Clownfish spend most of their lives nestling in the tentacles of their anemone homes. Now for the first time, researchers show that baby clownfish can travel marathon distances in the open ocean during the first few weeks of their lives. This study, published in PLoS One this week, is the farthest anyone has tracked the dispersal of reef fish, demonstrating how even distant populations can be well-connected.

Adult Omani clownfish (Amphiprion omanensis) provide high levels of parental care to their young, so newly-hatched babies have well-developed swimming and sensory capabilities. A team led by Stephen Simpson from the University of Exeter studied these fish throughout the southern coast of Oman. The area is isolated from the rest of the Arabian Peninsula, so lots of species there aren’t found elsewhere. Also, the coast has only two coral reef systems -- the northern province of Ash Sharqiyah and Dhofar to the south -- and they’re separated by 400 kilometers of surf beaches.

“In order to persist as a single species, we know Omani clownfish must occasionally migrate between these two populations,” Simpson says in a news release. During 92 dives, the team caught nearly 400 fish: 136 from the north and 260 from the south. After a small pectoral fin clip was taken, the fish were released back to their colonies. Then the researchers used DNA fingerprinting to identify local, long-distant migrant, and hybrid individuals throughout the species range. 

"Just like accents that allow us to tell an Englishman from an American, fish populations develop their own genetic signatures," study coauthor Hugo Harrison from James Cook University explains. "By looking at the signature of each fish we can tell whether it originated there or not. It's like finding an Englishman in New York, they stand out."

Being so well-developed allowed the babies to embark on an ocean-going larval phase of up to three weeks. During that timeframe, six percent of the fish sampled migrated the 400 kilometers from one population to another. That’s farther than New York to Boston or Washington, D.C.

Most who made the trek were headed south: Of the fish sampled, 5.4 percent from the northern population went the distance, compared with 0.7 percent from the south. This bias in offspring exchange may be due to prevailing currents, and the team’s oceanographic model for the region showed that their patterns of migration corresponded to dominant ocean currents driven by winter monsoons.

"This is an epic journey for these tiny week-old fish,” Simpson says. “When they arrive at the reef, they are less than a centimeter long, and only a few days old, so to travel hundreds of kilometers they must be riding ocean currents to assist their migration.”

The team also identified several second generation hybrids in both populations, indicating how migrants had successfully joined and reproduced with locals in their new homes. This exchange of genes between seemingly distant colonies means each outpost is constantly being topped up with new genetic material.

Images: 2014 Simpson et al. (top), Tane Sinclair-Taylor (middle)