Role of Capillary Forces on the Integrity of Sand-bubbler Crab Boluses

Sand bubbler crabs (Scopimera and Dotilla family) are curious burrowers known to decorate their burrows with curious galaxy patterns using near spherical balls made of sand. During high tide these crabs bury themselves in their cozy burrows and during low tides they come out of their hiding to forage on the microbial matter laden sand grains. The method to forage is quite curious too. Sand bubbler crabs scrape a thin layer of sand and put it in their frontal buccal cavity and continue to scrape and grow out a ball of sand. The microorganic material is filtered using spoon shaped setae in the mouth cavity of the crab and the wet sand grains are continuously pushed out of the mouth leading to the growth of the sand ball attached to the mouth. The ball is securely held using a pair of dangling antennae. These near spherical sand balls once grown to a certain size are eventually thrown off and pushed back using a series of back kicks. This exercise is performed continuously resulting in the formation of a galaxy structure around the burrow. We note that these spherical balls scale with the size of the crab i.e. the size of the buccal cavity and the antennae length. In this study, we explore the role of sand grain size, fraction of water in the sand and capillary action in the formation of these near spherical balls and the curious way of cleaning the particles off any unwanted microscale impurities.