Red Crabs
Gecarcoidea natalis
(Pocock, 1888)
The Red Crab is by far the most obvious of the 14 species of land crabs found on Christmas Island. Millions of these bright red land crabs live in their preferred shady sites all over the island.
Bright red is the common colour but there are the occasional orange specimens and more rarely some purple animals.
They are a big crab. An adult body shell (or carapace) may measure up to 116mm across. The carapace is round shouldered and encloses their lungs and gills. Their claws are usually of equal size unless one is a regrowing claw. Males grow larger overall than females, while females have a much broader abdomen and usually have smaller claws than males. However young crabs all have the characteristic narrow abdomen of the male. The broader female abdomen only becomes apparent in the third year of growth.
Red Crabs grow slowly, reaching about 40mm in carapace width after 4-5 years. They are sexually mature at this age and begin to participate in the breeding migrations.
The crabs moult their shells regularly during their early growth phases to match their increasing body size. Moulting usually takes place in the protected moist environment of their burrows. Mature crabs probably moult only once a year, as their growth rate slows.
Where do they live?
Although most common in the moist environment of the rainforest, Red Crabs live in a variety of habitats including coastal shore terraces, and even domestic gardens. Tall rainforest on deeper soils has the highest crab density. They dig burrows in almost every square metre of available soil or live in deep crevices in rock outcrops. For most of the year, a crab will settle in one place, living in their burrow.
The crabs' burrows have a single entrance tunnel which leads to a single chamber. Only one crab lives in a burrow and (except for the breeding season) Red Crabs are solitary, and do not tolerate intruders into their burrows.
Red Crabs are diurnal (active during the day) and almost inactive at night despite lower temperatures and higher humidity. They take great care to conserve body moisture and this seems to be the single most important factor influencing the crab activity. Sensitivity of crabs to moisture, combined with the seasonal climate on Christmas Island, create a distinct seasonal pattern of activity. Crabs retreat into the humid interior of their burrows during the dry season. They plug the burrow entrance with a loose wad of leaves to maintain a high humidity level, and effectively disappear from view for up to two to three months of the year.
What do they eat?
Red Crabs diet consists mainly of fallen leaves, fruits, flowers and seedlings. They prefer fresh green leaves but will eat any fallen leaves. They are not solely vegetarian however. They will eat other dead crabs and birds, the introduced Giant African snail and palatable human refuse if the opportunity presents itself.
They have virtually no competition for their food resource due to their high numbers and dominance of the forest floor.
Red Crabs and Forest Ecology
Red Crabs are important in the Christmas Island rainforest ecosystem. They eat fallen vegetation,leaves and fruit and recycle the nutrients contained in this material. Their cylindrical brown droppings scattered over the forest floor act as fertilizer. Their burrowing turns and aerates the soil and they are a major determinant of the unique structure and composition of the Christmas Island forest by their selective browsing on seeds and seedlings.
Scientific experiments in the forest have shown that the Red Crabs on Christmas Island remove seeds and seedlings from the forest floor at a far greater rate than do the animals present in other tropical rainforests. Long term scientific studies on the effect that Red Crabs have on the Christmas Island rainforest ecosystem are currently being carried out.
The Annual Breeding Migration
Crabs on their annual migration climb various different obstacles on their way to the sea.
Crab crossing grids have been built to help the crabs safely cross roads.
Plastic walls help funnel crabs to the crossings.
Crabs reach the shoreline and release their eggs, which hatch immediately on contact with the sea water.
Crabs on the rock faces.
Bumper stickers created by the local school children.
At the beginning of the wet season (usually October / November), 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 (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. 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 Red Crab Migration
Most of Christmas Island's adult red crabs begin their breeding migration to the sea as soon as the wet season rains have established. But we can never be sure when the wet season is going to begin!
The breeding migration comprises a series of separate actions on the crabs' part that follow on from one to the other in a programmed sequence. These separate actions in combination are the breeding migration and one can not occur unless the preceding action is accomplished.
The first action that occurs is movement of the crabs to the sea. The next occurs when the crabs reach the sea; they dip in the sea to replenish body moisture and salts.
The next action is the movement back on to the shore terraces where the males dig mating burrows. Mating then occurs. Following mating the males again dip in the sea and then start their return migration.
The females remain in the burrows to brood their eggs. A day or two before the spawning date the females emerge from the breeding burrows and move to the shoreline where they will again dip in the sea and then retreat to shade.
Before the turn of the high tide just before dawn the females will again move to the waterline and around the turn of the tide they will drop their eggs into the sea. After they have jettisoned their eggs the females commence their return migration.
The next phase of the breeding migration takes place in the sea. The eggs that the females drop into the sea hatch immediately into larvae. They grow through several larval stages into tiny prawn-like animals called megalops. After about 4 weeks the megalops emerge from the sea and they moult into baby crabs. The baby crabs then move inland and settle at suitable localities. The successful emergence of baby crabs is unpredictable but is incredible when large numbers emerge. Some years very few, or none, emerge. After about 4 years growth crabs will take part in the breeding migrations and the life cycle continues.
The crabs' breeding timetable is fixed around the phases of the moon. Spawning (the dropping of their eggs into the sea) must occur before sunrise on spring tides during the last quarter of the moon, regardless of any other factor. The timing of spawning is the only certain and predictable part of the whole migration; all other stages of the migration vary with the weather.
The crabs will start their migration as soon as the rains set in, if there is enough time for them to complete their downward migration, mate and develop eggs before the next suitable spawning date. To have sufficient time to be able to complete the development of eggs and to spawn on a particular date the crabs must start their downward migration at least 3-4 weeks before that date.
If there isn't enough time for them to be able to do all of these things, they will either delay the start of their migration, or begin moving slowly toward the coast, so that they are able to synchronize with the following month's spawning date.
If the rains stop or peter out, the crabs will delay the start of their migration, or, if they have started migrating, they will stop moving and stay wherever they are until the rains begin again.
It is rare that substantial rains will begin early enough in the year for a spawn during the last lunar quarter in October - but it has happened! Spawning in November or December are the more usual, which means that rain must commence in preceding months and continue. November is generally considered to be the beginning of the wet season on Christmas Island - but as always with nature one cannot be certain. Over recent years the rains have commenced later, delaying migrations till December.
The largest movement of crabs takes place in the first downward migration. Males farthest inland start this movement and are progressively joined by more and more crabs as the movement progresses toward the sea. The separate male and female crabs' return migrations are equally as impressive. The spawning event is also incredible and the emergence of baby crabs from the sea can be staggering. All phases of the crabs' breeding migration involve colossal numbers of crabs and usually occur all over the island.
If the rains continue, there is usually a second, and sometimes even a third, smaller, downward migration by crabs that did not join in the first migration. When this happens it is possible to see crabs on return journeys mingling with the crabs on their downward migration. It can become confusing for all concerned!
We are sorry that we can not be more explicit about the timing of the start of the red crab migrations, but the weather as you know cannot be accurately predicted. The best advice we can give is to be at Christmas Island during the last quarter of the moon in either November or December for the best chance of seeing something interesting happening in the annual red crab migration. If you are able to arrive earlier and to stay longer the more parts of the migration sequence you will be able to experience.
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