Page 71 - BBC Wildlife Volume 36 #06

LEMON SHARKS

TAKING TURNS

Sickle-fin lemon sharks, close relatives precise position in the hierarchy,” says
of lemon sharks from the Indo-Pacific, Pierpaolo Brena, the study leader.
have been studied for a decade in Higher-ranking sharks that got to eat
French Polynesia. Using cameras first were less submissive and tended
set up around a bait box, scientists not to veer of when swimming towards
recently discovered that they formed a others. “To our knowledge, it’s the first
pecking order based, surprisingly, not time that adult, free-ranging sharks
on body size but the shark’s behaviour. have been shown to adapt to the
“Every shark in the group had a very behaviour of their rivals,” Brena says.

AGGRESSIVE
BEHAVIOUR
A lemon shark asserts its
dominance by charging
at a perceived rival.

SUBMISSIVE
BEHAVIOUR
A lemon shark
encountering a rival
that’s higher in
the social hierarchy
gives way.

LEMON SHARKS their way back. Like all sharks, lemon sharks have Above left: The
electroreceptors ca lled ampullae of Lorenzini dotted, mangroves where
NOW JOIN THE LIKES like dimples, all over their snouts and along their lemon sharks
live are home
well as detecting weak electric
OF SALMON AND SEA flanks. As w m the twitching muscles of their to many other
fields fro
species, including
is may also let sharks sense
TURTLES AS MASTER OCEAN prey, th gnetic fields. the Bahamas
blue crab.
geomag
NAVIGATORS. BUT HOW
SLOW TO GROW
THEY DO IT REMAINS Female e lemon sharks only give birth
her year, producing between
every oth
A MYSTERY. four and 18
8 pups at a time. Their slow
d late sexual maturity at 11–13
growth rate and
years or even older, means these sharks are prone
to overfishing. Compare e this to fast-growing bony fish
species such as herring and anchovies, which have more
Between 1993 and 2012, scientists in Bimini caught of a scatter-gun approac ch to reproduction, taking only
newborn sharks in the mangroves, fitted them with a year or so to reach full l sexual maturity, and spraying
tags and took tissue samples to work out the DNA millions of eggs in to the sea.
fingerprint of each one. Then they let the pups go and These characteristics mean herring and anchovy
waited patiently until some of them began to come back. populations are able to recover from overfishing relatively
Six females returned 14–17 years later to give birth to quickly. Sharks, on the other hand – especially lemon
their own pups in the exact same spot where they were sharks – can take decades to replenish their numbers. As
born. It was the first time this behaviour, known as natal lemon sharks have been heavily fished throughout their
philopatry, had been directly observed in any sharks. range, it’s perhaps no surprise that they’re categorised
It means that lemon sharks now join the likes of as Near-Threatened (one step below Vulnerable on the
salmon and sea turtles as master ocean navigators. endangerment scale).
But exactly how they do it remains something of a Another concern is the lemon sharks’ dependence
mystery. It’s possible they imprint on the specific on mangrove forests. Globally, these ecosystems are
pattern of Earth’s magnetic field at their nursery area, coming under immense pressure. Huge stretches of
then use some kind of natural, built-in compass to find mangrove have already been cut down to create ponds
June 2018 BBC Wildlife 71