Red crab breeding migration

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Possible dates for 2019/20 breeding migration spawnings are;

  • spawning to commence around 23 October (this date has passed without rains establishing so the crabs will now look to commence spawning on the next possible date in November).

  • the next possible spawning date is 21 November. Rain will need to commence and establish late October or early November. If rains do not establish during this period then the crabs will look toward a December spawning.

  • the possible December date is 21 December. For the crabs to spawn around this date rains will need to have established by late November/early December. If this does not eventuate the crabs will delay spawning till January.

  • Spawning on these dates is dependant on rains

 

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Amazing annual breeding migration of the Christmas Island red crab

Every year, at the beginning of the wet season (usually during October / November), most adult Christmas Island red crabs suddenly begin a spectacular migration from the forest to the coast, to mate and for the females to then release eggs into the sea. This annual breeding migration has been described by Sir David Attenborough as one the 10 greatest natural wonders on Earth. The wet season rains provide moist and overcast conditions to sustain the crabs after they have left the safety of their burrow to make their often long and difficult journey to and from the coast to mate and spawn. 

Observation over time has shown that, while the commencement of the breeding migration clearly depends on establishment of wet season rains, the commencement is not, as was thought until recently, necessarily also synchronised with the lunar cycle. Rather, it is that fertilization of the eggs must occur at a crucial time in the lunar cycle and it is the date on which the eggs are cast into the ocean (spawning) that is fixed to conditions during a particular lunar phase, i.e., a pre-dawn receding high tide during the last quarter moon phase.

 

The crabs seem to know how far they have to travel, how long it will take them to get to the ocean and whether or not they have sufficient time to be able to complete the downward migration, mate and brood eggs ready for spawning at the optimum time during the last quarter moon phase. When the rains arrive late and the timing is challenging for them to be able to meet a spawning date they will migrate rapidly providing weather conditions remain constantly suitable, or if the rains arrive early and are consistent, the crabs will instead migrate at a comparatively leisurely pace to the coast, often stopping to eat and drink on the way. If there is not enough time for them to be able to meet the next spawning date, even though the weather may be suitable, they will
delay the start of their migration and remain at their burrow till the following month.

The breeding migration comprises a synchronised sequence of  actions by the red crabs; one action following on from another only when the preceding one has been accomplished. Should one sequence be delayed for any reason, then the following actions are necessarily delayed.

 

The sequences of a complete and successful red crab breeding migration are; the downward migration, dipping in the sea, digging of mating burrows, mating, males dipping and males return migration, females brooding eggs in burrows, females emerging from burrows with eggs, gravid females dipping in sea, females spawning, females return migration, larval development in ocean, emergence of baby crabs from ocean, baby crabs migrating inland and settling in forest. 

 

Following the onset of wet season rains  the urge to migrate is triggered. The crabs run the risk of dehydration after they leave their burrows if the humidity does not remain up around 100% for the entire period of the migration . The male crabs farthest from the coast become active first, and begin their migration. Their activity stirs the females into action and the males are progressively joined by more and more females as they move to the coast so that male and female crabs arrive at the ocean at the same time. Depending on the combination of available time to be able to meet a spawning date and/or prevailing weather, the main downward migration can be a frantic dash or a leisurely stroll. Some years the downward migration may be accomplished in around five days while in other years it has taken nearly three weeks.

 

To view videos of the red crab migration, filmed by the BBC Natural History Unit, click here

Red crabs migrate  from the forest to the coast

Male red crabs dig and defend burrows and mate with females

Gravid female red crabs release their eggs into the sea

Masses of juvenile red crabs emerge from the sea

Text and images contained in this site may be copied or reproduced for study, research, information or educational purposes, subject to inclusion of an acknowledgement of the source.

Images by Max Orchard unless indicated otherwise

Red crab migration | Murray Road