Finding Nemo’s Clownfish

Watch

1. Finding Nemo got the clownfish wrongyoutube.com/shorts/62srRdYYCAg
2. Nemo is probably not Marlin’s sonyoutube.com/shorts/5GJUhGE19qA

Word of the day

Larva (LAHR-vuh). An immature, free-living form of an animal that changes into a different adult form through metamorphosis.

• A tadpole is the larva of a frog or toad. Video 2 uses the plural, larvae.
• In ancient Rome, larvae were harmful ghosts.

Bonus words. Protandry (from prot-, “first,” and andr-, “male”) describes an animal that produces male gametes before female gametes. Philopatry (from Greek phílos, “loving,” and Latin patria, “homeland”), the word in the title of the Salles 2016 paper, describes an animal that stays near, or returns to, its birthplace to breed.

Predict

1. Océane Salles and the Kimbe Island team put together a family tree of 2,927 clownfish, then counted the parents and offspring living in the same anemone. Make a guess before you read the paper, and write one sentence on why you think so. What percent of parent-and-offspring pairs in the Kimbe Island family tree do you predict shared an anemone?

   Hint. In video 2, Raven gives a figure for relatives in general. Would parents and offspring share an anemone more often or less often than relatives in general?

Read the paper

Salles and others, “First genealogy for a wild marine fish population reveals multigenerational philopatry,” PNAS, 2016. The paper is free to read.

Divers on Océane Salles’s team sampled the orange clownfish on Kimbe Island in Papua New Guinea, home to 310 anemones, six times between 2003 and 2013. The team read 22 DNA markers from each fin clip and reconstructed a family tree of 2,927 fish. Table 1 of the paper counts the family links in that tree, then counts the related pairs living in the same anemone.

1. Quantitative reasoning Fill in the last column of Table 1. For the parent and grandparent rows, add the maternal and paternal links first. Which relationship in Table 1 shared an anemone most often, which percents in Table 1 rest on a single pair, and how much weight should a percent based on a single pair carry?

   Hint. Divide the related pairs in the same anemone by the family links in the tree. Watch for rows with a count of 1.

2. Process of science The Kimbe team used 22 DNA markers, accepted a parent pair only when its LOD score (the log of the odds that the match is true) topped 30, and rejected all matches to a single parent because generations overlap. What mistake does each of the Kimbe team’s three matching rules prevent, and what do strict matching rules cost the counts in Table 1?

   Hint. Think about two kinds of error, a false match and a missed match.

3. Evolution Philopatry can raise the risk of inbreeding, but Table 1 records 2 inbred matings and the paper reports an inbreeding coefficient of 0.018. Self-recruitment averaged 56%, and the other 44% of settlers arrived as larvae from other reefs. How do the inbreeding and self-recruitment numbers and the size order in each anemone explain why inbreeding stayed rare on Kimbe Island, and what would you predict if fewer larvae arrived from other reefs?

   Hint. Only the two largest fish in an anemone breed. How likely is a relative to reach the front of an anemone’s breeding line?

4. Science and society Kimbe Island is a marine reserve. The authors write that a reserve can protect several generations of one population, and they warn that climate change may shorten how far larvae travel between reefs. A reef manager has to decide which reefs to protect. What would you recommend? Write one paragraph that uses at least two numbers from the Kimbe Island paper.

   Hint. Think about the birthplaces of Kimbe Island’s settlers.

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