Migration of Red Crabs

What is migration?

When most people think of migration, they only think of birds migrating south for hr winter. There are however, tons of different species that take part in migration events for many different reasons. Animal migration occurs when individual animals move a relatively long distance, usually on a seasonal basis. Many different species take part in migration including birds, mammals, fish, reptiles, amphibians, insects, and crustaceans. There are multiple environmental factors that can trigger for the migration. Some of these include the local climate, the availability of food, what season it is, or mating reasons. To be considered a “true migration”, and not just a local dispersal or irruption, the movement of the animals should occur annually or seasonally. Examples of this include birds migrating from the Northern Hemisphere to the south for the winter or wildebeest migrating annually for seasonal grazing.

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“Cranes Migration” By Artur Rydzewski Under Attribution 2.0 Generic (CC BY 2.0)

Why migrate?

Migration is an adaptive response to certain seasonal or geographic variations of resources. The changes in seasons provides a drastic different in the duration and intensity of solar energy received in each hemisphere. It also allows the animals that migrate to take advantage of favorable food and weather conditions in different areas for different tine periods. Animals can travel hundreds to thousands of miles away from their current habitat during migrations.

How do the animals know where to go?

There are two main sources of environmental cues that allow animals to know where to go when they are migrating. These are visible cues and invisible cues. Visibly cues are used by birds, insects, crabs and a few more in order to detect polarized light patterns.  These light patterns are formed when light is scattered by airborne particles, and the patterns that they create changes as the sun’s position shifts throughout the day. This allows animals to know where they are going even on cloudy days when the sun is not visible. Invisible cues can consist of olfactory and magnetic cues. Magnetic cues are used by many species including birds, bats, whales, turtles, and many other species.  Some species are able to use their magnetic compasses to detect the changing angle of the magnetic force lines, the north-south polarity, or the strength of the Earth’s magnetic field.
The image below shows migration patterns in different species of birds and shows just how far some are able to travel.
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Migration of Red Crabs

Red is the most common color for Christmas Island Red Crabs but they can also be found in orange and very rarely purple. They are a large crabs, an adult crabs body shell  can measure up to 116mm across. This body shell encloses their lungs and gills. The claws of these crabs are usually  equal sizes however, they are able to regrow missing claws.

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“Christmas Island Map” By Ewan ar Born Under  Creative Commons Attribution-Share Alike 3.0 Unported2.5 Generic2.0 Generic and 1.0 Genericlicense.

The Christmas Island Red Crab is a species of land crab that is native to Christmas Island and Cocos (Keeling) Islands which are located  in the Indian Ocean. There were an estimated 43.7 million adult red crabs on Christmas Island alone, but this number dropped drastically after the yellow crazy ant was accidentally introduced. It is believed that these and have killed between 10–15 million crabs. Christmas Island red crabs are most known for their annual mass migration to the sea to lay their eggs in the ocean. The crabs only migrate during the rainy season, and if there is not enough rain, they will halt their migration.

The video below shows some great footage of the migration and explains a little more about it.