Interesting picture from Jon (see comments) of a Common Frog transporting both Common Duckweed and Water Fern.
Like all the best things, its obvious this could happen once you see it happening.
Go to:
http://pondat38nurseryrd.wordpress.com/2010/09/02/old-photos-of-frogs-and-duckweed-plus-azolla/
And the original post is here.
