Discovering: Migration part 2

H.W.Turner

This is the second part in the series on migration.

The previous article focused on birds and migration in general, this article considers the migration of invertebrates, fish, and reptiles and amphibians. 

Invertebrate migration

Although less obvious than bird migration, many species of invertebrates migrate, including some that travel vast distances, rivalling that of birds.

Two of the most obvious invertebrate migrations are the mass migrations of Christmas Island red crabs (Gecarcoidea natalis), and monarch butterflies (Danaus plexippus). Both involve huge numbers; 30 million red crabs and millions of butterflies. However, the reasons why these two species migrate and the distances travelled differ considerably. 

Red crabs migrate from the rainforest floor on Christmas Island to the coast where males and females mate, often in burrows that the male has defended. The males travel back shortly after mating, but the females remain at the beach while their eggs develop in burrows. Then at the high tide of the last quarter of the moon, the females release their eggs into the sea which hatch immediately on contact with sea water. Eventually, baby crabs will migrate from from the sea to the rainforest to mature into adults. The migration on Christmas Island is so extensive that at peak migration times, some roads are closed on the island. 

The migration of the monarch butterfly occurs in North America over vast distances. Each autumn, millions of the eastern population of monarch butterflies migrate south from North America to the Mexican state of Michoacan, a distance of 4,800 km for some butterflies.

Unlike the red crabs, which migrate to mate, the monarch migrate to survive. The winter temperatures in North America are too cold, so the butterflies migrate to the warmer climate of central America to overwinter in a state of diapause (which is when fat reserves are laid down and no breeding occurs). During this time thousands of butterflies cluster on single trees to stay warm. Come the spring, the butterflies start to migrate north again. However, no one individual butterfly will make the trip back, instead females lay eggs along the route and then die. These eggs then hatch and the new generation resumes the migration. By the time the monarch reach their summering grounds, it may be the 4th-generation that eventually makes it back. 

Another butterfly which migrates is the painted lady or cosmopolitan (Vanessa cardui), yet not all the time as this species is a occasional migrant. In some years, mass migrations of painted ladies occur as painted ladies move north from North Africa, and Southwest USA and Mexico (winter areas), and in other years, hardly any migrate at all. The reason for the disparity is due to conditions in the winter areas. When rains occur in these arid areas, the populations boom, due to an abundance of food. This results in increased competition and a subsequent depletion of the food supply, meaning the butterflies are forced to migrate north in search of more food.  

In the UK you can log your painted lady sightings at http://butterfly-conservation.org/612/migrant-watch.html

Fish migration

Two of the most well known fish migrations are the migration of European eel (Anguilla anguilla) and Salmon species. 

The migration of the European eel is complex. The eels spend most of their life in the fresh waters of European rivers and streams but migrate to the Sargasso sea to breed a fact that remained a mystery until Johannes Schmidt discovered the breeding grounds in the early 20th century. The young eels then travel with the gulf stream to European estuaries where they metamorphose into a transparent larval stage called a glass eel. Then, when these glass eels reach the rivers, they metamorphose again to elvers and then mature to eels, which will eventually migrate back to the sea to breed. 

The migration of salmon is a great spectacle. Again like the eel, the salmon migrate between river and sea, although this time the other way round. In the autumn, the salmon run occurs along the coast of Alaska as thousands of salmon migrate up the Yukon river. This glut of salmon provides a food supply to a range of predators including grizzly bear, wolf and bald eagle. The full dynamics of this event will be explored in another discovering article.

Reptile Migration

A number of species of reptile and amphibian migrate including; large sea turtles, which migrate as adults back to the same beach in which they hatched to lay  their own eggs, and common toad (Bufo bufo) which migrate in autumn from land back to the ponds where they were born. However, the migration of toads is halted in midwinter, when cold temperatures force the toads to hibernate. When the toads eventually reach their breeding ponds, mating occurs before the toads return to their land ranges, which can be up to 1600 m away from the pond.  

Sources

 

Published by H.W.Turner

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