Amazing Nature: Red Crab Migration and the Catatumbo Lightning
Amazing and Awesome Nature
Nature is both amazing and awesome. It may also be very intriguing. Animals, plants, the atmosphere, and the Earth are involved in some impressive natural phenomena. Two of these phenomena are the annual migration of millions of red crabs on Christmas Island and the "everlasting" Catatumbo lightning storm in Venezuela. Both are fascinating examples of nature in action.
Researchers estimate that forty to fifty million red crabs currently live on Christmas Island. When all of the adult crabs on the island migrate to the ocean at the same time in order to reproduce, as they do each year, the effect is spectacular.
The incredible Catatumbo lightning is seen over a very special lake in Venezuela. The flashes of lightning are visible on roughly 140 to 160 nights of each year, for around eight to ten hours each night, and up to 28 times a second at the peak of the season. The repeating light show has occurred for centuries.
Location of Christmas Island
Christmas Island and the Red Crab
Christmas Island is located in the Indian Ocean and is a territory of Australia. Its name comes from the fact that it was discovered on Christmas Day in 1643. The island is rich in biological diversity and contains some unique organisms. 63% of the island belongs to a national park.
The scientific name of the red crab is Gecarcoidea natalis. It's native to Christmas Island and the Cocos or Keeling Islands, which are also located in the Indian Ocean and are also a territory of Australia. Its carapace (the shell over its back) may reach up to 4.6 inches in width. Males are generally bigger than females. Although the animal is usually red in colour, some individuals are orange. Very rarely, a red crab may be purple in colour.
The Life of a Red Crab
The red crab lives on land and is active during the day. It breathes by using both lungs and gills. The animal is very sensitive to water loss from its body and digs a burrow for protection when its environment becomes unsuitable. It sleeps in the burrow and also uses it as a shelter during the day when the weather is too hot or dry. During the dry season the crab stays in the burrow and blocks the entrance with a wad of leaves.
Red crabs live mainly in forests but may also set up their home in people's gardens and in crevices in rocks. They feed on fresh or dead leaves, flowers, fruits, and seedlings. They also scavenge material from the bodies of dead animals.
Reproduction takes place at any time from October to January. November and December are the most common months for breeding, however. They are generally the rainiest months of the year. Males begin the journey to the ocean before the females but are joined by females during the trip. The largest males reach the sea first after a journey of five to seven days.
After dipping their bodies in the sea to replace moisture loss, the male crabs dig a mating burrow on the terraces by the seashore. When the females arrive they dip their bodies in the ocean. They then join the males in the burrows and mate there. Mating may sometimes happen outside of the burrows, however. After the mating process has finished, the males leave and return to the forests. The females stay to complete the reproductive cycle.
Annual Crab Migration on Christmas Island
The female lays her eggs about three days after mating with the male. She holds the eggs in the brood pouch on her abdomen. This pouch can hold up to 100,000 eggs. The female stays in the mating burrow while the eggs develop, which takes about twelve or thirteen days.
When the eggs are mature, the female releases them into the ocean. She vibrates her body in a dance-like motion known as a shimmy in order to release the eggs from the brood pouch. Once the pouch is empty, the female begins her return migration.
The youngsters go through several larval stages in their development. When the ones that have survived have reached the tiny crab stage, they emerge from the water. They perform their own migration in order to find a site where they can develop into an adult, as shown in the video below. The crabs are reproductively mature when they are about four years of age.
Migration of the Youngsters
Migration and Reproduction Problems
Migration is a dangerous time for the crabs. Dehydration and injury are both major threats. The crabs travel over roads as well as off-road areas to get to their destination. Officials erect barriers to try to guide the crabs along a route away from traffic, but some animals climb over the barriers. Roads are often closed during the migration to protect the crabs. In some places tunnels have been built under roads to allow the animals to travel safely.
The crabs take a break in their migration if the weather becomes too dry, creating a temporary burrow as a home until the situation improves. They also pause if the phase of the moon is incorrect. The eggs are released as the high tide is turning when the moon is in its last quarter. If this moment is missed, the adult crabs will wait for a month to complete their reproductive cycle. The behaviour of the crabs is truly a wonder of nature.
The Catatumbo Lightning in Venezuala
The amazing Catatumbo lightning can be seen from far away and was once used by Caribbean sailors as a navigational aid. They referred to it as "the Lighthouse of Catatumbo ". In 2014, Guinness World Records gave the Catatumbo lightning the award for the highest concentration of lightning in the world.
The Catatumbo lightning storm is very unusual because it always occurs in the same area and at the same time and because it occurs so frequently. There is nothing special about the lightning itself, though. People have noticed that the lightning storm has a different colour at different times, but researchers say this is because the colour is altered by dust particles and water vapour in the air. People also say that no thunder is created by the Catatumbo lightning, but experts say that this is simply because observers are too far away to hear the thunder. The repeated and frequent formation of a thundercloud over the lake is very intriguing, however.
Location of Lake Maracaibo
Formation of the Thundercloud Over Lake Maracaibo
The Catatumbo lightning occurs where the Catatumbo River flows into Lake Maracaibo. The cause of the thunderclouds that produce the lightning isn't known for certain, but the cloud formation is believed to be triggered by the unique combination of air currents and topography in the area.
Lake Maracaibo is located in northern Venezuela and is connected to the Gulf of Venezuela. It contains brackish water because it's fed by both the ocean and several rivers, the largest of which is the Catatumbo River. The lake is surrounded on three sides by mountains.
Warm winds from the Caribbean blow over Lake Maracaibo and meet the cooler air flowing off the mountains that surround the lake. The cooler air mixes with the warmer air over the Catatumbo River and Lake Maracaibo, which is probably the main contributor to the formation of a thundercloud. The evaporation of warm water from the lake probably feeds the cloud. The surrounding mountains are thought to trap the air mass over the lake. The combination of these factors likely leads to the creation of a thundercloud, which eventually discharges electricity and produces lightning.
The two videos below contain flashing lights and therefore may not be suitable for people with certain medical conditions.
Venezuela Catatumbo Lightning Show
Cause of the Lightning Over Lake Maracaibo
Once a thundercloud forms over Lake Maracaibo, lightning is believed to be created by the same mechanism that exists in other places on Earth. The explanation below is an overview of the leading theory for lightning formation. The theory may not be completely correct, however, and there are gaps in our knowledge of the process. Strange as it may seem, we don't completely understand the cause of lightning. Its production is a rapid, complex, and still somewhat mysterious process.
An Everlasting Storm
Charged Particles and Ions
Lightning develops due to the formation of charges in matter. It's helpful to know a little about the basic structure of matter in order to understand how these charges develop.
Matter is made of atoms. An atom contains a nucleus containing positive protons and neutral neutrons. Negative electrons orbit the nucleus. The number of protons and electrons in an atom is the same, so the atom is neutral. The electrons have a lower mass than the protons and neutrons.
Under certain conditions, one or more electrons may leave an atom. As a result, the atom has more protons than electrons and has become a positive ion. The released electrons may travel through a conductor or be absorbed by a different atom. An atom that has gained electrons is known as a negative ion.
Production of Charges in a Thundercloud
A thundercloud is very tall. Inside the cloud, turbulent winds transport air and water droplets up to the cold upper section of the cloud. Here the water in the air freezes, creating ice particles. The ice particles are then carried downwards by wind currents, colliding with other ice particles as they travel. Electrons pass between the ice particles during the collisions.
For a reason that is not completely understood, smaller particles of ice develop a positive charge while bigger particles develop a negative charge. The heavier negative particles collect at the bottom of the cloud while the lighter positive particles are left higher up. This charge separation is the key to lightning formation.
Lightning Production: An Overview of the Basic Process
Similar charges repel each other. The electron-rich, negative layer at the bottom of a thundercloud repels electrons in the surface of the Earth below the cloud or in the surface of an object projecting from the Earth. This gives the surface an unbalanced positive charge from the protons in its atoms.
Opposite charges attract each other. The negative electrons in the cloud are attracted to the positive surface of the Earth. They flow through the air towards the Earth in a channel known as a stepped leader. The electrons move in a series of steps that often branch.
Positive particles from the Earth are attracted to the negative particles in the cloud. They move up tall objects and then into the air through a channel known as a streamer or an upward leader.
When a stepped leader and a streamer meet, an electrical connection between the cloud and the ground is formed. Instead of consisting of a wire, as is often the case for electrical connections in our lives, this connection consists of ionized air. Ionized air allows for a much better flow of charged particles than normal air.
The electrons from the thundercloud accelerate towards the Earth through the connection that has been established and collide with air molecules. This causes the air to glow and produces the lightning flash, starting with the air closest to the ground. Although the negative charge moves from the cloud towards the ground, the lightning flash moves in the opposite direction. For this reason it's known as the return stroke.
How Does Lightning Work?
Natural Phenomena on Earth
Natural phenomena such as earthquakes and tornadoes can be dangerous and have tragic consequences. Phenomena like the Christmas Island red crab migration and the Catatumbo lightning are fascinating and enjoyable to observe, however. They can also teach us more about the amazing world of nature and its behaviour. The lesson is very interesting as well as useful.
- Facts about red crabs from the Christmas Island Tourism Association
- Red crab migration from the Government of Australia
- Venezuela's most electrifying lightning storm from BBC Travel
- The most electric place on Earth from BBC Earth
- Lightning facts from the Exploratorium
- Understanding the return stroke with animations from NOAA (National Oceanic and Atmospheric Administration)
Questions & Answers
© 2015 Linda Crampton