The opening scene of the 2003 Pixar film establishes the premise for everything that follows. Marlin and his wife Coral are guarding their eggs inside a sea anemone. A barracuda arrives, eats Coral and almost all the eggs, and leaves Marlin as a widowed single father raising his one surviving son. The rest of the film is the story of what happens when Nemo is captured and Marlin has to cross the ocean to get him back. It is a beautiful film. It is also, on the biology of the actual fish it depicts, so completely wrong at the level of the underlying reproductive system that no viewer familiar with anemonefish research can watch the first ten minutes without wincing.
The species Nemo and Marlin are drawn from is Amphiprion ocellaris, sometimes called the false clown anemonefish. It is one of about thirty species of anemonefish, all of which share the same broad reproductive strategy. And that strategy has one feature the film does not acknowledge and could not easily have shown without making the plot considerably stranger. Every clownfish is born male. When it changes sex, it changes in one direction. The dominant female in a colony was, at an earlier stage of her life, the dominant male.
Which means that in the specific circumstance the film depicts, meaning the death of the breeding female in an established pair, the biological outcome would not have been a widowed father. It would have been Marlin turning into a female within a few months.
How the social system actually works
According to the foundational 1977 paper by Hans Fricke and Simone Fricke, published in Nature under the title “Monogamy and Sex Change by Aggressive Dominance in Coral Reef Fish”, anemonefish live in extremely small and rigidly hierarchical social groups. A single sea anemone typically hosts one breeding pair and a small number of subordinate non-breeders. The breeding pair consists of a large dominant female and a somewhat smaller breeding male. The subordinates are smaller still, and they do not breed at all. The whole hierarchy is enforced through aggressive dominance. The female bullies the male. The male bullies the largest subordinate. The largest subordinate bullies the next-largest. And so on down the line.
The dominance is not incidental to the reproductive system. It is what maintains it. The stress the female’s aggression imposes on the male suppresses his development into a female. The stress the male’s aggression imposes on the largest subordinate suppresses that subordinate’s development into a reproductively active male. If either of the two dominant fish is removed, the aggression stops, and the fish immediately below in the hierarchy is released from whatever developmental brake was being held on them.
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The Fricke and Fricke study documented this experimentally by removing the dominant female from established groups of the anemonefish Amphiprion akallopisos in their natural coral-reef habitat, and watching what happened. Within days, the dominant male’s behaviour began to shift toward female-typical patterns. Within roughly two months, on average, his gonads had reorganised. The testicular tissue receded. Ovarian tissue developed in its place. What had been a functional male, capable of fertilising eggs, was now a functional female, capable of producing them. The formerly-second-ranked subordinate had, meanwhile, matured into the new breeding male. The colony continued as before, with the entire hierarchy shifted up one rank.
The process has since been documented across virtually every species in the genus Amphiprion, in laboratory tanks and in field observations, and the mechanism is one of the more thoroughly established phenomena in modern reef fish biology.
What is actually happening inside the fish
The gonadal transformation is the visible outcome, but it is not where the change begins. According to a 2016 study by Dr Laura Casas and colleagues at Spain’s Institute of Marine Research and the King Abdullah University of Science and Technology, published in Scientific Reports under the title “Sex Change in Clownfish: Molecular Insights from Transcriptome Analysis”, the first measurable changes happen in the brain. The researchers ran a genome-wide analysis of gene expression across the different tissues of clownfish at successive stages of the sex-change process, and found that a specific pattern of gene activation begins in the neural tissue days before any visible change has occurred in the gonads.
The central molecular actor across the whole process is an enzyme called aromatase, which converts testosterone into estradiol, one of the primary female sex hormones. In the brain, aromatase expression rises early. In the gonads, aromatase expression rises later, and drives the actual conversion of testicular tissue into ovarian tissue. The whole sequence, from the female’s disappearance to the completion of a functional new female, takes anywhere between roughly one and six months depending on the specific species and the individual fish involved.
Follow-up research has continued to refine the picture. According to a 2022 study by researchers at the University of Illinois Urbana-Champaign, published in Hormones and Behavior under the title “Stable and Persistent Male-Like Behavior During Male-to-Female Sex Change in the Common Clownfish Amphiprion ocellaris”, some aspects of the fish’s behaviour change quickly after the female’s removal, while others take considerably longer. Aggressive interactions with subordinates shift toward the female pattern within days. But some male-typical courtship behaviours persist through most of the transition period, before eventually being replaced. Which suggests, on the current best interpretation of the neural evidence, that different brain circuits shift at different rates, and that what looks from the outside like a single event is actually a coordinated cascade of changes across the endocrine, neural, and gonadal systems.
All of which brings the story back to Marlin and Nemo. In the biological version of the film, once the barracuda had eaten Coral, Marlin would have found himself the largest surviving fish in the anemone. Over the following weeks, his behaviour would have begun to shift. His aggression toward Nemo, the only other fish present, would have increased. Within a few months, his gonads would have reorganised, and he would have become the new breeding female of what remained of the colony. Nemo, meanwhile, growing up in that anemone as the only other fish present, would have eventually matured into the new breeding male. The reproductive pair the anemone lost would have been reconstituted, with the same two fish taking the opposite roles from the ones they occupied at the start of the film.
Which is not, on any honest reading, a family film Pixar was ever going to make. The biological version of Finding Nemo would have opened with a widowed single parent, transitioned into an ambiguous middle section during which the father slowly became a mother, and closed with the son growing up and reproducing with the transformed parent, all of it inside the same sea anemone their family had always lived in.
The real anemonefish colony, doing what anemonefish colonies have done for the roughly twenty million years since the group evolved, does not find any of this remarkable. It is simply how the species continues. And it is one of the more genuinely alien facts of the vertebrate world, sitting quietly inside a fish most people would recognise from a children’s film they saw twenty years ago.
Kiran Athar is a writer, not a marine biologist or an ichthyologist. This piece draws on peer-reviewed research in Nature, Scientific Reports, and Hormones and Behavior.
