Red Crab Mothers

A female red crab lays up to 100,000 eggs and scatters them into the ocean when the tide is calm, sometimes drowning from the effort in the hopes that some of her offspring will survive to adulthood.

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Red crabs are native to Christmas Island, Australia. The central plateau of Christmas Island is dominated by strands of rain forest. The island has a tropical climate and experiences both a wet season (December through April) and a dry season (May through November).

More than 120 million red crabs can be found on the rain forest floor of Christmas Island. Red crabs live alone in dirt burrows, or deep rock crevices. Crabs stay in the shade of their dwelling for most of the year. In October or November, when the wet season is about to return, crabs begin their migration to the shore. This timing coincides with the lunar cycle and the tides.

This video documents the second stage of the red crab migration, after the crabs have mated and before the eggs hatch in the ocean. At the start of this stage, females brood their eggs in dirt or coastal rock burrows for 12 to 13 days. A female red crab can lay up to 100,000 eggs, which she holds in her abdominal sac. With the arrival of the waning moon, females make their way into the sea. In what looks like a dance, females brace themselves at the waters edge, and release their eggs.
1. How many eggs can a female crab produce?
  
  Show Answer
  
  A female crab can produce up to 100,000 eggs.
2. How many days does the female red crab sit with her eggs?
  
  Show Answer
  
  The female red crab sits on and protects her eggs for 12-13 days.
3. What cues the red crabs to lay their eggs and begin migration?
  
  Show Answer
  
  The waning moon cues the red crabs to lay their eggs. A waning moon means that the moon is decreasing in size, moving from a full moon towards a new moon. During this time, the moon takes on a crescent shape.
4. What kind of tide is best for the female crabs to release their eggs?
  
  Show Answer
  
  Red crabs release their eggs during mild or low tides, coinciding with the waning moon. Although the tide is low, many female crabs drown as they attempt to release their eggs into the sea.
5. Do most of the eggs survive to return as hatchlings?
  
  Show Answer
  
  No, only a handful of the hatched eggs survive and return to shore.
Articles & Profiles
- National Geographic Magazine: Animal Migrations
Audio & Video
- National Geographic Channel: Red Crab Marathon
Websites
- National Geographic Animals: Migrations Quiz
- National Geographic Animals: Red Crab

Term	Part of Speech	Definition	Encyclopedic Entry
animal migration	Noun	process where a community of animals leaves a habitat for part of the year or part of their lives, and moves to habitats that are more hospitable.
burrow	Noun	small hole or tunnel used for shelter.
crab	Noun	type of marine animal (crustacean) with a flat body, hard shell, and pincers.
current	Noun	steady, predictable flow of fluid within a larger body of that fluid.	Encyclopedic Entry: current
lunar cycle	Noun	complete cycle of visible phases of the moon, approximately 29.5 days.
migration	Noun	movement of a group of people or animals from one place to another.
ocean	Noun	large body of salt water that covers most of the Earth.	Encyclopedic Entry: ocean
offspring	Noun	the children of a person or animal.
plateau	Noun	large region that is higher than the surrounding area and relatively flat.	Encyclopedic Entry: plateau
rain forest	Noun	area of tall, mostly evergreen trees and a high amount of rainfall.	Encyclopedic Entry: Rain forest
tide	Noun	rise and fall of the ocean's waters, caused by the gravitational pull of the moon and sun.	Encyclopedic Entry: tide

Credits

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Editor

Caryl-Sue, National Geographic Society

Producer

Nina Page, National Geographic Society

Page Producer

Makayla Trotter

Last Updated

Jan. 19, 2012

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