New to Nature No 59: Labuanium vitatum

A bright purple, tree-climbing crab has been discovered on several Indian Ocean islands. Peter Ng of the National University of Singapore and Peter Davie of the Queensland Museum report the new species,Labuanium vitatum, from Christmas Island, Nicobar Island, Nias and Java. Secretive habits had contributed to this conspicuously coloured species remaining unrecognised.

It appears to be active only in wet weather and at night and to spawn during full moon cycles between November and April. When active, these and related arboreal crabs are encountered near the bases of trees growing along the seashore. The carapace of mature crabs can reach 40mm in width. Its ecological low profile, however, is only part of the story. Specimens collected in the late 1970s were mistakenly identified as Sesarma oceanicum, a species that is itself now considered a junior synonym of the diverse crab species Labuanium rotundatum. In their study, Ng and Davie concluded that specimens long identified as L. rotundatum represent a species complex. In addition to describing N. vitatum as new, they elevated a subspecies first described in 1899 from New Guinea to full species status as L. papuomalesiacum. Scientific names must keep pace with the growth of knowledge. Based on new collections and evidence, experts periodically review earlier species descriptions to test whether they still reflect our best theories about the evolution of that group. With so many species and so few specialists there are many groups of invertebrates that, like arboreal crabs, will continue to yield biodiversity surprises as they are studied in greater detail in the field, museum, and laboratory. Even with these improvements, our knowledge of crabs of the genus Labuanium remains fragmentary. There are few specimens of the 10 known species, including the four known from the Indian Ocean. A colleague and former Christmas Island chief warden, Max Orchard, has a forthcoming book on Christmas Island crabs that will facilitate future observations on their fascinating natural history.