The barrier reef around Heron Island attracts many marine vertebrates, among them various members of the Class Condrichthyes, who hold the mantle of keystone predator.
At birth the lemon shark, Negaprion acutidens (Carcharhinidae) measures 60 – 65 cm in length and can grow to 380 cm TL (male). The average size of a litter is 6 – 12 pups. The lemon shark is an active predator whose behaviour is affected by the different conditions of its environment, for example, water temperature, currents, prey in the area and time of year, to name but a few. Its habitat includes tropical, shallow inshore and offshore waters in coral reef, lagoon and mangrove estuary environments.
Species within the Carcharhinid family have a complex social and reproductive behavior as compared to other families of shark. Studies have revealed this reef shark is geared to learn about its reef habitat due to a more developed telencephalon in the brain.
With growing maturity the lemon shark’s diet has been observed to change. As juveniles they feed on teleosts, crustaceans and molluscs around shallow waters but as adults, they switch to teleost and cartilaginous fish in pelagic waters. This may reflect the change in visual pigments, which occurs, rapidly enhancing its ability to hunt prey at night, along with its developing olfactory senses and important electro receptive ability. After hunting for prey all night, sharks seek shelter in the reef during the day.
Adult sharks hunt in pelagic or open ocean environments, while juvenile sharks hunt opportunistically during the day and twilight close to shore in the reef lagoon. Sharks have been observed to work as a group when hunting for food. They herd schools of reef fish toward the shoreline and with nowhere else to go the fish are vulnerable to attack. The sharks complete their hunt by ram feeding on the cornered fish.
Lemon sharks do not feed on a regular basis. They eat on average 2.68 % of their own body weight in short bursts and invest more time in digestion. Their stomachs are usually voided within 25 – 41 hours. The voiding mechanism is executed by the scroll valve, which everts to flush out any parasites.
Studies suggest a combination of factors: the sharks coral reef habitat, competition between different parasite species and the number of parasites living on a shark, contribute to the demise of a shark’s health, leading in some cases to its early death. Despite the sharks efforts to reduce the chance of infection, opportunities for parasites to infect a shark are increased by the shark’s own behaviour, for example, its hunting location and the type of fish it consumes.
Lemon sharks remain connected to a specific coral reef habitat throughout their lives. By nature they are quite a shy animal but dangerous if provoked. Current threats to lemon shark populations include: shark fisheries, dynamite fishing, pollution and mangrove deforestation due to its limited home range. The IUCN Redlist has listed the species as Vulnerable.
Lemon sharks are really fascinating marine animals and if you would like to know more about their biology and ecology, please refer to your local library or the World Wide Web.
Written by Gabrielle Ahern
Salty Wave Blue – Into all things ecology.
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IUCN Redlist – http://www.iucnredlist.org/details/41836/0
Wildscreen Arkive – http://www.arkive.org/sharptooth-lemon-shark/negaprion-acutidens/
FishBase – http://www.fishbase.org/summary/896
Atlas of Living Australiahttp://bie.ala.org.au/species/urn:lsid:biodiversity.org.au:afd.taxon:6f8bfc6c-b9ef-4bc5-8808-ae02105cb3f2#