Christmas Island - Red Crabs Migration

Red Crab Annual Breeding Migration

At the beginning of the wet season, usually between October and December, most adult Red Crabs suddenly begin a spectacular migration from the forest to the coast, to breed and release eggs into the sea. Breeding is usually synchronized island wide. The rains provide moist overcast conditions for crabs to make their long and difficult journey to the sea. The timing of the migration breeding sequence is also linked to the phases of the moon, so that eggs may be released by the female Red Crabs into the sea precisely at the turn of the high tide during the last quarter of the moon.

It is thought that this occurs at this time because there is the least difference between high and low tides. The sea level at the base of the cliffs and on the beaches (where the females release their eggs) at this time varies the least for a longer period, and it is therefore safer for the females approaching the water's edge to release their eggs. Sometimes there are earlier and later migrations of smaller numbers of crabs but all migrations retain this same lunar rhythm.

The main migration commences on the plateau and can last up to 18 days. Masses of crabs gather into broad columns as they move toward the coast, climbing down high inland cliff faces, and over or around all obstacles in their way, following routes used year after year for both downward and return migrations. Movement peaks in the early morning and late afternoons when it is cooler and there is more shade. If caught in open areas, in unshaded heat, the crabs soon lose body water and die.

The Breeding Sequence

Males lead the first wave of the downward migration and are joined by females as they progress. Larger males arrive at the sea first (after about 5-7 days) but are soon outnumbered by females. The crabs replenish moisture by dipping in the sea.

The males then retreat to the lower terraces to dig burrows. The density of burrows is high, about 1-2 per square metre, and fighting occurs between males for burrow possession. The females move to the terraces and mating occurs, usually in the privacy of the burrows that males have dug and fought for.

As mating, and fighting, abates, males dip again and begin returning inland. They move quickly, reaching the plateau in only 1-2 days

The females produce eggs within 3 days of mating and remain in the moist burrows on the terraces for 12-13 days while they develop. The eggs are held in a brood pouch between their extended abdomen and thorax. A single female can brood up to 100,000 eggs.

In the morning and late afternoon around the last quarter of the moon, the egg-laden females descend from the terraces to the shoreline. They pack into shaded areas above the waterline at densities of up to 100 per square metre in places.

The females usually release their eggs into the sea toward dawn, around the turn of the high tide.

Witnessing the release of eggs or spawning is a sight to behold. The process is akin to a dance, as the female crab lift its two claws upwards and shaking them as the eggs are released into the water.

Release of eggs may occur on 5-6 consecutive nights during the main breeding migration. After the first two days, eggless females may be seen crossing plateau roads, kilometres from the shore.

Larvae Grow To Baby Crabs

The eggs released by the females hatch immediately on contact with the sea water and clouds of young larvae swirl near the shore before being washed out to sea by waves and tides. Millions of the larvae are eaten by fish and plankton feeders such as Manta Rays and the enormous Whale Sharks which visit Christmas Island waters during the crab spawning season.

After about a month in the ocean, and after growing through several larval stages, the surviving larvae have developed into prawn-like animals called megalopae. The megalopae gather in pools close to the shore for 1-2 days before changing into young crabs and leaving the water.

Although only 5mm across, the baby crabs begin their march inland, taking about 9 days to reach the plateau. Here they seem to disappear and are rarely seen, living in rocky outcrops and under fallen tree branches and debris on the forest floor for the first three years of their life.

In many years, very few or no baby crabs emerge from the sea, but the occasional very successful year (perhaps only one or two every ten years) is enough to maintain the Red Crab population to a high level.

Managing Human Impacts

Certain human activities have led to increased numbers of Red Crabs being killed during their annual migration to the sea. As well as there being a greater risk of crabs dehydrating when forced to cross areas cleared of forest cover, many thousands of adults and young are crushed by vehicles while crossing roads. Some have to negotiate up to three or four such hazards on their migration to and from the sea each year.

Sections of roads where crabs cross in high numbers have been identified and conservation measures have been implemented to help reduce this high death toll. To reduce the number of crabs killed by vehicles during the migration, 'crab crossing' grids have been built into the roads which cross main crab migration paths, for crabs to pass through. Walls or plastic fencing that the crabs can not climb over are erected alongside the roadside to prevent the crabs getting on the road, and to 'funnel' the migrating crabs through the grids and under the road so that they may continue their migration unhindered by traffic. These crab crossings may be seen on most major roads.

Other conservation measures used by the community are road closures and traffic detours around the major migration paths during peak migration periods.

Viewing the Migration

The annual spectacle of the Red Crab migration attracts the interest of national and international visitors as well as the local community. This unique natural event may be observed at close quarters.

During peak migration times, sections of roads where crabs cross in high numbers may be closed to vehicles for short periods of time. It is then possible to park the vehicle and carefully walk amongst this moving seas of crabs as they relentlessly make their way to and from the shore. Convenient sites to experience the crab migration are at Drumsite, Flying Fish Cove, Ethel Beach and Greta Beach.

Flying Fish Cove, Ethel Beach and Greta Beach are also the easiest places to watch the females spawn (deposit their eggs into the sea) later in the migration timetable. Updates on crab movements are given on public notice boards and broadcast over local radio. Specific queries can be directed to staff at the Parks Australia office at Drumsite.

| video of crab migration |