Tiny Mandarin Fish Reveal Surprisingly Complex Spawning Ritual

Hillary Mayell
for National Geographic News
April 27, 2001

Each year, more than 40 million fish are removed from the wild to fill the tanks of marine aquarium hobbyists. With the increasing popularity of reef aquariums, that number is likely to grow.

What this popularity means to ocean fish populations worries scientists and conservationists. There is a real danger that excessive removals of wild fish for the aquarium trade could have a devastating impact on local populations, said Yvonne Sadovy, a biologist at the University of Hong Kong.

Very little is known about the life cycles and biology of many reef species. This makes it difficult to determine whether the fish need to be protected, and if so, how.

To help provide answers, Sadovy, along with Mariella Rasotto from the University of Padova and George Mitcheson from the University of Hong Kong, have studied populations of exotically colored mandarin fish in Palau, Micronesia. The project was funded with assistance from the National Geographic Society's Committee for Research and Exploration.

Insight Into Other Species

Mandarin fish are highly popular among aquarium owners, divers, and marine photographers because of their striking beauty. Their body designs of swirls, squiggles, dots, and stripes in improbable neon colors of orange, blue, yellow, and green make them look like something drawn by a five-year-old.

For the researchers, the fishes' exotic markings make it easier to detect individual specimens and observe their behavior.

Mandarin fish are one of the tiniest species among ocean spawners, which reproduce by depositing thousands of eggs into the water. Wide-ranging species such as tuna, whitefish, and sardines all belong to the same group, but are much harder to track and study than the tiny mandarin fish, which are only one to two inches (20 to 55 millimeters) long and live close to shore in the shallow waters of coral reefs.

What the scientists have learned so far about the mandarin fish has surprised them. "We never anticipated the complexity of the social structure and mating system that we encountered," said Mitcheson. "We also didn't expect that individuals could be so distinctly different, one from the other, in terms of their habits and behaviors."

As the researchers monitored mandarin fish populations, they found that each evening, within 20 minutes of sunset, groups of three to five females gathered. Each group had its own "street corner" to which its members returned night after night. Males visited the sites and displayed courtship behavior, hoping to attract females.

To spawn, a female joins the male, resting on his pelvic fin, and the pair rises slowly about three feet (one meter) above the coral reef. At the peak of their rise, a cloud of eggs and sperm is released.

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