Why does the Sand bubbler crab form tiny sand balls?

Even though I go to the beach quite often, until recently I have never realized that on some of Krabi's beaches are tiny cute crabs that seem to work really hard. What I discovered during low tide were mysterious patterns, created from sand balls, that made me wonder why they have been formed over and over again.

After swimming in the ocean at Klong Muang beach, Krabi, I went back home, still wondering about these sand patterns. Hence, I started searching for details about this phenomenon on the internet and found a quite interesting explanation indeed .

These tiny animals are called Sand bubbler crabs (Pu Lom in Thai). Beside the fact that they can run very fast, they also form these tiny sand balls around their hole. The ball's size is as big as their body. Each ball takes about 10 seconds or even less to form. What in fact interested me most is an answer to the question: "Why do they do this?"

The answer is simple. It is all about food.
High tide brings organic substances onto the beach. At low tide, these crabs will come out of their holes, looking for food.
They use their claws to take the sand through their mouth and after having eaten all organic materials from it, they will release the sand again. During this process, however, they will shape the sand into tiny balls. These crabs will form about 360 balls per hour, and these balls are not just being scattered around. Amazing patterns are being created that soon, after 3 or 4 hours, cover most of the beach.

So, why do they form these patterns? In this way they make sure that they don't come back and eat the sand again that has already been stripped of organic material. Smart, isn't it?

Let's take a look their work!

@GeoVenture, @Tripbod