Clownfish are the most popular reef fish in the world. Known for their striking orange, white and black color scheme, these fish are popular in saltwater aquariums as well. The movie franchise, Finding Nemo is based around the fascinating social lives of these reef-dwellers.

Recent studies have shown that the bars (or white stripes) on a clownfish is not just an aesthetic addition but serve as an important indicator. These bars appear as clownfish grow and mark the change from larvae into adults through a process called metamorphosis, but the new findings show that they indicate a much deeper and more important characteristic of this species. 

Clownfish have an amazing bond with sea anemones. Immune to the paralyzing sting, these fish use anemones as their home and a source of protection from larger predators. The speed at which clownfish develop these distinctive patterns, according to scientists from Okinawa Institute of Science and Technology, indicates the species of sea anemone in which the clownfish live. 

The scientists also discovered that thyroid hormones, which play a key role in metamorphosis, drive how quickly their stripes appear, through changes in the activity of a gene called duox.

“Metamorphosis is an important process for clownfish — it changes their appearance and also the environment they live in, as clownfish larvae leave life in the open ocean and settle in the reef,” said senior author Professor Vincent Laudet, who leads the Marine Eco-Evo-Devo Unit at the Okinawa Institute of Science and Technology Graduate university (OIST) in a media release. 

“Understanding how metamorphosis changes depending on the sea anemone host can help us answer questions not only about how they adapt to these different environments, but also how they might be affected by other environmental pressures, like climate change,” he added.

The study, published in PNAS journal, states that clownfish usually pick two species of sea anemone. They are found in the magnificent sea anemone (Heteractis magnifica) or the more toxic giant carpet anemone (Stichodactyla gigantea). During the survey conducted off the coast of France, the team of researchers saw a distinct difference between the time taken by clownfish on both sea anemones to develop their stripes. Clownfish that called the giant carpet anemone home developed the stripes much quicker than clownfish living in the magnificent sea anemone.

“We were really interested in understanding not only why bar formation occurs faster or slower depending on the sea anemone, but also what drives these differences,” said first author Dr. Pauline Salis, a postdoctoral researcher at the Observatoire Océanologique de Banyuls-sur-Mer, Sorbonne Université Paris, who studies color patterning in coral reef fish.

The study then moved on to assessing the thyroid levels in both sets of clownfish to understand what triggers the faster release of this hormone, known to be the primary component that triggers stripe development in these fish. By studying the fish in a controlled lab setting and exposing them to only one of the sea anemone, 

The researchers then exposed both sets of fish to different doses of thyroid hormones. When the fish received a higher does of thyroid hormone,, it accelerated the development of white stripes on the body. Similarly, it was also noted that lower doses led to slower development of white stripes, leading to the definitive link between the hormone and metamorphosis of clownfish.

The team also noted the importance of iridophores in the formation of white bars in clownfish. The thyroid hormones accelerated the iridophore subset of the genetic makeup of clownfish. The activation sent the pigment cells into overdrive and accelerated the  physical transformation, proving to be the key determinant in clownfish metamorphosis. To obtain clear and indisputable results, the team also experimented with live clownfish populations in reefs in Kimbe Bay.  Levels of thyroid hormones were much higher in the clownfish from the giant carpet anemone than in the clownfish from the magnificent sea anemone, Dr. Salis confirmed from the field. 

“The big surprise was that out of all these genes, only 36 genes differed between the clownfish from the two sea anemone species,” said Prof. Laudet. “And one of these 36 genes, called duox, gave us a real eureka moment.”

Duox, which makes the protein dual oxidase, plays an important role in the formation of thyroid hormones, previous research has shown. The duox gene showed higher levels of activity in clownfish from the giant carpet anemone, compared to clownfish from the magnificent sea anemone.

A further look into the role of duox by Professor David Parichy from the University of Virginia, U.S., confirmed its role in developing iridophore pigment cells in clownfish. When the duox gene is inactivated in mutant zebrafish, development of the iridophore pigment cells is delayed, the study found.

The study combined the results from the duox study with the role of thyroid to generate a clearer picture of pigment metamorphosis in clownfish. The combined data revealed that increased activity of duox in clownfish living in the giant carpet anemone results in higher levels of thyroid hormones, and thus the faster rate of white bar formation as iridophore pigment cells develop quicker.

There were also speculations regarding how the giant carpet anemone triggered a faster reaction from juvenile clownfish. Some argue that the toxicity increases stress levels, which in turn could kick hormone production into a higher gear. With thyroid hormone levels increasing as a response to stress, the clownfish could be forced to grow and become adults faster to enhance their chances of survival. The researchers treated larval clownfish with different doses of thyroid hormones. The higher the dose of thyroid hormones, the faster the clownfish developed the white bars, the team reported. Conversely, when the researchers treated the clownfish with a drug that stopped thyroid hormones from being produced, bar formation was delayed.

“Here at OIST, we’re starting to delve into some possible explanations,” said Prof. Laudet. “We suspect that these changes in white bar formation are just the tip of the iceberg, and that many other differences are present that help the clownfish adapt to the two different sea anemone hosts.”