Wednesday, 5 May 2010

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.

18 comments:

Offthebeatentrack said...

Great shot and information, thanks for highlighting an aspect of our natural heritage and biodiversity that is often missed by everyone!

Chavie said...

That is so cool Amila. Lovely shot. I had no idea that crabs carried their young in a pouch! :O

Tabib said...

Oh.. This a new thing for me - Tree-Climbing Crab.
Thanks for sharing this wonderful world of nature.

Gallicissa said...

Hi Naren,
A popular guide is needed to popularise our inland crabs, and I am sure Bahir will do it.

Hi Chavie,
They do take parental care seriously.

Hi Tabib,
No problem! We as many crabs as you have in P.Malaysia and Singapore put together. SL is half the size of that land mass.

chandanie said...

Hi Amila,
Good to see you back with the pics and all the info. Thanks for that.
Have been visiting your blog regularly and admiring the photos and sighing over them :(
But finally got the 50D with the kit lens 18-200mm :)))) Am having a ball!!! Eying the 100-200 but.......

Gallicissa said...

Hi Chandanie,
I wish to warmly welcome you to the exciting and expensive world of dSLR photography!

I am very happy for you.
I know at least one more person who may be happier :)

Aim high (I appreciate you've already done that) and go for 100-400mm f4.5-5.6lens if possible. You will not regret it. Nice to hear that you admire my pics - Thank you!

Me-shak said...

crabs carry their young in a pouch? cool. And it's a pity to see all these beautiful creatures pass away. i hope at least some one will take the initiative to preserve them :D
nice post Amilla. looking forward for more :D
Cheers!

Andrea said...

Great Picture.
I am happy to discover your really interesting blog.

Lady divine said...

I always trust you to surprise me with your posts and pictures! :)

Great work. Awesome picture!

Gallicissa said...

Hi Shak,
We should be grateful to WHT for bring our crabs to the fore. There's more to crabs than their culinary appeal!

Ciao Andrea!
I am quite happy to have you commenting. Gràzzie!

Hi Lady Divine who turned 26,
Thank you! You are too kind.
I am glad the effort I put in for the picture was worth it.

Stu said...

I admire your dedication.

Leeches....................yuk.

Robby said...

Very cool stuff as usual.

Gallicissa said...

Hi Stu,
Leech socks help to prevent bites to legs, which are mostly vulnerable for bottom-up attacks. I also wear wellies with them, so they aren't too much of a worry for me now. I do not use insect repellents, but they too help.

Hi Robby,
Thanks a lot - it is always nice to hear from you!

mboi said...

Hi Amila, I have a funny story about these freshwater crabs. When i was about 8 years old i used to go and catch these guys from a mountain stream near our house in kadugannawa. After hearing about biologists discovering new species and getting them named after them, I thought maybe these crabs were unknown to science and get them named after me. Well guess what, a few years ago i was reading that western ghats-sri lanka biodiversity paper and couldn't help notice a freshwater crab that was described from kadugannawa.

now the sad part, after going back to sri lanka last year, i went to look for the stream i used to catch these guys at, someone had bulldozed the entire block of land where the water used to flow and built a house right on top. what used to be a wet marshy area had completely dried up, i could find a single crab.

Gallicissa said...

Hi mboi,
They started naming crabs for scientists and after running out with names they turned to collectors....Perbrinckia gabadagei was named to in honour of my friend Dinesh Gabadage - who was "the collector of the type series...".

Therefore, it's a shame you couldn't get a crab named after yourself :)

Very sorry to hear the loss of that patch and crabs. Thanks for your comment!

Amila Kanchana said...

How little one knows! Never knew of a tree climbing critter. Thanks Amila!

Gallicissa said...

No problem Amila!

Amila Kanchana said...

Hi Amila,could you pleas help me to identify the crabs that I've videoed and uploaded here and here ?

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