Why are there small balls of sand on the beach?

The small balls of sand commonly found on beaches are primarily created by the feeding activities of sand bubbler crabs.

The Architects: Sand Bubbler Crabs

These distinctive sand formations are the tell-tale sign of sand bubbler crabs, a group of animals that can belong to genera such as Scopimera or Dotilla. These crabs are small, semi-terrestrial creatures that inhabit sandy intertidal zones, particularly in tropical and subtropical regions worldwide. They are incredibly efficient at processing sand, leaving behind unique patterns on the beach surface as the tide recedes.

How Sand Balls Are Formed

The creation of these characteristic sand balls is an integral part of the sand bubbler crab's survival and feeding strategy. Here’s a breakdown of the process:

• Sifting for Food: Sand bubbler crabs feed on the microscopic organic matter, detritus, and diatoms found within the sand. They scoop up batches of sand and meticulously sift through it using specialized mouthparts.
• Extracting Nutrients: As they filter the sand, they extract the tiny food particles, separating them from the larger, inedible sand grains.
• Discarding Waste: After a batch of sand has been processed and all the nutritional value extracted, the crabs cleverly discard the "cleaned" sand. To avoid re-sifting the same sand and to efficiently manage their workspace, they roll this spent sand into small, spherical pellets.
• Characteristic Size: These discarded balls of sand are typically uniform in size, about half a centimetre (approximately 0.5 cm) across. This consistent size creates the neat, patterned appearance seen on many sandy beaches.

This fascinating behavior allows the crabs to efficiently feed while simultaneously creating the distinctive landscape of countless sand balls.

Key Characteristics of Sand Bubbler Crab Activity