Walking a few days ago on the beach in Vung Tau (South Vietnam) it just happened to walk by a stack of small sand balls which just laid down on the beach in front of me. Looking more carefully I had noticed that next to each agglomeration of sand droplets there is a small hole and after waiting a few minutes, some tiny creature has shown up and gave me a smile.
This was the famous sand crab which burrows holes in the sand and feds itself with phytoplankton and small organic debris brought by the waves to the beach. The eating procedure consists in just catching the organic particles with their tentackles, most of them already present between the sand grains. The crabs are of different sizes growing up to 35 mm long and 25 mm wide, building nice sand patterns on the beach:
Sand crabs are used as bait by fishermen. Some sand crabs are also used by humans to monitor the concentrations of domoic acid in water, a toxin produced by a type of phytoplankton. When sand crabs eat the toxic substance, they become toxic for the birds and fishes that consume them. By measuring the concentration of domoic acid in crabs’ flesh one can estimate the toxin level in the water. The sand crabs reproduce by eggs (about 45.000) which are carried by the female crab on her abdomen for about 30 days.
Some more info on sand crabs, also known as mole crabs:
- digging kinematics (legs coordination) of sand crabs: Faulkes and Paul, 1997
- description, natural history and monitoring of Pacific mole crabs
