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Negaprion acutidensBroadfin shark(Also: Sharptooth lemon shark; Sicklefin lemon shark)

By Elizabeth Lee

Geographic Range

Sicklefin lemon sharks (Negaprion acutidens) are native to the Indian Ocean and the Pacific Ocean. They are found in tropical and subtropical areas of the oceans, specifically the western and eastern parts of the Indian Ocean, and the western central, eastern central, and northwest parts of the Pacific Ocean. Historically, these lemon sharks were found in areas around India and Thailand. (Schultz, et al., 2008; Simpfendorfer, et al., 2021)

Habitat

Sicklefin lemon sharks are found in tropical, saltwater habitats. They are predominately found in shallow coastal habitats and around reefs in areas known as atoll lagoons. Atoll lagoons are those enclosed by a ring of coral. Lemon sharks have been found around sandy plateaus near coral intertidal areas. These sharks utilize these areas for their nursery, but as they grow older, they do not tend to stray far from these spaces.

Sicklefin lemon sharks exhibit predictable daily movements with tide changes. They move into the shallow flats at high tide and into the deep lagoons at low tide. Both sexes of lemon sharks move further away from shore during winter months. The range of depths where lemon sharks are found is between 0 to 92 m, and they tend to swim along the substrate of the sea floor. (Byrnes, et al., 2021; Filmalter, et al., 2013; Lea, et al., 2020; Pillans, et al., 2021; "Sharks of the world", 1984; Simpfendorfer, et al., 2021)

  • Range depth
    0 to 92 m
    0.00 to 301.84 ft

Physical Description

Sicklefin lemon sharks reach their maximum size at about 310cm to 340cm total length. Male lemon sharks reach maturity at about 240cm in length, and females at about 220cm. At birth sicklefin lemon sharks are 45 to 80 cm long and they grow about 12 to 15 cm a year. Lemon sharks are sexually dimorphic with the males being larger than the females. Further, males possess a pair of claspers on their abdomens to enable internal fertilization of females.

Lemon sharks' color uniformly ranges from a pale yellow to a light brown, being even lighter on their ventral side. These patterns do not change by sex or age. Sicklefin lemon sharks are short-nosed and stoutly built. Lemon sharks' total vertebral count is 224 to 227. The mass of sicklefin lemon sharks has not been reported.

They have two prominent dorsal fins, two pectoral fins, two pelvic fins, one small anal fin, and one hypocercal caudal fin. Lemon sharks are ectothermic, as their body temperatures match the temperature of the surrounding water.

Lemon sharks have 27 to 33 rows of teeth on both their upper and lower jaws. Their upper anteroposterior teeth are erect and slim, with slender cusps and no cusplets. Their lower teeth do not have cusplets, but are erect and long with slightly curved cusps. The lower teeth have no serrations, and the upper teeth have serrations that are confined to the crown of the tooth. Sicklefin lemon sharks share all of these features with lemon sharks (Negaprion brevirostris). The identifying feature of sicklefin lemon sharks is their titular fin shape; the trailing edge of their pectoral fins are sickle-shaped. (Buray, et al., 2009; Mourier, et al., 2013; Parsons, 2006; "Sharks of the world", 1984; Simpfendorfer, et al., 2021)

  • Sexual Dimorphism
  • sexes alike
  • male larger
  • sexes shaped differently
  • Range length
    220 to 340 cm
    86.61 to 133.86 in

Development

Sicklefin lemon sharks are viviparous and typically follow a biennial (every two years) reproductive cycle, giving birth between August and October. The gestation period of lemon sharks is typically 10-11 months, and average litter size is 9.3 (range 6-12). Neonatal lemon sharks are identified by the presence of an open or healing umbilical scar. The umbilical scar heals shortly after birth, so those without the presence of this scar were identified by size. Neonatal lemon sharks range in size from 45 cm to 80 cm and have a growth rate that ranges from 0.2 to 28.2 cm in their first year of life. Lemon sharks exhibit indeterminate growth, growing 12 to 15 cm a year until they reach maturity (at ca. 16.5 years). Lemon sharks have been recorded at lengths of 380 cm at 27 years old.

Male lemon sharks reach maturity at about 240 cm, while females reach it at ca. 220 cm. Male sicklefin lemon sharks possess claspers, and females do not. Their size can also help determine the sex, because males are larger than females; this size comparison is typically done using the fork and total length measurements. Female lemon sharks can sometimes be sexed by indicators of current or past pregnancy like presenting an extended abdomen or possessing postpartum scars. (Hodgkiss, et al., 2017; Mourier, et al., 2013; Pillans, et al., 2021; Simpfendorfer, et al., 2021; Weideli, et al., 2019)

Reproduction

Sicklefin lemon sharks are polygynandrous, as both sexes will have multiple mates throughout their lives. Female sharks emit chemicals, known as pheromones, that signal to male sharks in the area that they are ready to mate. This is necessary as sharks are solitary, so these pheromones lead the males to the females. Male sharks are known to bite female sharks that are potential mates, as well as biting the females during the act of mating to remain stable. Male sharks use organs called claspers to transfer their sperm into the oviduct of the female sharks. The male sharks will bite on or around the females' pectoral fin and use this to stabilize themselves to insert one of their claspers into the female. Some females are found with multiple bite marks at a time, leading to the conclusion that multiple males will mate with one female during one mating period. The existence of litters with multiple paternities also lends credence to this theory. (Clua, et al., 2010; Feldheim, et al., 2002; Feldheim, et al., 2004; McCelland, 2020; Mourier, et al., 2013; Parsons, 2006; Pillans, et al., 2021)

Male and female sicklefin lemon sharks are iteroparous, mating during late January to April, and males employ internal fertilization. Female lemon sharks can store the sperm of multiple mates at a time; some litters have been noted to have multiple paternities.

Male and female lemon sharks reach sexual maturity at 12 to 16 years old. Females have a 10 to 11 month gestation period and give birth to their young between October and early January. Male lemon sharks mate yearly, but the females only get pregnant biennially (every two years).

Lemon sharks are viviparous and give birth to 6 to 12 pups in a litter. The average litter size for lemon sharks is 9.3 pups. Lemon sharks' birth length is typically 45 to 80 cm.

Female lemon sharks give birth in nurseries and then leave their young to develop on their own. Lemon sharks' independence begins at birth as they grow and learn in their nursery. Female lemon sharks have shown to return to the same nursery site each time they give birth. (Clua, et al., 2010; Feldheim, et al., 2002; Feldheim, et al., 2004; Filmalter, et al., 2013; McCelland, 2020; Mourier, et al., 2013; Parsons, 2006; Pillans, et al., 2021; "Sharks of the world", 1984; Simpfendorfer, et al., 2021)

  • Breeding interval
    Males breed yearly, Females breed biennially (every two years)
  • Breeding season
    Late January to April
  • Range number of offspring
    6 to 12
  • Average number of offspring
    9.3
  • Range gestation period
    10 to 11 months
  • Range time to independence
    0 (low) days
  • Range age at sexual or reproductive maturity (female)
    12 to 16 years
  • Range age at sexual or reproductive maturity (male)
    12 to 16 years

Very little information is known about the parental investment of sicklefin lemon sharks. Female sharks provide pre-fertilization provisioning and protecting, as well as pre-birth provisioning. After giving birth to their young, the female sharks leave their young in nurseries where the juveniles grow and learn with their fellow juvenile sharks. The sharks stay in these shallow-water intertidal nurseries until they reach about 90 cm in length, but they occasionally stay around their nurseries for additional years. Males provide no parental investment beyond the act of mating. (Clua, et al., 2010; Feldheim, et al., 2002; Feldheim, et al., 2004; McCelland, 2020; Mourier, et al., 2013; Parsons, 2006; Pillans, et al., 2021)

  • Parental Investment
  • female parental care
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female

Lifespan/Longevity

Lifespan is not reported specifically for sicklefin lemon sharks but others in the genus, lemon sharks (Negaprion brevirostris) are reported to live 20 years in the wild. These lemon sharks have been recorded to reach the age of 30. Although lemon sharks are occasionally held in captivity, their lifespan in captivity is not reported. (Garcia, et al., 2008)

Behavior

Sicklefin lemon sharks are considered solitary, natatorial, and motile creatures. They typically only interact with other lemon sharks for mating, foraging, and defense. Young lemon sharks interact with each other as juveniles in their nurseries, as they are left by their mothers to grow and mature on their own. Although they are solitary, they demonstrate a dominance hierarchy by using social information about other sharks, like size and sex, to increase their chance to feed. Sharks lack eyelids and do not fully sleep. Instead, they exhibit resting phases in daylight hours, still able to be cognizant of threats around them. Sharks are nocturnal and crepuscular, as they do most of their feeding and active swimming at dusk, overnight, and at dawn.

Sicklefin lemon sharks are migratory animals. In late July to September, they move directionally south, to warmer areas that are as far as 140 km away within 1 to 2 days. Lemon sharks return north typically from March-April. Males typically begin this move 1-2 months earlier than females. The females' migration periods vary annually due to breeding; on years where they give birth, they leave at the later end of the migration period.

A change in lemon shark behavior has been directly caused by humans who feed them for fishing and tourism. Consequently, these sharks have become more aggressive to other sharks and humans, leading to multiple shark attacks on humans and shark aggressive interactions. It also has caused more inbreeding amongst lemon sharks because it decreases shark mobility.

Sharks communicate with each other by arching their backs and remaining non-confrontational. Male lemon sharks bite the females when attempting to find a mate and bite them during the act of mating to remain stable. (Clua, et al., 2010; Hodgkiss, et al., 2017; Lea, et al., 2020; Pillans, et al., 2021; Simpfendorfer, et al., 2021)

Home Range

Sicklefin lemon sharks stay within 10 km^2 of initial capture across month-long periods. Pillans et al. (2021) reported an average core home range of 1.7 km^2. Female sharks tend to shift their core home ranges about 0.45 km further offshore into deeper areas during winter months. They do not defend a territory. (Hodgkiss, et al., 2017; Lea, et al., 2020; Pillans, et al., 2021; Simpfendorfer, et al., 2021)

Communication and Perception

Little is known regarding the specifics of the communication and perception of sicklefin lemon sharks. However, they possess many features that are typical of all sharks. Sharks can see underwater about 3m in front of them, depending on the depth and lighting. They perceive some colors like blue and green. Sharks use ampullae of Lorenzini, pores on their bodies that are used to detect electric fields. Lemon sharks use this to locate prey in the substrate beneath them and at night when vision is limited. The electric fields are felt by the sharks and can be used to determine the size, shape, and location of the prey they are sensing.

Sharks use their tastebuds, located in the roof, walls, and floor of their mouths to determine if food is palatable and to help them avoid noxious substances. There are large olfactory bulbs in sharks' brains that give them a considerably good sense of smell. They can use their sense of olfaction to smell prey that are as far as 1.7km away. Sharks hear and detect vibrations and they are most sensitive to low frequency vibrations, like those from injured fish and boats. They use the detection of these low frequencies to find food and to avoid humans. Another way sharks detect vibrations is by using their lateral line system. The lateral line system consists of pores that line the flanks of sharks that they can use for detecting vibrations of a distance about 1 to 3m. Sharks communicate to each other by arching their backs to be non-confrontational, and occasionally even biting each other when attempting to find a mate. (Linzey, 2020; Parsons, 2006; "Sharks of the world", 1984; White, et al., 2004)

Food Habits

Sicklefin lemon sharks mainly feed on bottom-dwelling fish. Some examples of these are stingrays (family Dasyatidae) and other small elasmobranchs, porcupine fishes (family Diodontidae), and other teleost fishes. They occasionally hunt birds that land in the water and other, smaller sharks of any species that is small enough to eat.

They also consume bottom-dwelling aquatic crustaceans such as squid (superorder Decabrachia), crab (infraorder Brachyura), and lobster (family Nephropidae). No dietary differences between sexes have been reported in this species. (Byrnes, et al., 2021; "Sharks of the world", 1984; White, et al., 2004)

  • Animal Foods
  • birds
  • fish
  • mollusks
  • aquatic crustaceans

Predation

The only known predator to adult lemon sharks is humans (Homo sapiens). Humans fish for lemon sharks to eat. Predators to juvenile lemon sharks include humans and larger sharks (superorder Selachimorpha) that feed on them. It is not known which shark species will eat juvenile lemon sharks, but it can include other sicklefin lemon sharks.

The one anti-predator adaptation that lemon sharks exhibit is that they are darker colored on their dorsal side, and lighter on their ventral side. This allows the sharks to blend in with the surface of the oceans if looking from the sky and blend in with the sky light if looking from underwater. (Byrnes, et al., 2021; Clua, et al., 2010; Mourier, et al., 2013; Simpfendorfer, et al., 2021)

  • Anti-predator Adaptations
  • cryptic
  • Known Predators
    • Humans (Homo sapiens)
    • Larger sharks
    • Sicklefin lemon sharks (Negaprion acutidens)

Ecosystem Roles

Sicklefin lemon sharks mainly feed on bottom-dwelling fish. As a top predator in their system, only humans catch and kill them.

Sicklefin lemon sharks can have a number of internal parasites, all tapeworms in the class Cestoda. They include Paraorygmatobothrium roberti, Paraorygmatobothrium arnoldi, Alexandercestus masoumehae, Ceratomyxa negaprioni, Pseudogrillotia spratti, and unidentified tapeworms in the order Tetraphyllidea. (Beveridge and Justine, 2007; Byrnes, et al., 2021; Gleeson and Adlard, 2011; Maleki, et al., 2020; Ruhnke and Thompson, 2006; Ruhnke and Workman, 2013)

Commensal/Parasitic Species
  • Cestoda (Paraorygmatobothrium roberti)
  • Cestoda (Paraorygmatobothrium arnoldi)
  • Cestoda (Alexandercestus masoumehae)
  • Cestoda (order Tetraphyllidea)
  • Cestoda (Ceratomyxa negaprioni)
  • Cestoda (Pseudogrillotia spratti)

Economic Importance for Humans: Positive

Sicklefin lemon sharks' meat is used for human consumption. It is typically used fresh, dried, salted, frozen, or smoked. Lemon sharks' fins are cut off and used for shark-fin soup bases. Lemon sharks' hides are used for leather and their liver is used to extract vitamin oil which is used in modern medicine to help boost the human body's immune system.

Shark feeding is used as a tourist attraction and lemon sharks are a popular species for this. These acts of tourism generate the local economies for the areas that do it. Occasionally, lemon sharks are displayed in aquariums for educational and tourism reasons. In the Western and Central Pacific, sicklefin lemon sharks are used for diving and interactions with tourists. According to Simpfendorfer et al. (2021), in one shark feeding ecotourism operation in French Polynesia, sicklefin lemon sharks have been estimated to generate about $5.4 million USD a year. (Clua, 2010; Clua, et al., 2010; Gallagher and Hammerschlag, 2011; Simpfendorfer, et al., 2021)

Economic Importance for Humans: Negative

Sicklefin lemon sharks rarely attack and injure humans without provocation. There are no reported fatal attacks on humans by lemon sharks. (Clua, 2010; Clua, et al., 2010; "Sharks of the world", 1984)

  • Negative Impacts
  • injures humans
    • bites or stings

Conservation Status

Lemon sharks are classified as "Endangered" on the IUCN Red List. They have no special status on the US Federal List, CITES, or the State of Michigan List.

Sicklefin lemon sharks are sought out by fisheries, and this is leading to the decline in their populations. Lemon sharks' meat is being used for food, their hides for leather, their livers for vitamin oils, and they are used for tourist attractions. In Australia, and some island nations in the Western Pacific, and the Indian Ocean, their populations appear stable. However, in Asia, Africa, parts of the Pacific, Indonesia, the Philippines, Thailand, and in the Red Sea, as well as other Arabian Sea regions, populations are in decline. Across their range, sicklefin lemon sharks are estimated to have experienced a decrease of 50-79% from 1971 to 2021.

Sicklefin lemon sharks are commonly caught and killed in gillnets, longlines, trawls, and handlines both intentionally and accidentally. Sicklefin lemon sharks live by coral reefs and are negatively affected by coral bleaching. According to Simpfendorfer et al. (2021), due to coastal development sicklefin lemon sharks are losing their nurseries in mangroves.

There are many measures taken to manage the conservation of sharks. These measure work to control and monitor the fisheries that target and seek out sharks. Sicklefin lemon sharks are common throughout coral reef areas, and coral reef areas are commonly considered Marine Protected Areas. Marine Protected Areas are spaces that focus on conservation of the plant and animal life in those areas, and human activity like fishing, diving, and netting are strictly prohibited. There are areas known as Exclusive Economic Zones (EEZs), where places have completely banned target shark fishing within these zones. Many areas manage the number of sharks caught by using a catch or harvest limit, and harvest and trade management measures. These work by monitoring the amount of sharks that can be caught, harvested, sold, traded, and kept within certain areas and time spans. (Simpfendorfer, et al., 2021)

Contributors

Elizabeth Lee (author), Radford University, Sierra Felty (editor), Radford University, Bianca Plowman (editor), Radford University, Karen Powers (editor), Radford University, Victoria Raulerson (editor), Radford University, Christopher Wozniak (editor), Radford University, Genevieve Barnett (editor), Colorado State University.

Glossary

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

crepuscular

active at dawn and dusk

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

drug

a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease

ecotourism

humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

electric

uses electric signals to communicate

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

food

A substance that provides both nutrients and energy to a living thing.

indeterminate growth

Animals with indeterminate growth continue to grow throughout their lives.

internal fertilization

fertilization takes place within the female's body

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

migratory

makes seasonal movements between breeding and wintering grounds

molluscivore

eats mollusks, members of Phylum Mollusca

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

piscivore

an animal that mainly eats fish

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

reef

structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

sperm-storing

mature spermatozoa are stored by females following copulation. Male sperm storage also occurs, as sperm are retained in the male epididymes (in mammals) for a period that can, in some cases, extend over several weeks or more, but here we use the term to refer only to sperm storage by females.

tactile

uses touch to communicate

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

vibrations

movements of a hard surface that are produced by animals as signals to others

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

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