The phenomenon of neuroplasticity is perhaps no better exemplified than in the anemonefishes and many other coral reef marine fishes which change sex depending on the outcome of territorial contests. For the past 10 years, we have been maintaining a colony of Amphiprion ocellaris, a species of anemonefish easily kept and bred in the laboratory, to study sex differences in the brain and behavior, and the mechanisms by which the brain and behavior become feminized. We have discovered that males are the primary care-givers of the eggs, that peptide neurotransmitters oxytocin and arginine vasotocin are crucially involved in shifting the males behavior between aggressively defending the nest from predators versus providing nurturing direct care to the eggs by fanning and nipping them. Most recently we discovered that the non-breeders and male anemonefish will step-father eggs that are not their own, if given the opportunity. Hence, anemonefish males have a fathering instinct, that needs only the egg stimulus and the opportunity for it to be displayed. Recently, we published our first sex change paper, which documents the cellular changes in the preoptic area (part of the brain that controls the gonads and reproductive behavior) and gonads during sex change from male to female. We discovered that as males change sex into females, their brains change first, sometimes long before their gonads. Hence, anemonefish can exist with a female brain but male-like gonads and circulating sex steroids , questioning the way we define sex based on the gonads only.