I just read an amazing web page about a fish that suckles its young while they are still inside the female's body.
The European Eelpout, Zoarces viviparus, is the first fish known to do this. Many fishes have internal fertilization and are live bearers. Guppies, the Eastern Gambusia (a pest species in south-eastern Australia - see images) and many sharks spring to mind. None of them, however, suckle their young with nutrients that flow from an ovarian follicle.
The 'gestation period' of Zoarces viviparus is about 6 months.
The remarkable discovery was made by a team of researchers led by Professor Peter Skov of the University of Copenhagen. They have published the findings in the Journal of Experimental Marine Biology and Ecology. The BBC Earth News site gives a summary of the findings.