Fairly recently, in the huge scale of time when tiny transparent young eels ( glass eels or elvers) have been quietly infusing our estuaries, swimming (the comparison is unavoidable) sperm-like each spring up every river, into every lake and ditch and wetland, feeding otters and bitterns, bringing an injection of fertility from the far Sargasso, it turned out to have been a bad plan for an adult eel to look a bit like a penis.