attract groups of sharks, suggesting they use sound detection for predation. Group feeding behavior such as pack hunting or communal scavenging was observed in a study in which pieces of the same stingray were found in the stomachs of several lemon shark individuals that were caught and examined.

Social behavior

Lemon sharks are found in groups based on similar size. Passive sorting mechanisms such as the ontogenetic habitat shift discussed above have been postulated to contribute to the formation of groups organized based on size or sex. One exception to this behavior is that sharks up to 1 year old show no preference for groups of matched or unmatched size. One hypothesis for this finding is that it is beneficial for the small young lemon sharks to associate with the larger individuals because they have an easier time gathering information about the habitat regarding elements such as predators and local prey. Lemon shark groups form due to an active desire to be social rather than a simple attraction to the same limited resources such as the mangrove habitat and prey associated with such a habitat.

Many studies have related brain size with complex social behaviors in mammals and birds. The brain of a lemon shark, being comparable in relative mass to that of a mammal or bird, suggests they have the ability to learn from social interactions, cooperate with other individuals, and have the potential to establish dominance hierarchies and stable social bonds.

Reproduction

Lemon sharks congregate for reproduction at special mating grounds. Females give birth to their young in shallow nursery waters to which they are philopatric. Lemon shark young are known as pups and they tend to remain in the nursery area for several years before venturing into deeper waters. Lemon sharks are viviparous, meaning that the mother directly transfers nutrients to her young via a yolk-sac placenta and the young are born alive. Fertilization is internal and occurs after a male lemon shark holds a female, bites her, and inserts his clasper into her cloaca. Female lemon sharks are polyandrous and sperm competition occurs due to their ability to store sperm in an oviducal gland for several months. Several studies suggest that polyandry in female lemon sharks has adapted out of convenience, rather than indirect genetic benefits to offspring. This type of polyandry is termed as convenience polyandry because females are believed to mate multiple times to avoid harassment by males. Females have a biennial reproductive cycle, requiring a year for gestation and another year for oogenesis and vitellogenesis after parturition. Lemon sharks reach sexual maturity around 12–16 years of age and have low fecundity. The maximum number of pups recorded in a litter is 18.

Importance to humans

This species of shark is best known in its behavior and ecology, mainly due to the work of Samuel Gruber at the University of Miami, who has been studying the lemon shark both in the field and in the laboratory since 1967. The population around the Bimini Islands in the western Bahamas, where Gruber's Bimini Biological Field Station is situated, is probably the best known of all shark populations. As of 2007, this population was experiencing a severe decline and may disappear altogether as a result of destruction of the mangroves for construction of a golf resort.

The lemon shark is targeted by commercial and recreational fishermen along the U.S. Atlantic Ocean, Caribbean, and in the eastern Pacific Ocean due to its prized meat, fins, and skin. Lemon shark skin may be used for leather and its meat can be consumed and is believed to be a delicacy in many cultures. Concern exists that over-fishing has led the lemon shark populations in the western north Atlantic and eastern Pacific Ocean to decline.

Lemon sharks do not represent a large threat to humans. The International Shark Attack File lists 10 unprovoked lemon shark bites, none of which were fatal.

Facts about Lemon sharks

• Lemon sharks have electroreceptors concentrated in their heads. These receptors detect electrical pulses emitted by potential prey and the lemon sharks hunt them.
• Lemon sharks are one of the most studied sea animal and shark on the planet. Therefore, scientists have had the opportunity to observe more of their behavior than any other shark.
• Lemon sharks are bottom dweller and they often track their prey by churning up the ground for rays, bony fish, and sometimes even sea birds.
• Like most sharks, Lemon sharks have very sharp teeth but they are shaped differently. They are curved rather than straight up and down. This allows them to easily catch fish that are slippery.
• They have magnetic sensors in their nose that allows them to attract prey. They depend on this sense of smell because they have very poor eyesight.
• Lemon sharks use their yellow color to successfully hide in their natural habitat. They are found swimming along the sand at the bottom of the water so they blend in with ease, so they can hide from predators.
• It is believed that this shark has a special set up with the retina. This enables them to be able to see very clearly even in the murky depths under the water.
• Most Lemon sharks do fairly well in captivity and can live a long time in captivity. They are very social though and they develop a hierarchy based on gender and size that is very complex.
• Lemon sharks are classified as Near Threatened on the IUCN Red List. However they are not currently to be at risk from extinction, still measures are being taken to save them from hunters.

Taxonomy

The lemon shark was first named and described in 1868 by Felipe Poey. He originally named it Hypoprion brevirostris, but later renamed it Negaprion brevirostris. The lemon shark has also appeared in literature as Negaprion fronto and Carcharias fronto (Jordan and Gilbert, 1882), Carcharias brevirostris (Gunther, 1870), and Carcharhinus brevirostris (Henshall, 1891).

The name may be related to the French word for shark, requin, which is itself of disputed etymology. One derivation of the latter is from Latin requiem ("rest"), which would thereby create a cyclic etymology (requiem-requin-requiem), but other sources derive it from the verb reschignier ("to grimace while baring teeth").

Family members have the usual carcharhiniform characteristics. Their eyes are round, and the pectoral fins are completely behind the five gill slits. Most species are viviparous, the young being born fully developed. They vary widely in size, from as small as 69 cm (2.26 ft) adult length in the Australian sharpnose shark, up to 5.5 m (18 ft) adult length in the tiger shark.

Requiem sharks are responsible for a large proportion of attacks on humans; however, due to the difficulty in identifying individual species, a degree of inaccuracy exists in attack records.

Classification

The 60 species of requiem shark are grouped into 12 genera:

• Genus Galeocerdo J. P. Müller & Henle, 1837
  • Galeocerdo cuvier Péron & Lesueur, 1822 (tiger shark)
• Genus Scoliodon J. P. Müller & Henle, 1838
  • Scoliodon laticaudus J. P. Müller & Henle, 1838 (spadenose shark)
  • Scoliodon macrorhynchos Bleeker, 1852 (Pacific spadenose shark)
• Genus Carcharhinus Blainville, 1816
  • Carcharhinus acronotus Poey, 1860 (blacknose shark)
  • Carcharhinus albimarginatus Rüppell, 1837 (silvertip shark)
  • Carcharhinus altimus S. Springer, 1950 (bignose shark)
  • Carcharhinus amblyrhynchoides Whitley, 1934 (graceful shark)
  • Carcharhinus amblyrhynchos Bleeker, 1856 (grey reef shark)
  • Carcharhinus amboinensis J. P. Müller & Henle, 1839 (pigeye shark)
  • Carcharhinus borneensis Bleeker, 1858 (Borneo shark)
  • Carcharhinus brachyurus Günther, 1870 (copper shark)
  • Carcharhinus brevipinna J. P. Müller & Henle, 1839 (spinner shark)
  • Carcharhinus cautus Whitley, 1945 (nervous shark)
  • Carcharhinus cerdale C. H. Gilbert, 1898 (Pacific smalltail shark)
  • Carcharhinus coatesi Whitley, 1939 (Coates' shark)
  • Carcharhinus dussumieri J. P. Müller & Henle, 1839 (whitecheek shark)
  • Carcharhinus falciformis J. P. Müller & Henle, 1839 (silky shark)
  • Carcharhinus fitzroyensis Whitley, 1943 (creek whaler)
  • Carcharhinus galapagensis Snodgrass & Heller, 1905 (Galapagos shark)
  • Carcharhinus hemiodon J. P. Müller & Henle, 1839 (Pondicherry shark)
  • Carcharhinus humani W. T. White & Weigmann, 2014 (Human's whaler shark)
  • Carcharhinus isodon J. P. Müller & Henle, 1839 (finetooth shark)
  • Carcharhinus leiodon Garrick, 1985 (smoothtooth blacktip shark)
  • Carcharhinus leucas J. P. Müller & Henle, 1839 (bull shark)
  • Carcharhinus limbatus J. P. Müller & Henle, 1839 (blacktip shark)
  • Carcharhinus longimanus Poey, 1861 (oceanic whitetip shark)
  • Carcharhinus macloti J. P. Müller & Henle, 1839 (hardnose shark)
  • Carcharhinus melanopterus Quoy & Gaimard, 1824 (blacktip reef shark)
  • Carcharhinus obscurus Lesueur, 1818 (dusky shark)
  • Carcharhinus perezi Poey, 1876 (Caribbean reef shark)
  • Carcharhinus plumbeus Nardo, 1827 (sandbar shark)
  • Carcharhinus porosus Ranzani, 1839 (smalltail shark)
  • Carcharhinus sealei Pietschmann, 1913 (blackspot shark)
  • Carcharhinus signatus Poey, 1868 (night shark)
  • Carcharhinus sorrah J. P. Müller & Henle, 1839 (spot-tail shark)
  • Carcharhinus tilstoni Whitley, 1950 (Australian blacktip shark)
  • Carcharhinus tjutjot Bleeker, 1852 (Indonesian whaler shark)
  • Carcharhinus sp. A not yet described (false smalltail shark)
• Genus Glyphis Agassiz, 1843
  • Glyphis fowlerae Compagno, W. T. White & Cavanagh, 2010 (Borneo river shark)
  • Glyphis gangeticus (J. P. Müller & Henle, 1839) (Ganges shark)
  • Glyphis garricki Compagno, W. T. White & Last, 2008 (northern river shark)
  • Glyphis glyphis (J. P. Müller & Henle, 1839) (speartooth shark)
  • Glyphis siamensis (Steindachner, 1896) (Irrawaddy river shark)
  • Glyphis sp. Not yet described (Mukah river shark)
• Genus Lamiopsis Gill, 1862
  • Lamiopsis temminckii (J. P. Müller & Henle, 1839) (broadfin shark)
  • Lamiopsis tephrodes (Fowler, 1905) (Borneo broadfin shark)
• Genus Nasolamia Compagno & Garrick, 1983
  • Nasolamia velox (Gilbert, 1898) (whitenose shark)
• Genus Negaprion Whitley, 1940
  • Negaprion acutidens (Rüppell, 1837) (sicklefin lemon shark)
  • Negaprion brevirostris (Poey, 1868) (lemon shark)
  • †Negaprion eurybathrodon (Blake, 1862)
• Genus Prionace Cantor, 1849
  • Prionace glauca (Linnaeus, 1758) (blue shark)
• Genus Rhizoprionodon Whitley, 1929
  • Rhizoprionodon acutus (Rüppell, 1837) (milk shark)
  • Rhizoprionodon lalandii (J. P. Müller & Henle, 1839) (Brazilian sharpnose shark)
  • Rhizoprionodon longurio (D. S. Jordan & Gilbert, 1882) (Pacific sharpnose shark)
  • Rhizoprionodon oligolinx V. G. Springer, 1964 (grey sharpnose shark)
  • Rhizoprionodon porosus (Poey, 1861) (Caribbean sharpnose shark)
  • Rhizoprionodon taylori (Ogilby, 1915) (Australian sharpnose shark)
  • Rhizoprionodon terraenovae (J. Richardson, 1836) (Atlantic sharpnose shark)
• Genus Loxodon J. P. Müller & Henle, 1838
  • Loxodon macrorhinus (J. P. Müller & Henle, 1839) (Sliteye shark)
• Genus Isogomphodon Gill, 1862
  • Isogomphodon oxyrhynchus (J. P. Müller & Henle, 1839) (daggernose shark)
• Genus Triaenodon J. P. Müller & Henle, 1837
  • Triaenodon obesus (Rüppell, 1837) (whitetip reef shark)

† = extinct