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Explained: Losing Nemo? His world is changing too fast for his genes to adapt

Clownfish typically live at the bottom of shallow seas in sheltered reefs or in shallow lagoons. It is this habitat that is under threat.

Written by Kabir Firaque | New Delhi |
Updated: November 30, 2019 2:14:30 pm
Clownfish, Finding Nemo, Finding Nemo fish, clownfishes in India, clownfish in India, Express Explained, Indian Express Clownfish on anemone in lagoon in Papua New Guinea. (Simon Thorrold/WHOI)

The clownfish, made so popular by the animated film Finding Nemo and its sequel Finding Dory, cannot be expected to be able to adapt to a rapidly changing environment, a new study has concluded. It does not have the genetic capacity to do so, scientists report in the journal Ecology Letters.

Habitat under threat

While clownfish are found in various parts of the Indian and Pacific Oceans, including the Great Barrier Reef, only some species are widespread and most of the others have restricted distributions. Clownfish typically live at the bottom of shallow seas in sheltered reefs or in shallow lagoons. It is this habitat that is under threat.

Clownfish breed only in sea anemones, sharing a symbiotic bond. “It is a strong, obligate symbiosis,” study co-author Geoffrey Jones said, by email. “Clownfish shelter in the anemone and are the only fish that do not get stung by the nematocysts of the anemone. The anemone benefits because clownfish can defend the anemone from fish that might eat it. They never live anywhere but in the anemone,” said Professor Jones, of the Australian Research Council (ARC) Centre of Excellence for Coral Reef Studies at James Cook University.

And now the anemones, like coral reefs in general, are under direct threat from the impacts of climate change. It works like this: The anemones share another symbiotic bond, with algae. Under stress in warming waters, the algae leave the anemones. If the algae stay away too long, the anemone starve to death. Which leaves the clownfish without a home.

Clownfish fail test

What the study sought to find out was whether there are genetic variants of clownfish that can breed faster than others. There aren’t, it concludes after 10 years of research on the coral reefs of Papua New Guinea.

Family trees were established for the entire clownfish population at an island in Kimbe Bay. Working with about 280 breeding pairs, the scientists identified each fish individually and sampled its DNA to establish who was related to whom over five generations. It was comprehensive — all individuals including adults and juvenules were sampled; offspring were almost always assigned to both parents who cohabit in the same anemone.

From the family tree, the researchers were able to assess the ability of the population to persist and the genetic potential to adapt to increasingly rapid environmental change. The potential is almost nil.

“… We find that Nemo is at the mercy of a habitat that is degrading more and more every year. To expect a clownfish to genetically adapt at a pace which would allow it to persist is unreasonable,” co-author Dr Serge Planes, a Director of Research at France’s National Centre of Scientific Research (CNRS), said in a statement from the ARC Centre of Excellence.

The home, not the genes

“There are no particular genetic variants that contribute more offspring to the next generation. The quality of the host anemone contributes most to the ability of the clownfish to renew its population,” Prof Jones said in the statement.”

“Their future depends on our ability to maintain the quality of their habitat,” the authors conclude.

Apart from the scientists from the Australian and French institutions, the team included researchers from the United States (Woods Hole Oceanographic Institute), Saudi Arabia (KAUST) and Chile (Universidad Austral de Chile).

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