Well since merpeople are considered fish, I'd assume that they would reproduce in the same way that fish do, thus, the uke wouldn't need a penis since (according to a quick google search, don't quote me) females usually release eggs into the open and males release the sperm which fertilise the eggs by themselves rather than direct insertation which would require penile penetration.
Just what im assuming tho!

Okay but if his mermussy is up there where is his penar....genuine question