Freaks of Nature: Clownfish

Making the switch from Marlin to Coral

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Welcome back to Freaks of Nature! This week we have a particularly innocent and inconspicuous freak: the clownfish.

Also known as anemonefish, the majority of clownfish species belong to the genus Amphiprion. Clownfish are common aquarium fish, and were made popular by the Pixar movie Finding Nemo.

Clownfish live in a mutually beneficial, close association with sea anemones. The two organisms help each other out in a number of ways.

Both organisms may leave around leftovers of food for the other to eat. This kind of arrangement may not work well for roommates, but clownfish and anemones are not too picky.

The clownfish are protected from their predators by living inside the anemone, which have toxic stingers. The clownfish are protected from the anemone’s stingers by a layer of mucus on their skin.

Anemones, in turn, are protected from their predators by the presence and actions of their resident clownfish. These fish can scare off animals, like turtles, who attempt to make a meal out of their home anemone.

Clownfish also help protect their anemone from infectious parasites.

All clownfish are sequential hermaphrodites which are born male. Sequential hermaphroditism is a process of sexual determination in which individuals change sexes at some point in their life.

Clownfish are protandrous hermaphrodites, meaning they transition from male to female.

Clownfish are monogamous, with a single breeding female and male pair per group. This monogamy may have developed due to the small living spaces available in anemones.

Along with the two breeding fish, several non-breeding juvenile males may reside within an anemone as well.

The fish residing in an anemone adhere to an aggressive hierarchy based on size. The large alpha female is aggressively dominant over the males. The beta fish, the breeding male, is, in turn, aggressive toward the smaller non-breeding males, which are all ranked based accordingly by size.

Many non-breeding males are unable to produce sperm. These juveniles are physically deterred from attempting to mate with the female. As well, the female’s dominance over the males prevents the formation of another female. This helps establish and maintain the hierarchy found within the groups.

This is where things get interesting! Upon the death or loss of the female, the breeding male will change sex and become the new breeding female.

A large non-breeding male will rapidly grow and assume the role of breeding beta male. In this way, non-breeding fish eventually inherit the territory possessed by the breeders upon their death. This future benefit most likely contributes to why non-breeders tolerate aggressive oppression from the breeding pair.

This inevitably brings us to our short discussion on Finding Nemo. What should have happened in everyone’s favourite animated sea tale not starring the Beatles?

Well, Nemo’s father Marlin, upon the death of his beloved wife Coral, became the most dominant fish in the anemone. As a result, Marlin should have turned into the resident breeding female.

Nemo, the only juvenile fish remaining in the anemone, may have eventually grown into the next breeding male, and Marlin’s next mate.

I, for one, am happy that Pixar decided not to go for scientific accuracy on this one.

Although they might not appear to be very unorthodox, the sequential hermaphroditism of clownfish is equal parts fascinating and weird.