A purple crab stares down the camera in the Philippine island of Palawan (map) in an undated picture. The colorful crustacean, dubbed Insulamon palawanense, is one of four new species in the Insulamon genus described in a recent study.
The crab's brilliant hues may simply help the species recognize its brethren, said study author Hendrik Freitag, of the Senckenberg Museum of Zoology in Dresden, Germany.
"The particular violet coloration might just have evolved by chance, and must not necessarily have a very specific function or reason aside from being a general visual signal for recognition," said Freitag, whose study was published in February in the Raffles Bulletin of Zoology.
Freitag described the four new crabs—each between about an inch (2.5 centimeters) to 2 inches (5.3 centimeters) wide—from museum specimens and individuals collected during two field surveys in Palawan. Only one other species, I. unicorn, is already known in the genus, and it was identified in 1992.
Photograph courtesy Hendrik Freitag
A male I. palawanense is seen on a rock in Palawan. The species is widespread throughout the Philippine island, unlike the other three newfound species, which are restricted to small home creeks or rivers.
Including the previously known I. unicorn, all five Insulamon species are exclusively freshwater creatures, spending their days burrowing in muddy holes and emerging at night to feed.
That's an unusual life cycle for a crab—most of the world's known crabs migrate to the sea at some point to spawn, Freitag said.
Most Insulamon crabs—including I. magnum, seen above—are more threatened than other freshwater crabs because they're confined to specific creeks and small rivers, Freitag said.
Also, because the crabs live in or close to running water their whole lives, "they cannot just escape or migrate via the ocean or dry land to other places when their original habitat [is] destroyed," he said.
In Palawan, farmers are clearing tracts of forests, which dries up rivers and streams—a "major threat" for most freshwater species, including the crabs. Likewise, planned mining projects may harm water bodies in the region, particularly via pollutants.
A view of the underside of an I. magnum female reveals the eggs she's carrying.
After finding the new Philippine species, Freitag and students from Ateneo de Manila University plan to survey the Philippines' Mindoro Island (map), where many habitats are already pressured by mining.