Clawing at a life on land

At some 75cm across and capable of cracking open a coconut shell with its formidable claws, the land-dwelling coconut crab is your beach lounger's worst nightmare.

Fortunately for the sunbather, the world's largest terrestrial arthropod has seemingly always been confined to tropical islands across the Pacific and Indian oceans.

Geerat Vermeij, professor of geology at the University of California, Davis, believes the coconut crab's inability to get a foothold on continental shores, while thriving on tropical islands, explains why switching home from a marine to a terrestrial environment, or vice versa, is so difficult.

Few organisms, from insects to flowering plants, have managed this transition. Most scientists have explained this fact in terms of specific problems that individual species face with the sheer physical task of adapting between a life underwater and life in the open air.

But in research published with Robert Dudley, at the University of Texas, Vermeij argues that there may be a more significant factor in play - competition.

When an invading species attempts the transition, it is faced with competition for resources with incumbent species better adapted to the environment.

The number of known transitions between land and freshwater environments, which typically have lower levels of competition, are many times more than between land and sea water.

An analysis of times and places in the fossil record where species have successfully made a marine-terrestrial switch reveals environments with low levels of competition and predation, such as the terrestrial communities of the middle Palaeozoic, some 350-400 million years ago, or oceanic islands.

It seems that when the marine ancestors of the coconut crab tried to adapt to life on land, they could not compete with predators on the continents, but did not encounter such problems on isolated islands with a much more limited range of competitors.

The exceptions are the tetrapod vertebrates, the lineage that gave rise to the reptiles, mammals and amphibians. These creatures, once established on land, seem to possess an ability, possibly linked to their high metabolic rates, to survive the switch back to the sea and have done so on a number of occasions, whales being a comparatively recent example.

Vermeij believes the barrier could be crumbling as an unprecedented number of species are driven by mankind to extinction.

He warns that this could remove much of the competition and, in the long run, open fresh possibilities to would-be invaders.

Perhaps the coconut crab will prevail after all.

The research is published in the Biological Journal of the Linnean Society.