The One and Only Tree-climbing Freshwater Crab in Sri Lanka

Until mid 90s, the inventory of freshwater crabs in Sri Lanka stood at a modest eight species, belonging to four genera. This was following the first description of a couple of species in 1880.  Since mid 90s, this tally has seen phenomenal improvements, thanks largely to the work of Wildlife Heritage Trust of Sri Lanka (WHT), which began exploring this less-studied fauna in collaboration with Carcinologist, Peter Ng from the National University of Singapore. At present the freshwater crab inventory stands at an impressive fifty-one species—all of which are endemic!

According to a conservation assessment done of our crab fauna by WHT, a total of thirty-seven species are threatened with global extinction, and twenty-six species are found in areas less than 100 sq.km, with two species found in an area less than 1 sq.km. Clearly, our crabs are in peril!

The one and only tree-climbing freshwater crab in Sri Lanka, Perbrinckia scansor Ng, 1995.

All freshwater crabs in Sri Lanka belong to the Family Parathelphusidae. They are currently assigned to seven genera of which five are endemic to Sri Lanka. The species shown above, which I hereby refer as Sri Lanka Tree-climbing Crab for lack of a readily available vernacular, was discovered by the good men at WHT from Sinharaja 'World Heritage' rain forest, and was originally described by Peter Ng in 1995 as Ceylonthelphusa scansor. In 2005, the Sri Lankan Biologist, Mohamed Bahir of WHT and Ng assigned this species to the endemic genus, Perbrinckia in a paper published in the Raffles Bulletin of Zoology (supplement No.12), which described ten new freshwater crabs. The species name, 'scansor' means climber, which refers to this crab's ability climb trees. It is typically found in tree-holes containing rain water in the rain forests, in the highly biodiverse 'wet zone' in South West Sri Lanka. In addition to tree holes, I have also found this species in water-trapped holes in rocks in the forest.

The eggs of the freshwater crabs hatch directly into first crab stages and they do not have a sea-faring stage. All freshwater crabs brood the newly hatched young under their abdomen in a sort of a trapdoor hinged pouch. The young are released when they are grown up to a certain stage and are more active. The patch of black in the belly, out of focus in the above picture are a cluster of crab babies or crabbies as I prefer to call them.

A quick note here on the background details of this photograph and tools used may be in order. I photographed this individual at Sinharaja in April while guiding Dr. Gil Ewing - a serious birder from US-CA on a 14-day Absolute Birding tour. Our hits and misses and other juicy details of this tour will be blogged in due course, after clearing the backlog of reports of tours done since Jan.

We encountered this crab species on two days - on April 19 and 21, both during late afternoons. On both occasions we found it on the main track - away from its typical arboreal habitats.

I used my Canon 40D and Canon 100mm f2.8 Macro lens with Canon Macro Twin Lite MT-24EX Flash for this. Settings were ISO:400, 1/250, F 5.6, FEC -2/3. To get this angle I remained postrate on the track, ignoring all leeches and other creepy crawlies for a moment. I usually do not carry my macro rig in the field when I am with birders, but on this occasion I had them handy. This was because we'd managed to bag all our endemic target birds by 9.00 a.m. on the first of the three mornings at Sinharaja. Our quick progress with the target birds found us in a more broad-minded and trigger happy mood, and gave us time to look for other natural history delights that Sinharaja had in store.