Finding Nemo put clownfish on the map, now we know how they get their stripes
/Finding Nemo, the story of a cute orange and white-striped baby fish who goes on an adventure down the east coast of Australia, put clownfish on the map.
Key points:
- Clownfish like Nemo develop a distinct pattern of white stripes as they get older
- Scientists discovered fish living in particular species of sea anemone develop stripes faster than others
- The researchers believe their choice of home triggers thyroid hormones, which control stripe development
But in reality, young Nemo may not have as many stripes as his cartoon pal.
And just when he got his stripes could depend on where he calls home, according to research published today in the Proceedings of the National Academy of Sciences.
"The real travels of Nemo are much more fascinating and mysterious than the movie," study co-author Vincent Laudet said.
The real story of Nemo
LoadingClownfish spend most of their life hanging out in the stinging tentacles of sea anemones.
After floating around as transparent yellowish blips in the ocean for about 10 days, baby fish swim back to the reef to join adult fish living in sea anemone colonies.
If they are accepted by the colony, they go to the end of the reproductive queue, which is headed by the dominant female and male (who turns into the next matriarch when the female dies).
"The new recruits are almost never the babies of [the dominant female or male] couple. It is really random [which colony they enter]," says Professor Laudet of Okinawa Institute of Science and Technology Graduate University.
Around day 15, clownfish start developing the first of their three stripes, which appear in a precise order.
"First the one is on the head, then the trunk, then the tail," Professor Laudet said.
"All except a few species have stripes."
Some species have more stripes than others, and there is individual variation in stripe shape — possibly to help the colony recognise their new recruit.
"They need to know who is in the queue in the colony and who is a foreigner. And if you are a foreigner, you're expelled," Professor Laudet said.
The process of developing stripes, known as metamorphosis, is driven by thyroid hormones.
The white stripes appear due to pigment cells, called iridophores, which are activated by genes stimulated by thyroid hormones.
Number of stripes differed in PNG colonies
But scientists working with a clownfish species (Amphiprion percula) that lives in Kimbe Bay in Papua New Guinea noted something strange.
The clownfish there either live in the magnificent sea anemone, Heteractis magnifica, or the more toxic giant carpet anemone, Stichodactyla gigantea.
The baby clownfish living in giant carpet anemones, which have short tentacles, developed stripes faster than those that lived in the magnificent sea anemones with their long, wavy tendrils.
Professor Laurent and his team, led by Pauline Salis at Oceanographic Observatory of Banyuls-sur-Mer, Sorbonne University Paris, thought this might have something to do with the thyroid hormones.
"I said OK, we have to try it, and the result was 'wow', incredible, we have a story here," Professor Laudet said.
The team injected another closely related species of clownfish, Amphiprion ocellaris, with thyroid hormones, and found their stripes appeared faster.
When they injected fish with a product that inhibited the hormone, the stripes appeared later.
When the team took juveniles of the species back to Papua New Guinea, the fish exposed to the giant carpet anemone had higher levels of thyroid hormones.
The team pinpointed increased action of a single gene called duox, which is important in the production of the iridophores.
So why did fish respond differently to their homes?
Just why the fish, which are genetically the same, should respond so differently to the type of sea anemone they live in is a mystery.
"Perhaps the fish feel less secure in the giant carpet anemone because they are less hidden than the [long-tentacled magnificent anemone], so they have more stress, and this induces an increase in thyroid hormones," Professor Laudet hypothesised.
But given that thyroid hormones are important for a multitude of metabolic and immunological functions, the findings could have deeper significance that could change the way we understand how the fish respond to their environment.
"Maybe the fish have less food, or maybe they have to cope with more stinging in the giant carpet anemone than in the magnificent anemone, and increasing thyroid hormone is a good way of solving these problems," Professor Laudet said.
Professor Laudet suspects this phenomenon may also happen to other fish.
"I think this is occurring in many situations in the world, but we never see it because we don't have fish prisoners in a micro-environment like that which are easy to detect."
Luckily for researchers, apart from a brief stint on the open seas before Nemo earned his stripes, he never leaves home.
"We know that how you change your appearance, metabolism and phenotype to your local environment changes occurs, but we have very few convincing demonstrations of how this might work.
"Now we have one."