By Deepa Padmanaban
A striking new species of crab has been found living in tree-holes high above the ground. The animal, which fits in the palm of a human hand, has a deep bluish black body that stands out against the tree bark that it prowls for worms and seeds to eat.
Scientists discovered the crab—named Kani maranjandu—in the lush forests of the Western Ghats in south India. It’s an entirely new genus and species named after the Kani, the tribal community that noticed the crabs, and maranjandu, the local colloquial term for tree crab.
The forest-dwelling Kani first reported sightings of “long-legged crabs” on trees in 2014. A. Biju Kumar, a professor of aquatic biology at the University of Kerala, was at that time leading a project to survey the Western Ghats of Kerala for freshwater crabs. After months of tracking the tree crabs with the help of the tribesmen, Kumar and his student Smrithy Raj recently managed to catch a couple of these elusive crabs.
In the Journal of Crustacean Biology, the scientists describe Kani maranjandu as having a distinct hard outer shell or carapace that is broad, swollen, and convex. Most conspicuously, the legs are extremely long, with slender, curved, sharp ends that help them get a good grip on the tree, making them effective climbers.
The crabs live in water-filled hollows of tall evergreen and deciduous trees. The Kani tribesmen detect their presence by looking for air bubbles coming out of the hollows. Outside the hollows, the crabs move rapidly on tree trunks, using their pincer-bearing thick front legs to propel themselves.
The crabs are shy creatures, retreating deep inside the hollows when approached. The younger ones take shelter in the canopy of the trees, up to about 30 feet. That’s unusual for crabs, which don’t normally climb more than a few feet into trees.
“This lifestyle of tree living indicates that, since they cannot disperse widely through the sea, their range tends to be limited to a very narrow area,” says Tohru Naruse, an expert on crab biodiversity at Japan’s University of the Ryukyus. He not involved in the discovery.
This geographical restriction could mean that any impact on their habitat could put the species at greater risk.
Biju Kumar also stresses the importance of the crab’s habitat: the large trees and forest ecosystem of the Western Ghats. The crabs’ existence hinges on rainwater collected in tree hollows, and the crabs have been observed to change trees if the hollows dry up. The broad, swollen carapace is an adaptation that helps them hold water in their gill chambers.
“It also suggests that the tree-climbing behavior and morphology of Kani maranjandu, and possibly other related, undiscovered species, has evolved where they are distributed,” adds Naruse.
For Peter K.L. Ng, a National University of Singapore biologist who helped classify Kani maranjandu, the species’ most alluring feature is how it illustrates crab evolution. “The exciting thing for me is that these crabs, regardless of where they have been found, and how they are related (or unrelated) to each other, they have nevertheless evolved to use specialized habitats to enhance their survival—in this case, tree-holes and climbing,” he says.