Page 305 - (DK) Ocean - The Definitive Visual Guide

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Red Crab Migration

The remarkable annual migration of the red crab
STAGES OF MIGRATION
(Gecarcoidea natalis) from the forests of Christmas
Island (southwest of Indonesia) down to the sea to MAN-MADE OBSTACLES
spawn is one of the wonders of the natural world. Although the distance to the
shore is only about 1,600 ft
Until recently, about 120 million of the crabs made (500 m), red crabs have
the journey each year, spending the rest of the time to negotiate a number of
obstacles, including roads
on a forested plateau about 1,200 ft (360 m) above and hot train tracks. In the
sea level. Red crabs are mainly herbivorous, feeding past, it was not unusual for
on fallen leaves, fruit, and flowers gathered from the one million crabs to perish
each year, yet this had little
forest floor, but they also eat other dead crabs and impact on the population.
birds when the opportunity arises. They conserve TRAVELING ACROSS LAND Today, various measures
such as road closures and
water by restricting their activity to times of high concrete underpasses offer
humidity (over 70 percent), retreating to their some security to the crabs,
but they still run the risk
burrows during drier periods. Like their marine of dying from dehydration
counterparts, red crabs have gills for respiration, during their journey.
but the gill chamber of this species is lined with tissue
EGG RELEASE After incubating
that acts as a lung and maximizes gaseous exchange.
up to 100,000 eggs for 12–13
Red crabs on Christmas Island have undergone days, the females leave their
a noticeable decline in numbers, largely due to the burrows and gather on the shore
to disperse them directly into the
accidental introduction of the yellow crazy ant in sea. They do so at night, as the
the 1930s. Since the mid-1990s, about 20 million high tide turns, by raising their
claws and shaking their bodies
red crabs are thought to have been killed by the ants, RELEASING EGGS vigorously to free the eggs from
which squirt formic acid on the crabs as a defense their pouches. Crabs on the cliffs
mechanism when they are disturbed. There is also may be 25 ft (8 m) above the water.
pressure to increase the number of phosphate mines
on the island, which would involve deforestation, MEGALOPAE LARVAE Red crab eggs hatch as soon as they hit
depriving the crab of its habitat in the affected areas. the water. The young remain in the ocean for up to 30 days, and
pass through several larval stages before returning to the shallows
as shrimplike megalop larvae. They metamorphose into tiny crabs
after 3–5 days and leave the water to start life on land.
The Sequence of Migration

The onset of the wet season on Christmas
Island, usually in early November, signals
the start of the red crabs’ migration, which
takes place over three lunar cycles. The
males set off first, followed by females.
It takes about a week for the crabs to reach
the shore. After dipping in the ocean, the
males compete fiercely for space to dig their burrows, where it FROM SEA TO LAND
is thought mating takes place. The males then return inland,
leaving the females to brood their eggs in the burrows. They
emerge after about two weeks to release their eggs into the
sea at high tide at night during the last quarter of the moon.

full moon new moon
Phases of the
moon over a
3-month period
from November to females
January and males
males
in early November, females
the crabs start to
move down from females return
forest plateau to
the shore to breed to the forest
young crabs
first wave emerge after
of crabs about a month,
reaches the having matured
shoreline in the water OCEAN LIFE

in the second and largest migration, after mating, males dip a females move to the shoreline,
the crabs dip in the sea to replenish second time, then either releasing eggs into the sea at the
body salts, then the males fight to return inland immediately turn of the high tide during the
establish burrows or remain and feed last quarter of the third lunar cycle