Sunday, November 30, 2014

Nature’s artistic patterns at Borneo sandy beaches


MAY I ask the reader for the answers to two simple questions? Question 1: What have the following words in common: hermit, fiddler and bubbler? Question 2: What have Bako, Damai, Sematan and Tanjung Aru in common? Whilst there are no prizes for the answers, you will most likely already have guessed crabs and sandy beaches.

Hermit crabs are usually seen in rocky foreshore pools and fiddler crabs on many a mudflat but the bubbler crab (Scopimera globosa) is only found on the sandy beaches mentioned earlier, where it is the most artistic of all crab species.

Its intricate artistic patterns are only found at low tide.

It was first described in detail by a Dutch zoologist Wilhem de Haan in 1835.

For many years, I have been fascinated by an intricate piece of Indonesian batik which hangs on my sitting room wall – given to me by a former student.

I have marvelled at the little gold painted spots on the outline of figures on this folk artwork.

It was much later, in 1992, when strolling along the public beach at low tide at Tanjung Aru in Kota Kinabalu that I observed small rounded pellets of sand aligned in an orderly fashion and not dissimilar from the detail of my Indonesian batik.

I have more recently seen similar sand pellets on the Nature’s artistic patterns By Alan Rogers beaches of Sematan and Damai Central and on other sandy beaches around the Indo-Pacific Ocean.

Amongst these minute sand ball pellets are two to three small holes in the sand from which the lines of sand balls radiate in arcs.

Each pattern is uniquely created on the damp sand by an individual artist, but how did this pattern emerge between high and low tides on the beach? The minute, shy, Scopimera crab is not unlike the street artist Banksie, who has painted talented works of art on blank walls in England but has never been seen in action! Scuttling out of their holes, these bubbler crabs specialise in sieving microscopic organisms, such as stranded plankton, trapped on the surfaces of sand grains.

Scooping up the grains with their small claws into their mouths they only ingest organic material and then spit out the sand, as residue to create the sand balls that we see.

When threatened by the squelching of human feet or the shadows of humans or by preying seabirds, this crab scuttles very quickly into its burrow – its air-raid shelter.

Measuring from one to 1.5 centimetres in width across its upper exoskeleton, with eyes on very short stalks to be lowered into its body whilst scurrying into its burrow, its upper meral joints (equivalent to our humerus bones leading down to our elbows) have membranous disks.

These disks are breathing organisms, used in gas exchange, when the crab is in its burrow or when on the sand surface to absorb atmospheric oxygen.

Zoologists frequently refer to these membranes as gas windows.

When the tide has ebbed, the crabs quickly emerge from their burrows, creating pathways or grooves as they eject indigestible sand in pellet form.

Almost chameleon-like in its ability to change colour, the bubbler crab adopts the same colour of the sand from which it feeds.

Continue reading (Incl. Pics) at: Nature’s artistic patterns at Borneo sandy beaches
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