Actually, quite a lot of species
do physically change sex. Many fish can do it, amphibians, some reptiles, insects, many molluscs are actually both male and female at once. I'm not suggesting that this is the case with scrabs, just that it is possible.
By subversive I mean smaller males that do not fight the dominant scrab. At least until the time is right...
Some animals, specifically a type of batfish, have three male castes in the shoal. A dominant one who is the largest and most obvious, who mates with all the females and doesn't allow the males to do so. There are males who look like females, so the dominant male does not notice them, who secretly mate with the true females, but run the risk of the alpha male trying it on with them. And there is the theif, a small, quick male who darts in, spawns, then retreats before the alpha male can stop them.
