While diving around the dives site in Koh Tao, whenever your swimming over sand its quite common to see small burrows in the sand, these are usually the home to Gobies. Gobies are part of the largest families of fish, with more than 2000 species.
Usually in these burrows they live with a shrimp, this is called a symbiotic relationship. This relationship works by the shrimp maintaining a burrow where the shrimp and goby both live. The shrimp has poor eyesight so the Goby looks out for any danger at the entrance of the burrow, thats why they are sometimes called watchmen or prawn gobies. The goby alerts the shrimp to any potential danger by swimming past it into the burrow.
Each party gains from this relationship; the shrimp gets a warning of approaching danger, and the goby gets a safe home and a place to lay its eggs.
So the next time your swimming over sand have a look for some of the smaller things to see.