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 the 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
behavior, 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. The IUCN Redlist has listed the species as Vulnerable. 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.
Lemon
sharks are really fascinating animals, and if you would like to know more about
their biology and ecology, please refer to the following sources of
information, local library or the World Wide Web.
Written
by Gabrielle Ahern
Salty Wave Blue
– Into all things ecology.
Follow
@SaltyWaveBlue on @Instagram and @Twitter
Web Links
IUCN
Redlist
Wildscreen
Arkive
FishBase
Atlas
of Living Australia
