How Nemo got his stripes decoded

Scientists have decoded the evolution of the distinctive orange and white patterned clownfish -- the coral reef fish species that was featured in the popular animated movie 'Finding Nemo'.

Coral reef fishes, including clownfish, display a wide variety of colours but it remains unclear how these colours evolved or how they develop throughout a fish's life.

Research published in the journal BMC Biology sheds new light on the evolution of different stripe patterns in clownfish and on how these patterns change as individuals from different species grow from larvae into adults.

"We show that the ancestor of today's clownfish possessed three white stripes," said Vincent Laudet, from the Sorbonne University in France.

"Then, as some species evolved they lost stripes and we reveal a surprising similarity between this loss of stripes during species evolution and the development of different stripe patterns in individuals from different species today," said Laudet.

Studying two species of clownfish -- Amphiprion ocellaris and Amphiprion frenatus -- that have three stripes or a single head stripe, respectively, researchers found that shortly after hatching, the larvae of neither species had any stripes.

Subsequently, both species acquired stripes on head and trunk at the same time, with A oscellaris acquiring a third stripe near the tail and A frenatus losing the trunk stripe before reaching adulthood.

Examining development information for 26 additional species of clownfish, researchers observed that at least nine species have more stripes as juveniles than they do as adults, which prompted researchers to investigate the development of stripes across the evolution of clownfish.

"Every clownfish species gains stripes from front to back after they are born, before individuals of some species lose stripes again from back to front as they grow into adults, which is similar to the loss of stripes observed during clownfish evolution," said Laudet.

"While all clownfish started out with three stripes -- that is their last common ancestor had three stripes -- as they diversified into what are now 30 different species, some clownfish lost stripes in a pattern that is similar to how today's clownfish lose stripes as they grow up," he said.

While clownfish species vary in their number of stripes from zero to three, there is limited variation in how these stripes are organised.

In all two-striped species, the stripe nearest the tail has been lost, while the head and the trunk stripes are retained. All one-striped species have retained the head stripe and have lost the trunk and tailfin stripes, researchers said.

In order to investigate the molecular mechanisms that underlie stripe formation and loss, researchers treated clownfish larvae with a substance known to suppress stripe development in zebrafish.

The substance works by targeting certain receptors in iridophores -- the cells that produce a reflective or iridescent colour.

Researchers that larvae treated with the substance did not fully develop stripes or developed no stripes at all in a dose-dependent manner.

The findings suggest that the white stripes in clownfish are produced by iridophores and that a decrease in the number of these cells will inhibit stripe formation.

"Unravelling the mysteries of why pigmentation patterns from coral reef fish are so diverse, how they evolved and where their diversity originated will help us to understand the formation of very complex phenotypes," said Laudet.

Researchers suggest a possible purpose for the different stripe patterns; they may allow clownfish to recognise individuals belonging to the same species, including potential partners for reproduction.