A SIXTH of the world's freshwater crab species are at risk of extinction, experts have warned.
It is feared pollution and habitat loss could be wiping out many types of crabs.
The creatures help keep tropical aquatic ecosystems healthy by recycling animal and plant remains.
Their disappearance would break the nutrient cycle, according to scientists, and have knock-on effects on water quality, animal populations, and human communities.
Freshwater crabs are the main catch for small-scale fisheries in many parts of the tropics and often provide a key source of protein for local people.
They are also an important food source for fish and other animals.
Dr Ben Collen, from the Zoological Society of London, who co-led a new assessment of freshwater crab populations worldwide, said: "The loss of freshwater crabs threatens to interrupt the processes that provide benefits to humans such as nutrient cycling and maintaining water quality.
"We must set clear goals to reverse these trends and ensure that our enduring legacy is not to wipe out the small things that provide us with great benefits, such as climate regulation."
His colleague Dr Neil Cumberlidge from Northern Michigan University in the US, added that a wide range of predators, from herons to snakes, depend on freshwater crabs for survival.
The British and US scientists conducted the research for the International Union for Conservation of Nature Red List of Threatened Species.
The Red List categorises different animal and plant species according to extinction risk.
One species of particular concern is the Singapore freshwater crab, Johora singaporensis, which has been listed as "critically endangered".
It has not been sighted in nature reserves in Singapore since the early 1990s. Acidification of water is believed to be responsible.
Freshwater crabs are not fussy about what they eat but extremely choosy about where they live. The study showed the widest range of freshwater crab species lived in rain forests.
Most threatened species were highly sensitive to habitat loss, pollution, and water siltation.
The increasing destruction of tropical rain forests and freshwater ecosystems meant the crabs were in need of urgent protection, said the scientists.
The research is published today in the journal Biological Conservation.