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Homeowners have been known to have gardens on rooftops.  Some turtles had that idea long before people put out those plant pots.

Some green algae have found a turtle’s outer shell, its carapace, is just fine for living and staying wet most of the time.  The alga is an epiphyte; it probably gets no nutrients directly from the turtle, but just hangs out, reproduces, and grows there.  It is impossible for the turtle to reach around and feed on the greens.

Those algae who are not floaters, like these on a snapper’s back, have special holding cells to keep them attached.  Algae have no r...