Expunging a rival's sperm is crucial in the bid for fatherhood, and the male shark has his own way of acheiving this, The shark's sexual organ is double-barrelled. One tube squirts a powerful jet of seawater into the female to flush out sperm from previous matings, while the second introduces his own.
In this way, he ensures that the offspring will carry his genes. Unless, of course, another male intervenes and flushes away his sperm in turn.