Thursday, 14 April 2016

Sand bubbler Crabs and Sand Galaxies

Sand bubbler crab gets its name as it produces tiny spherical sand balls like solid bubbles of sand. It produces these sand balls from the sand that it 'eats'. It actually eats microscopic food within the soft sand on a sea beach. It then discards the sand balls towards its back, two to three balls every second. And it does not push all the balls in one direction but adjusts its position continuously. The result is an amazing pattern of sand balls arranged artistically. See the picture.
Since there are hundreds of these tiny crabs (just one cm wide) working simultaneously, all the patterns together appear like a galaxy. Since this galaxy is in sand and not in space, it is called a 'sand galaxy'. We had explained this behaviour of sand bubbler crabs in our earlier story in December 2014 (click here to browse). Here is one part of the sand galaxy for you.
Jacob Abraham, a regular reader of the blog, has sent the following picture of a real galaxy taken through Hubble's telescope. Does it not resemble our sand galaxy? 

While some patterns appear similar, we could see many new patterns. The crab goes into the burrow as high tide comes in. It remains there till the tide recedes. It then starts to forage for food digging the sand and discarding the remains in the form of sand balls. We know that the tides change every six hours. Therefore, these tiny crabs work tirelessly every 12 hours. And in the bargain, create artistic patterns like the one below.
We also saw patterns having radial spokes of almost equal length! Amazing, how much intelligence this tiny one-centimetre creature has? See the picture.
 During this visit to Suryalanka, we could not spot any spiral pattern made by the crabs. But in Dec 2014, we had spotted spirals galore.  See a file photo below. Does it mean that the patterns made by the crabs vary with the season? Or, the temperature of the water? Or, the direction of the Sun? Would be grateful to any reader who can throw some light on this aspect.


Look at the pattern below-- like the headgear of an ancient Chieftain ! See the arcs and the symmetry.
 How does our tiny craftsman look like? It is so agile and alert that it is difficult to capture on the camera. Here is one picture. See nature's camouflage to deceive the predators.



Depending on the pattern made by the crab, it's burrow lies either in the centre or in one corner. See another pattern below with the burrow in the centre.
 The next pattern looked like the famous Palm-Jumeirah property in Dubai which has been created on land reclaimed from the sea. Notice the burrow in one corner of the pattern.
So much for the tiny crabs. We also saw its elder cousin with similar camouflage and agility. But the burrow is larger and the discarded sand is not in the shape of balls but as sand pellets. It makes it burrow on high ground. The burrows can be seen every morning. See the pictures below.
 Sand pellets produced by crabs larger than the sand bubbler crabs.


While observing the larger crabs, we saw them playing with the surf- much the same way we humans do. We captured this crabs-and-surf game on video. A short 30 seconds clip for you.



If you are unable to see the video, click here to browse it on youtube.

The sand bubbler crabs are found on the tropical beaches of India, Malaysia and Australia. But within India, we haven't seen clips or blogs of sightings on the west coast. Could a reader substantiate this please?

Hope you saw that sea beaches have so much to offer beyond the surf and the sand. Next time you are on a beach, look out for these tiny creatures and their sand galaxies.

Thanks for browsing

   Harsh-the-Ghumakkad/ 14th April 2016

Postscript: 21st April 2016--The mystery of creating varying patterns with season remains. However, a reader M Susree pursuing PhD at IIT, Hyderabad has sent an interesting link to Aquatic Ecology-- a scholarly journal. The article explains the Population Biology and Breeding Period of the Sand-Bubbler Crab from Southern Mozambique. So, see where did we find a match in this tiny creature? Thanks Susree.