Can Clownfish Change Sex? The Science Behind Nemo

How clownfish sex change works

Every clownfish group lives on one sea anemone and follows a strict pecking order based on size. At the top is the breeding female, the largest fish. Second is the breeding male, her only mate. Below them are usually a few smaller non-breeders, which cannot reproduce at all.

The system only changes when someone at the top is lost. If the female dies or disappears, the breeding male changes sex and becomes the new female. The largest non-breeder then matures into the new breeding male, and every fish below moves up one place. If the male is lost instead, the female simply pairs with the largest non-breeder, which develops into a breeding male.

This was demonstrated in experiments in the Red Sea in the 1970s, when the biologists H. Fricke and S. Fricke removed females from groups of two-band anemonefish and watched their partners turn into females. The same pattern has since been confirmed in the ocellaris clownfish, the orange clownfish and other species.

Why the biggest clownfish is the female

The reason is size and eggs. A bigger female can produce many more eggs, so being large pays off far more for a female than for a male. Because each anemone holds only one breeding pair, a male never competes with crowds of rivals for mates, and his success depends little on his size.

Biologists call this the size-advantage model. When a fish is small, it does best as a male; when it becomes the biggest fish in the group, it does best as a female. A clownfish female can lay hundreds of eggs, up to about 1,000, in each clutch, and the number rises as she grows.

What happens inside the fish

Young clownfish are immature, and their gonads hold both ovarian and testicular tissue. When a fish becomes the breeding male, the testicular tissue grows and pushes the ovarian tissue aside. When a male turns female, the opposite happens: the ovarian tissue expands and the male tissue is absorbed.

The change begins in the brain and with behavior. Soon after the old female disappears, the male starts acting dominant and grows faster. Hormone levels then shift, with female hormones such as estrogen rising, and the ovaries develop over the following months. The whole transition can take months or longer, and once it is complete it cannot be reversed.

The waiting line in every anemone

Non-breeding clownfish are essentially waiting in line for a chance to breed. They cannot join a breeding pair elsewhere, because nearby anemones are already occupied, and they are too small to challenge the fish above them. A fish moves up only when a fish above it dies.

The female and male keep control through chasing, displays and sounds. Researchers have found that lower-ranked fish actually regulate their own growth to avoid trouble. In orange clownfish, each fish is roughly a quarter larger than the one ranked just below it. A subordinate that grows too close in size to the fish above risks being chased out of the anemone, which on a reef usually means being eaten.

What Finding Nemo got wrong

The 2003 film Finding Nemo is a story about a clownfish father and son, and it bends the biology in several ways:

• Marlin would have changed sex. When Nemo’s mother died, Marlin, as the breeding male, would have become a female.
• Nemo would not have grown up with his father. Clownfish hatch as tiny larvae that drift away into the open sea for about 8–12 days before settling on a reef, usually in an anemone with unrelated fish. Clownfish fathers do guard the eggs carefully, but they provide no care after hatching.
• The setting is off. Nemo and Marlin look like ocellaris clownfish, which do not naturally live on the Great Barrier Reef, where the film takes place. The very similar orange clownfish lives there. How the different species are spread across reefs is covered in Where do clownfish live?

The film did get some things right, including the close bond between clownfish and their anemone, which is explained in Why don’t clownfish get stung by anemones?

Other fish that change sex

Clownfish are not unusual among fish. Hundreds of fish species can change sex, but most go in the opposite direction, from female to male. This is called protogyny. Many wrasses, parrotfish and groupers start out as females, and the largest fish become males that defend territories and mate with several females. In those species a big male gains the most, because he can monopolize many mates.

A few small fish, such as some coral gobies, can switch in both directions depending on who else is around. Clownfish remain the best-known example of the male-to-female route, largely thanks to their starring role in film.