Today I found out clownfish are all born males and the dominant male of a group will turn female when the female of that group dies.

Clownfish, also known as anemonefish, are sequential hermaphrodites that first develop into males. In fact, they live in regimented schools made up of all males and just one female- the lone female being the dominant and generally the largest fish in a given group.  The second in command of the school is usually the largest, and most aggressive, male of the group. Due to his dominance of the other males, he’ll tend to ensure they stay small via taking the best food opportunities for himself and the like.  Besides the perks of more food, he’s also the only one that gets to breed with the female.

During breeding, the female will lay sometimes thousands of eggs, depending on the species and her size, usually on a pre-cleaned rock or coral close to the anemone they live in. After the eggs are laid, the male will go along and fertilize them.

From here, if a given egg is poor quality, whether damaged (i.e. Nemo) or it becomes infected by a fungus or the like, the male will typically eat the egg. Likewise, if the egg is infertile, or if the clownfish is an inexperienced breeder or stressed, he may also simply eat the eggs.

Otherwise, the male, and rarely the female, will guard the fish for the 6-10 days it takes to hatch, as well as regularly fan them, which increases the chances of the eggs successfully developing. After the eggs hatch, the parent’s job is done and they have nothing to do with the baby fish, who ascend to the surface and feed on plankton until maturation.

If the female dies, the dominate male will get first choice of food and begin to gain weight, ultimately becoming female.  She will then choose a breeding partner among the available males, which is usually the largest and most aggressive male available. At the same time, everyone else moves up one in the pecking order.  The two will pair off and breed together until something happens to one or the other, at which point the cycle begins again.

It is thought that this peculiar ability to change sex developed because, unlike many other types of fish, clownfish almost never stray very far away from their homes in sea anemones, which they form a symbiotic relationship with. Thus, if not for their ability to adapt to potentially limited mating partners in their local area, they may not get a chance to breed at all.

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Bonus Facts:

• Other sea life that are known to change their gender include wrasses, which is a type of fish, and moray eels.  Unlike clownfish, wrasses switch from female to male with the largest female switching to male and taking over a group of females.
• As mentioned, clownfish form a symbiotic relationship with the sea anemone they live in.  The clownfish eat various small invertebrates and algae that could harm the anemone.  Their feces also serves to help fertilize the anemone.  The sea anemone offers a great deal of protection for the clownfish from predators.  The clownfish also gets food in the way of scraps from the anemone’s food. The clownfish return the favor by using their bright coloring to lure fish into the anemone, which are then killed by the anemone’s poison and eaten, with the scraps going to the clownfish.  Finally, it is thought that the clownfish’s frequent activity in and around the anemone provides better water circulation, which helps the health of the anemone.
• It isn’t exactly known how the clownfish survive the anemone’s very potent poison.  It is thought that the mucus coating their skin must be made up of some sort of sugar compound, rather than of proteins, which perhaps makes it so the anemone doesn’t recognize the clownfish as a potential food source and so doesn’t sting them.  Clownfish are also somewhat resistant to the toxins in the anemone, but not wholly.  When directly exposed to the toxins, when it penetrates their mucus layer, they will die like any other fish.
• Clownfish live in warmer water regions in the Indian and Pacific oceans.  They can particularly be found in the Great Barrier Reef and in the Red Sea.
• Because clownfish breed so easily in captivity and exhibit near constant, quirky activity while awake, they have become extremely popular aquarium fish.
• Occasionally, humans can also change sex naturally, at least in terms of changing external body parts.  In the vast majority of these cases where the change occurs naturally, a human born with female genitalia, will turn male after puberty.  This is usually due to a 5-alpha-reductase deficiency (5alpha-RD-2) or 17-beta-hydroxysteroid dehydrogenase deficiency (17beta-HSD-3).  Even more rarely, a person born with male parts will naturally turn female after puberty.  How this occurs from a genetic standpoint isn’t currently well understood.  It can also occasionally happen that a male or female human can be born with the opposite body parts and general physical appearance than their chromosome pairs indicate.