Bluegill sunfish have pretty bizarre life histories. Males exhibit three completely different types, with different behaviors and appearances. These types seem to be differentiated on environment, rather than genetics.
Some male bluegills grow to be large, and start building nests. They defend territories and the females will visit the nests of various males, and decide whether or not to deposit eggs there. If they do, the males will fertilize them and then proceed to care for the infants. These males are light in body color with dark yellow-orange breasts.
Other males do not find a territory and build a nest - instead they use a 'sneaking' tactic. They try to hide from the highly territorial nesters and dash in to fertilize eggs laid in another males nest. If successful, they manage to completely bypass all of the energy required to build and defend a nest, and then care for the young. These males are much smaller and are light in body color.
The final type of males actually mimic females. Some data suggests that sneakers become mimics as they age and grow larger. These types pretend to be females, and use this tactic to approach nests without being attacked by the nesters. As males often accept eggs from more than one female, they then quickly fertilize as many eggs as they can and leave.
Believe it or not, tactics like these are not unique in the animal kingdom. Many animals mimic another sex in order to gain a reproductive advantage - some actually physically change their sex. To learn more about this, see here: http://bit.ly/VnFcfL