If you’ve seen the popular animated film Finding Nemo, you may remember the character Dory who sang the catchy tune “Just Keep Swimming” to help her clownfish friend Marlin make the long journey from the Great Barrier Reef to Sydney.
In this case, art imitates life. Marathon swimming performance is an essential part of the early life of the vast majority of coral reef fish. Baby (larval) reef fish – smaller than the size of your thumbnail – hatch from eggs laid on the reef and spend a few weeks in the open ocean before swim back to the reef.
But how does such a small creature make this impressive journey? Our research published today on the way to answering this question.
We found that larval clownfish dramatically change their physiology to complete their journey from the ocean back to the reef. In particular, they take in more oxygen per breath and at a faster rate than any other fish species studied to date. Essentially, this makes baby clownfish one of the smallest athletes in the world.
Read more: Dazzling or Deceptive? Coral reef fish tags
Mini athletes swim 10-50 body lengths per second
Reef fish are vital to coral reef ecosystems. They play an important role in the food web, helping keep the reef clean and recycling nutrients. In addition, their vibrant colors attract millions of tourists every year.
Adult reef fish keep to a small area. Their eggs are carried by wave action to the open ocean, where they hatch and develop.
Within a few weeks, the small fish larvae should return to the reef. It’s a long, hard journey that can take weeks to months. Depending on the species, they cover distances of up to 64 kilometers. So how do they do it?
Until the 1990s, scientists believed that the development of reef fish larvae was similar to that of other fish such as herring, cod and flatfish. These species “go with the current”, passively riding the ocean currents until they grow large and are sufficiently developed to actively swim against the current on their own.
However, groundbreaking studies from the early 1990s documented the impressive swimming abilities of baby reef fish. As it turns out, reef fish are not passive particles finally.
Previous research has provided overwhelming evidence of what coral reef fish are capable of great swimming performance as babies.
Some of these little athletes are capable of that swim 10-50 body lengths per second as a larva. By comparison, Olympic multi-gold medalist Michael Phelps races just below two body lengths per second.
Combined with well-developed sensory systems such as sight and smell, such impressive athletic feats allow these babies to “just keep swimming” with or against ocean currents until they find an optimal reef to settle on.
But 30 years after its discovery, we’re still wondering how they did it. Now we know.
Measuring the attributes of an athlete
My colleagues and I measured the physiological characteristics required to be an athletic swimmer throughout the larval stage of a clownfish. These traits include swimming speed, oxygen uptake rates, gene expression patterns, and low oxygen tolerance (hypoxia).
Why hypoxia? At night, when it is no longer possible to use sunshine and carbon dioxide to make energy through photosynthesis, corals and plants breathe in oxygen to make energy. This lowers oxygen levels on reefs. Larval reef fish returning home from the open ocean must prepare for such conditions.
We found that larval clownfish have the highest oxygen uptake of any fish to date. This supports elite swimming, growth and development.
As they develop faster and swim faster, thousands of genes change. Genes encoding proteins that transport and store oxygen, such as hemoglobin and myoglobin (also present in our body), are particularly important. They allow oxygen to be transported and stored during intense exercise and help retain oxygen in tissues when the fish experience hypoxia in their reef habitats.
The changes in hemoglobin and myoglobin genes also correspond to when these baby fish begin to increase their hypoxia tolerance.
We’ve seen this before, in reverse. Salmon are one of the most studied fish of all time and as adults they are pretty great athletes too. Baby salmon, however, have to contend with low-oxygen conditions in the first few weeks of life, immediately after hatching, while hiding in the gravel of the freshwater riverbeds.
And indeed, in the 1980s, research showed salmon change their hemoglobin also – exactly when the baby salmon should transition from hypoxia tolerance to training to become elite swimmers.
Why our research matters
The changes in physiological machinery we’ve discovered are key to clownfish survival. It is likely that other coral reef fish follow similar developmental paths.
Reef fish – of all shapes, sizes and colors – are integral to maintaining coral reef health and the survival of future coral reefs. This is critical as climate change threatens these beautiful, fragile ecosystems.
Read more: I studied what happens to reef fish after coral bleaching. What I saw still makes me sick