Coral Reef, © USFWS Pacific

Counting Marine Life – How Many Species Have We Missed?

Dan Thornhill, Conservation Scientist 

Coral reefs are widely known as the most biodiverse marine ecosystem on the planet. Despite covering less than 0.2 percent of the sea floor, scientists estimate that reefs are home to up to nine million species throughout the world! Only tropical rainforests rival coral reefs in numbers and types of wildlife.

Coral reefs can be home to tens of thousands of marine species. (©Hagal Nativ)

Coral reefs can be home to tens of thousands of marine species. (©Hagal Nativ)

Although the rich diversity of coral reefs has been known for decades, determining exactly how many species spend their lives on or around reefs is quite a challenge. Coral reefs are scattered across many remote parts of the world; just traveling to these places takes time and money. Moreover, working underwater is technically difficult and potentially dangerous, which limits the time that scientists can safely work there. Many species of coral reef wildlife are small and cryptic, making them hard to find and identify. Adding to this identification challenge, some species may even look alike despite their genetic and behavioral distinctiveness. (In fact, even something as big as a whale can be hard to categorize: two genetically distinct but physically similar species are both known as minke whales). Finally, few scientists study coral reef wildlife, so not enough effort has been devoted to cataloging and learning about these fascinating animals and ecosystems.

Two years ago, Laetitia Plaisance and her colleagues set out to measure the diversity of coral reef wildlife using DNA forensics. They focused on crustaceans – shellfish like lobster, crabs, and shrimp- that are important marine species throughout the world. Their approach was straightforward : they examined either the surface of dead corals or placed a lattice of small plates in the water on several reefs across the Pacific Ocean, waited one year, and then examined the DNA of the animals present on the coral surface or plates.

Researchers place Autonomous Reef Monitoring Structures (ARMS) around coral reefs. Later, they can return and study the organisms that have grown on the plates. (©Christopher Meyer)

Researchers place Autonomous Reef Monitoring Structures (ARMS) around coral reefs. Later, they can return and study the organisms that have grown on the plates. (©Christopher Meyer)

The researchers sampled a tiny total area of just 6.3 square meters, or about the size of a small bedroom. Even with such a small sample area, they found a staggering number of unique species – 525 different crustaceans in total! Fewer than 4% of these species had been recorded previously; most of these animals were entirely new to science. And yet this number is just the tip of the biodiversity iceberg. Most of the 525 species were found only one time and in one location. It would take much more surveying all across the tropics to get a reasonably reliable headcount on the total number of coral reef crustaceans.

The diversity of marine life is truly astounding. Clearly there is still much to learn about coral reefs and the wildlife that depends on them, yet these important ecosystems are rapidly changing and increasingly threatened. Will we even be able to identify the diversity of wildlife present before it is too late? I certainly hope so. As Plaisance and her colleagues demonstrated, there is still much to learn about marine wildlife.

Categories: Coral Reef, Wildlife

One Response to “Counting Marine Life – How Many Species Have We Missed?”

  1. Annemette Jensen

    Dear Daniel,

    I am working with studens from a school who are going on a freediving trip to a small (coral reef) island in the Philippines called Pamilacan. We are focused on different things during this week on the island – culture, plastic pollution projects with a local organization, friendship school, and as mentioned, freediving. The problem is, we would like to focus on the coral reefs as well, although we have little time.

    We will be visiting the island over a period of once a year over 4 years (with different students). I was wondering if you had any good ideas as to how we could engage the students in e.g. measuring marine life in the coral reefs. Not as too create “scientific results” as such, but as a means to learn more about how scientists do this as well as a means to create a more direct awareness among the students of what is going on with coral reefs around the world.

    I was wondering if you have any suggestions as to how we can go about this? We have perhaps one or two afternoons available..

    I would love to hear if you have any suggestions for us.

    Thank you,


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