Photo: Caroline Armon

The arrow points to a mud circle. How did it get there?

A mud circle is the muddy area on the water's surface left by a whale that is "snacking." In the nursery lagoons of Mexico, the bottom sediment has very little whale food. But the water is rich in plankton and algae. One type of algae has a cottonlike texture. A scientist named Dr. Swartz suggests this algae gets stuck on a whales' baleen, so maybe this whale was taking a mouthful of sand and sediment to clean its baleen! (Whales gulp mouthfuls of sand and sediment from the bottom and squirt it out through their baleen. Any food gets trapped and swallowed, but the sand could also "scrub" the baleen and get the sticky algae off.)