Christmas Island-Crab out of water

by Matthew Norton

Christmas Island is home to a strange group of crabs that live in the island’s rainforests, completely out of water, for most of their lives. The red land crab, Gecarcoidea natalis, is the most well-known, and abundant species, with their mass movements through human settlements being captured on video (https://www.youtube.com/watch?v=gR02_MFpOYY).

(Rev) christmas island crab article image 1 — *Gecarcoidea natalis* (top) is the most well-known land crab species on Christmas Island, a small island in the Indian Ocean (bottom). Despite being closer to Indonesia it an Australian territory.

These are unusual surroundings for what is usually an aquatic animal and so they have unique difficulties to overcome. For example crabs would normally struggle to breathe in air as the individual sections of their gills, called lamellae, are very thin and kept close together to maximise the surface space over which dissolved oxygen and carbon dioxide can be exchanged with the surrounding water. This works in aquatic animals because the buoyancy of the water keeps the gills upright, but in air this delicate structure collapses and the lamellae fold in on each other, reducing the surface space for gas exchange. The Christmas Island land crabs thus have stiffer gills and thicker lamellae so that they remain upright in air, presumably the greater availability of oxygen in air compare to water compensates for any hindrance on gas exchange.

The gills also provide aquatic crabs with the means to exchange dissolved salts, which they need to maintain a favorable balance of in their bodies. Land crabs cannot get these salts from air and so they rely on the salts in their food and the redirection of their urine over their gills from which additional salts can be recovered. They balance the salts in their body by releasing, or suppressing the release of hormones to control the volume of urine they produce and the intensity of the salt recovery process over their gills.

Another unique problem for land crabs is that they need to keep their bodies from drying out. They manage this by simply retreating into their burrows when they are at risk, so much so that the humidity at the forest floor virtually controls how active these crabs are.

However, they must leave the safety of their burrow and make an arduous journey to the coast in order to breed, so that their young can develop and grow in seawater. Gecarcoidea natalis carefully time this migration with the start of the wet season, so that they don’t dry out on the way, and in sync with the lunar cycle so that they all get there at roughly the same time.

(Rev) christmas island crab article image 2 — The march of the red crabs in action (left), sometimes aided by their human neighbours who have installed fences and open grids to give them safe passage underneath our roads.

Still, walking such a distance is tiring for them, and with a high rate of oxygen consumption, even at their average walking speed, they often resort to anaerobic (non-oxygen) means of getting the energy they need. This in turn causes lactic acid to build up in their muscles, as it does in our muscles when we are aching from exercise, and so they have to take regular breaks so that this lactic acid can clear.

From a human perspective

The various species of land crab do not directly affect anyone living Christmas Island, but they are critical to the island’s rainforests. They make a substantial contribution to recycling nutrients into the soil by feeding on plant material in general, and by selectively feeding on the seeds of trees and vines they affect which species dominate the rainforest. This arrangement keeps the rainforest ecosystem healthy and stable, which in turn supports the charismatic wildlife that draws tourists to the island. Also the vast numbers of young land crabs, which develop in the sea, are a source of food for charismatic marine wildlife, such as whale sharks.

It is therefore unfortunate that these crabs are under siege from invasive species that humans have introduced into their environment, albeit accidently, which could then thrive at their expense. They can resist some of these invaders, for example most snails of the giant African species Anoplolepis gracilipes are killed by Gecarcoidea natalis, the exception being areas of rainforest that are too damaged to support large numbers of land crabs. Unfortunately, the yellow crazy ant (Anoplolepis gracilipes), is a far more aggressive invader, overpowering these land crabs with sheer numbers and killing them in their millions.

(Rev) christmas island crab article image 3 — The yellow crazy ant, Anoplolepis gracilipes, is named for its frantic movements.

(Rev) christmas island crab article image 4 — A group of yellow crazy ants photographed on Christmas Island overpowering a single beetle. A lone land crab is unlikely to fare much better against a similar attack.

The loss of these crabs, and the direct consumption of plant material by the invasive ants, will have a knock on effect on the rainforests, altering which plant species dominate the invaded rainforest areas. The benefits and drawbacks of these changes to the rest of the plants and animals are debatable, but there is little doubt that it will be an extreme shift in the structure of the rainforest as the influence of the land crabs is weakened.