In the second Star Trek movie, The Wrath of Khan, the character Khan uses Ceti eel larvae to subvert the self control of two people, Terrell and Chekhov. Khan gets the larvae from an adult eel that he has in a cage within his domicile. Why did Khan have these eels? Did Khan and his crew eat them, or was Khan keeping them to use in the event that someone would eventually return to the planet? Or, most unlikely, was Khan using them on his own people?
Is there a canon explanation for this?