We tend to think that sandy beaches are more or less sterile, but it isn't true. Life exists everywhere. During our walks along the gorgeous beach at Riviera Nayarit at low tide, we found Keyhole sand dollars. Those we have here on Vancouver Island have complete discs, with no holes, but those in Mexico have 5 slits through the disc. They move about slowly
just below the surface of the sand, and I could watch them sieving the sand through the slits
as they filtered it and the water for food particles. The trails they left were often quite amusing, because they had a little 'owl face' at the end, as you can see in some of the photos.
Or sometimes they looked a bit like an animal footprint. Their name is Mellita (not spelled like the coffee), and there are thousands of them, all along the shoreline.