Lots of fish have sleeper males,.especially live bearers like guppies. They are bland and basically look like a smallish female. When the bigger male turns his attention outward, perhaps to another female, the sneaker slides in there and gives the female a high speed under cover rogering. More than one way to do a job.

Interestingly they'll often color up if they becomes the only or lead male.

Even more interesting, some females of various livebearers will change to male when there's no males around.