Human beings are big on balls. From footballs to meatballs we can’t get enough, and neither, it turns out, can sand bubbler crabs. Across beaches in the tropical Indo-Pacific where these curious critters are common, it’s no surprise to locals to walk down at low tide and find the sand strewn with tiny sand balls. Turfed out in their millions, these tiny balls are the result of the sand bubbler crabs’ unique feeding technique, and the process behind it is facilitated by adaptations that have rendered these crabs expert sand sifting machines.
Sand bubbler crabs are from the genera Scopimera and Dotilla in the family Dotillidae, a family of crabs that contains 59 species. Sand bubblers are notable for their unique mouthparts, which have evolved to make them expert sand sifters. They gather sand from inside a small burrow and run it through their mouth, using water from their body to sculpt the rejected sand into a ball. As the sand is processed, the crab filters out small bits of organic matter and tiny organisms that live in the sand. The video below demonstrates some of the microscopic but stunning microorganisms on the sand bubbler’s menu, with a commentary well worth switching your sound on for.
Making sand balls is easier said than done when you’re a delicious crab dashing across a beach frequented by hungry birds. While their calcified exoskeleton is well camouflaged, they can still be spotted when rolling their balls and so an easy exit strategy is vital to survival. As such, they work in a pattern that can resemble a star or a spiral, with a clear runway back to their central burrows should any suspect characters approach. It’s yet another example of how evolution results in patterns in nature that look as if they’ve been crafted by an artist but are actually just happy accidents.
Forming spitballs of sand is thirsty work, but the sand bubblers have evolved specialized legs that help them maintain their water levels and breathe while their mouthparts are busy. The tops of their legs are kitted out with “gas windows” that facilitate aerial respiration, while hair legs suck water out of the sand as the crab walks, keeping it plenty stocked with the wet stuff for sculpting sand balls.
Sand bubbler crabs hustle for the length of low tide, but as the water starts to come back in again they must find a suitable spot to hunker down. Here, they dig out a shallow burrow over which they build a dome-like structure. The hidey-hole, which eventually resembles a sandy igloo, is sealed with wet sand so that the crab remains in a bubble of air during high tide. Once the tide goes back out again, the crabs emerge from their domes and it’s time to get rolling again.