Female Amazon mollies need to mate with a male to reproduce. On the face of it, that’s not a terribly surprising statement – it’s on par with “Humans breathe air.” But the tricky secret of these strange fish lies in the details.

The phrase “sexual parasite” might conjure up memories of unpleasant exes, or the irritating hitchhikers that rely on intimate liaisons to jump ship between hosts, but the term truly signifies unusual, ephemeral species that exploit other individuals to propagate their own genetic legacy. Beyond that basic principle, though, the details of sexual parasites are a bit squishy. In species with distinct males and females, there can be sexual parasites of either sex. Whereas Amazon mollies block the genetic contribution