The evolution of functional specialization within aquatic animals is discussed in this Casey Dunn speech. The Assistant Professor of Brown University talks about the evolving biodiversity within colonial organism -- animals that produce bodies asexually but have those bodies remain attached to the original organism. An example of a colonial organism that many people are familiar with is coral reef.
While the majority of colonial organisms produce bodies that perform the same tasks, Siphonophores are found to create separate bodies that are genetically identical but carry out different tasks. Humans have different organs for a variety of distinct functions but all of those organs are part of a central body. Siphonophores create separate bodies for separate functions -- one body is used for movement, another for digestion, etc.
This division of labor between cells is changing the way scientists approach genetic modification and the response to evolutionary pressure.