We’ve all seen the epic movie Finding Nemo and admired Dory in her debut lead role in the blockbuster Finding Dory. These fish are famous … but, after watching Nemo and his legendary struggle to free himself and his friends from the clutches of the terrifying, fish-shaking, maniacal daughter of the dentist who owned the aquarium, I got to thinking about some of the biological discrepancies of the movie’s plot – which is strictly the reserved pastime of geeky marine biologists like me.
Truth is, when Nemo’s mum was eaten by the barracuda, Nemo’s dad, Marlin, should have undergone a sex change to replace her, and taken up a new relationship with a close family friend.
You see, clownfish are protandrous hermaphrodites, which simply means they are all born male and the largest eventually undergo a sex change to become the breeding matriarch. So, when you are next admiring a family of clownfish, look for the largest, and often most aggressive, member of the group and you will have the pugnacious female. Next in line in size is often her mate, followed by a posse of other diminutive, non-breeding males that occupy the lower rungs of the hierarchical ladder.
So, when that barracuda comes along and eats Mrs Doubtfire, her mate – after a period of socially appropriate grieving – will receive hormonal cues stimulating a total transformation of his gonads to change from him to her. A reshuffle of the hierarchy then takes place and one of the non-breeding males increases in size to become the next to procreate with the new female.
Freaky? As it turns out, many species of reef fishes employ this type of breeding strategy … although, the more common practice is structured the other way around: protogynous hermaphroditism (born female). Parrotfish and wrasses are recognised devotees of this latter method and, although the details of their strategy are a little more complex, the critical thing is that the large and showy ones are ostentatious males dressed in tuxedos, flashing the gadgets. It’s just that this 007 might not have started out as Bond.
All of this fishy business is a very clever trick to ensure there is always someone with whom to mate, no matter who gets eaten.
But what of the curious relationship that clownfish exhibit with their host anemones?
There are roughly 30 species of clownfish. Some will only associate with one species of anemone while others, like the Clark’s anemonefish, show no such loyalty and can be seen coquettishly frolicking in the tentacles of up to 10 different kinds of anemone.
How does this bizarre friendship get started in the first place?
Clownfish are demersal egg layers, which means the eggs are glued to a nearby surface and the babies are born in the presence of mum and dad, but extreme tides at full moon sweep them away to a new life and, hopefully, a new anemone home. One of the theories as to how the fish survive the anemone’s stinging cells is to embed their host’s chemical signature within their extra-thick mucus lining. This cunning strategy ensures that the anemone does not detect them as foreign intruders and their venom-filled nematocysts fail to fire. Therein lies the genius of this symbiotic relationship – the clownfish remain relatively safe from harm, provided they remain close to the protective tentacles of their host.
The whole story is rather remarkable, really. Such intricate and intimate threads make up the life story of this characterful fish, many details of which you would never have gleaned from Nemo’s fanciful odyssey.