2002 WINNER

Why women drink the tree crab 'potion'

By Clodagh O'Brien

In a village in the East Usambara Mountains in Tanzania, the rhythmic beating of drums echoes through the tree canopy. The local people are celebrating the safe arrival of a new baby. But the drums are not heralding the birth. Instead they are giving thanks to an unlikely creature, a crab that lives in tree holes.

During a biodiversity survey project in Tanzania, East Africa, Julian Bayliss, a researcher at Oxford Brookes University made an amazing discovery. He found that the native people use an unusual medicine to treat pregnant women, particularly those at risk of miscarriage. They call this medicine ''Mazi yangodi'', which roughly translates as ''water from holes with crabs''.

It is not a new thing for indigenous people to know of natural methods to treat medical conditions. But what is amazing about this discovery is how and why this ''potion'' is formed. The key, according to his paper in the African Journal of Ecology, lies in the odd behaviour of a new species of crab.

Crabs belong to a group of animals known as the Crustacea and originally lived only in saltwater environments.But due to fluctuating salt levels,some of the crabs gradually moved into freshwater streams and rivers. Now it seems that these strange crabs have taken a bigger step by moving into tree crevices.''We are experiencing a new step in crab evolution where these crabs seem to be adapting to a new environment and exploiting the resources around them,'' says Bayliss.

These onshore crabs still have to undergo a process called ''moulting ''. They can moult up to 20 times during their lifetime dependent on water temperature, food availability and the age of the crab. They have a hard outer shell called an exoskeleton, which acts as a shield and protects the soft tissue underneath. Unfortunately, this hard shell does not grow. Eventually, a crab grows too big and has to shed its old shield, leaving it soft, naked and vulnerable. The crab will form a new shell, but it takes time to harden.

Calcium is essential for the new body armour to form.The crab's new soft skin turns into a living sponge that sucks up any available calcium. However, this is no easy task for a tree-dwelling crab living in a closed canopy forest with little or no calcium. But they have found a recipe for success. As darkness falls, they scuttle in all directions in search of molluscs, such as snails, that live in the leaf litter. The crabs dine on the snail flesh, but the secret ingredient they are searching for lies in the shell of their victim.

The water in the crab's tree hole is naturally acidic. After finding a mollusc it returns to its watery home clutching pieces of shell between its long claws. Once safe inside, the crab drops the shell fragments into the water. Precious calcium seeps into the water, reducing the acidity and creating a well of armour-building water. The crab uses this calcium-rich solution to hasten the development of its hard outer shell and is often found submerged in its private plunge pool.

And it is this water that the pregnant women of the East Usambara Mountains drink. But how does it help them? Women need calcium while pregnant. It has been found to reduce the risk of pregnancy-induced hypertension (PIH) and pre-eclampsia, a potentially fatal disorder of high blood pressure and kidney failure. These disorders have been found to affect many women worldwide and are the leading causes of caesarean sections and low birth-weight babies. This may explain why the Mazi yangodi ''potion'' helps babies survive to term. Bayliss thinks that as the local people have problems finding calcium, the high levels in the crab-made water could provide the pregnant women with the essential calcium they need.

Another rare behaviour has emerged from Bayliss 'study. These tree-hole crabs don't just unknowingly help people reproduce, they also ensure the survival of their own offspring. Bayliss found that the crabs use these holes as miniature nurseries. Young crabs are less able to collect the leaf-litter molluscs, says Bayliss, so the parents will raise several generations of them until they are old enough to leave and find holes of their own. ''This behaviour is extremely rare in crabs and it could be several years before the young crabs leave the care of their parents.''

The discovery of this highly adapted and efficient crab opens a wealth of possibilities. If these tree-dwelling crabs in East Africa hold the key to preventing miscarriages, what other secrets lie hidden in the depths of the natural world?