How the clownfish earned its stripes

How quickly clownfish get their white stripes depends on which anemones they live with, according to a new study published in PNAS.

The pretty orange fish develop their white bars when they mature from larvae into adults – a process called metamorphosis – but how this happens was previously unknown.

Now, a team of researchers, led by Pauline Salis of the University of Paris, found that certain genes and thyroid hormones control how fast the fish grow their stripes, but hormonal levels are influenced by the type of anemone the clownfish live in.

“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,” says Salis.

The team studied the clownfish species Amphiprion percula in Kimbe Bay, Papua New Guinea. This species lives in two different types of anemone: the magnificent sea anemone (Heteractis magnifica) or the giant carpet anemone (Stichodactyla gigantea).

Two fish side by site. The fish on the left is yellow and has one white stripe. The fish on the right is orange and has two white stripes.
Clownfish living in the long-tentacled anemone, Heteractis magnifica, (left) have fewer stripes than clownfish of the same age and size living in the shorter, carpet-style anemone, Stichodactyla gigantea (right). The image shows the typical appearance of clownfish aged 150-200 days. Credit: Fiona Lee, Academia Sinica, Taiwan.

They saw that the juvenile fish living in the giant carpet anemone developed their stripes faster.

Back in the lab, the team treated a related clownfish species (Amphiprion ocellaris) with different doses of thyroid hormone, which is known to trigger metamorphosis in frogs. Fish given a higher dose developed their stripes more quickly.

They then analysed the hormone levels of the A. percula fish from Kimbe Bay, and found – consistent with the lab results – that the fish living in the giant carpet anemone had higher levels of thyroid hormones than the fish living in the magnificent sea anemone. This may be why their stripes developed more quickly.

Two fish. The top fish has translucent tail and finds and body. It is yellow underneath its skin and the skin has black sport. The bottom fish is white at the back and pale orange at the front, with one white stripe behind the eye
The image shows a control clownfish larvae (top) and a larvae five days after it was given a dose of thyroid hormones (bottom). Credit: Pauline Salis.

To further investigate what causes the different hormone levels, the team measured the activity of genes of the clownfish.

“The big surprise was that out of all these genes, only 36 genes differed between the clownfish from the two sea anemone species,” says senior author Vincent Laudet of Okinawa Institute of Science and Technology, Japan. “And one of these 36 genes, called duox, gave us a real eureka moment.”

The duox gene plays an important role in thyroid hormone formation, they say, and this gene was more active in clownfish that lived in the giant carpet anemone.

Altogether, this means that clownfish that live in giant carpet anemones promoted higher activity of duox, which makes more thyroid hormone, which makes the fish grow stripes faster.

“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,” says Laudet.

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