When it comes to reproduction, most fish are external fertilizers, crop dusting eggs in a cloud of sperm. But swordtails (Xiphophorus) aren’t like most fish. These fish fertilize eggs internally and “give birth” to live young. To help this whole operation, males have evolved external genitalia for transferring sperm—a…
Source: Gizmodo – For Fish Penises, Bigger Isn’t Always Better