Red crabs and migration | Christmas Island National Park
Red crab migration - when, where and why?
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 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 will vary with the prevailing weather.
The crabs will start their migration if there is enough time for them to complete their downward migration, mate and develop eggs before the next suitable spawning date.
The red crab 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 make up the breeding migration and one action will not occur unless the preceding action is accomplished. If there isn't enough time for them to be able to do all of these things before the next spawning opportunity, they will delay the start of their migration and attempt to meet the following month's spawning date.
The first action that occurs is movement of crabs to the sea. The largest mass movement of crabs takes place in this first downward migration. Males farthest inland start this movement and are progressively joined by more and more crabs (both males and females) as the movement progresses toward the sea.
When the crabs arrive at the shoreline, they dip in the sea to replenish body moisture and salts. The male and female crabs then move back on to the shore terraces where the males dig burrows for mating. Mating takes place and then the males again dip in the sea and then they will start their return migration.
The females remain behind in the mating burrows to brood their eggs. This takes a couple of weeks. A day or two before the spawning date the females emerge from the breeding burrows with ripened eggs and move to the shoreline where they again dip in the sea and then retreat to shade.
Before the turn of the high tide and 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 four 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 four years growth crabs will take part in the breeding migrations and the life cycle continues.
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 the preceding month and continue.
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.