Movie science: Finding Nemo and the gender-swapping abilities of clownfish

Tom Llewellyn April 14, 2020
Movie science: Finding Nemo and the gender-swapping abilities of clownfish

We all remember the Disney classic Finding Nemo as a wholesome animated comedy about a father’s perilous journey across the open ocean in the hope of finding his missing son.

With an all-star cast and an overcoming-the-odds story of family reunion in the deep blue, this movie was an instant classic for millions of people across the world… so let’s ruin that with some science!

Clownfish have the incredible ability to change genders based on their social environment and, by explaining how this change happens, we’re going to see why Finding Nemo isn’t as wholesome as you think.

The film opens with two clownfish, Coral and Marlin, discussing their new sea anemone home and the hundreds of eggs they have laid in a nearby hole. When a barracuda appears from nowhere, instead of hiding in the safety of the anemone Coral dives for the eggs and the predator strikes, knocking Marlin out cold in the process. When he awakes, he finds Coral, the barracuda and all the eggs gone, bar one.

Finding Nemo cr: Disney

Several years pass and we’re introduced to Nemo, a young clownfish who’s dangerously curious of the world outside his reef. One day Nemo is captured by a diver and Marlin sets out on a daring voyage across the sea. With help from a range of hilarious characters, Marlin navigates the threats of the open ocean and is eventually reunited with his son.

‘In the end, the storyline for Finding Nemo was obviously entertaining for children but, as a fisheries scientist, I must admit I find the biologically accurate storyline a bit more entertaining’

Patrick Cooney, marine biologist

The first thing we need to know is all clownfish (Amphiprioninae), also known as anemonefish, are born as sequential hermaphrodites that first develop into males. This means while they are males during early development, they also carry the capacity to produce female reproductive organs.

Specifically, clownfish are protandrous sequential hermaphrodites, changing from male to female, whereas other fish such as the California sheephead (Semicossyphus pulcher) are protogynous sequential hermaphrodites, changing from female to male.

Arguably the most well-known fact about clownfish is they live within the tentacles of highly toxic sea anemones, with whom they have a symbiotic relationship. The fish are immune to the anemone’s venom, which provides protection from predators in return for helping their host breathe by improving water flow between its tentacles.

Anemones can often host a large school of clownfish, which consists of a group of sexually immature males, one breeding male and a single, dominant female. Females are the anemone’s defenders, being noticeably larger and more aggressive than the breeding male, who is himself larger and more aggressive than the other males.

During reproduction, females can lay thousands of eggs on pre-cleaned rock or coral, which the male will fertilise by producing a large cloud of sperm. From this point on, the female has little to do with the offspring, returning to her role as anemone security guard while the male becomes a quality-assurance checker.

If an egg is damaged, infected or infertile, the male will often eat the egg to recover nutrients he lost in reproduction. Similarly, the male may also eat some of the eggs if he is an inexperienced parent or particularly stressed.

The male will guard the eggs for between six and ten days, regularly fanning them to increase their chance of developing by improving the supply of oxygenated water. After the eggs hatch, the male also loses interest in his offspring, which float to the surface to feed on plankton until they mature.

The storyline issues for Disney start when we learn about the amazing gender-swapping abilities and social structure of clownfish. Since the fish never stray far from their anemone because of predators, if the alpha female dies it would be incredibly difficult for the males to find another female to breed with.

Instead of putting the survival of the group at risk, the dominant male would transform into the new alpha female by maturing the ovarian reproductive tissues he had been carrying since birth. At the same time, each immature male moves up the hierarchy, meaning the cycle can repeat continuously.

‘Because of the sex change, the same individual can have an opportunity to breed as a male and a female’

Dr Ricardo Beldade, marine biologist

It’s unclear exactly what the signals are that start this transformative process, whether behavioural or olfactory, but according to Justin Rhodes, professor at the University of Illinois, this change occurs through the “brain interpreting the social [environment] change” and then “encouraging one of the tissues to develop”.

Unfortunately, understanding the social behaviour and changing physiology of clownfish drastically alters how we view the once-wholesome Disney storyline of Finding Nemo.

As fish biologist Patrick Cooney explains: “Nemo hatches as an undifferentiated hermaphrodite while his father transforms into a female (now that his female mate is dead).

“Since Nemo is the only other clownfish around he becomes a male and mates with his father, who is now a female. Should his father die, Nemo would change into a female and mate with another male.”

Even if the events of the movie were still to take place, evolutionary biologist Dr Suzanne Mills says because there are a number of weeks between the fish-knapping and family reunion, “when Nemo finally gets back to his anemone at the end of the film, he’s actually meeting his mum”.

On the other hand, there’s also an argument Nemo wouldn’t have survived long enough to experience his ocean adventure since his egg was damaged during the barracuda attack. In truth, Marlin was likely to have eaten Nemo to recover nutrients required for his forthcoming gender swap.

Either way, that would have been an early – and gruesome – ending for Finding Nemo and it certainly wouldn’t have become one of the most successful animated films of the 21st century for Disney.

The ability of clownfish to change genders based on social structure is amazing but, considering the limited parent-offspring bond in this species, it also explains why poor Marlin is so determined to find his missing son.

In conclusion, if Nemo doesn’t want to start a new family with his own father, he should set off in the opposite direction to his home and “just keep swimming”.

Finding Nemo is available now on Disney+

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Tom Llewellyn is the Head of World Content, Anime, and Comics at HITC Entertainment and also helps oversee all things deemed ‘nerd culture’ at Forever Geek. After gaining a BSc degree in Animal Behaviour, Tom continued his education with a Master’s degree in Science Communication (MSc) and post-graduate research; with his own independent project being published in the Journal of Zoological and Botanical Gardens in 2021. Since joining GRV Media in early 2020, Tom has amassed over 48 million views on his content and has conducted interviews with various actors and filmmakers from the wonderful world of anime.