Such a strange, elegant creature—the sea star! The mystery: its tiny larva, floating in the plankton, has bilateral symmetry, but its adult form is radially symmetrical—a radical transformation! How could that happen? Donald Williamson, an English zoologist, suggested a theory of “larval transfer”, saying that in the past, genes for body shape and for life cycle were exchanged when hybridization occurred between marine species with the two distinct body plans. This would be possible due to external fertilization, which, in the ocean means millions of sperm and eggs of different species are floating around and bumping into each other. Williamson demonstrated in the lab that cross-fertilization between species is possible in those circumstances!
See Ryan, Frank, “The Mystery of Metamorphosis—A Scientific Detective Story”, White River Junction, VT, Chelsea Green Publishing, 2011, print