The larvae of coral fish in the ocean are millimetre-sized. And yet they find their way over kilometres back to their native reef. Oldenburg biologist Gabriele Gerlach has investigated how this is possible on Australia's Great Barrier Reef.
Every Australian summer, from December to February, they go on a migration: the larvae of coral fish, millions of which hatched a few weeks earlier in the Australian Great Barrier Reef. Flushed into the open water, the so-called pelagic, at low tide, their journey begins.
The journey is reminiscent of the migration of salmon, which return to the river in which they were born to spawn. Oldenburg biologist Prof Dr Gabriele Gerlach recently spent several weeks at the "One Tree Island" research station on the Great Barrier Reef to study the orientation abilities of coral reef fish larvae.
Her stay at the "One Tree Island" research station on the Great Barrier Reef was made possible by the Australian science network ANNiMS. It selected Gerlach as a Visiting Scholar 2012 in recognition of her work on coral reefs, migration and orientation of fish. ANNiMS stands for Australian National Network for Marine Science and is a co-operation project between the universities in Townsville, Perth and on the island of Tasmania.
How can navigation of coral reef fish back to their home reef be explained?
"Despite all the dangers, the chances of survival of coral reef fish larvae are still higher in the open ocean than on the reef or near adult fish," says Gerlach. However, it is still an unsolved mystery where exactly the larvae are and how far they move away from the reef.
Depending on the species, they have to find a reef again after a while, as the tiny creatures cannot continue to develop in the open sea. For a long time, it was believed that it was pure chance where the currents and storms caused the larvae to drift. A good decade ago, the first reports appeared that many larvae were returning to the reef where they were born. Since then, Gerlach has also been studying the orientation behaviour of coral reef fish. According to the scientist, navigation back to their home waters cannot be explained without a special sensory system.
To track down the fish, Australian scientists use a special marking method: they expose the eggs and newly hatched larvae to an antibiotic for a few hours. This causes the daily growth ring in the ossicles to turn green. The returnees can be easily identified by their colouring.
The population of origin of the larvae was determined using DNA microsatellite analysis.
The scientists calculated a return probability of 35 to 60 per cent. Gerlach, an expert in biodiversity and animal evolution from Oldenburg, chose a different method to track the migrations of the fish larvae: "Similar to how criminologists identify criminals using DNA traces, DNA microsatellite analysis can be used to determine the population of origin of the larvae." Cardinalfish (Ostorhinchus doederleini) were used as test animals.
Gerlach's hypothesis: if the larvae return to their native reef, then as adult fish they should also be genetically distinguishable from their conspecifics on the neighbouring reefs. During her investigations in Australia, the biologist was able to prove exactly that: Within a reef, the fish are closely related to each other, while conspecifics in neighbouring reefs differ genetically.
Researcher found that larvae have a sense of smell.
The question remains: How do the fish larvae, which are only a few millimetres in size and often move kilometres away, find their way back to their natal reef? Gerlach had already provided an initial answer in 2007 in a sensational publication in the renowned American scientific journal PNAS (Proceedings of the National Academy of Sciences).
Together with the Australian marine biologist Michael J. Kingsford from James Cook University in Townsville and the American scientist Jelle Atema from Boston University, she discovered that the larvae have a sense of smell that helps them to navigate in the seawater.
"When they are on their way home again after the three-week migration in the open ocean that is typical for their species, the larvae show an odour preference for their natal reef. The water that flows out of the reef lagoon at low tide therefore has odour characteristics typical of reefs," explains Gerlach.
Larvae smell the odour of "their" reef.
In an experimental set-up, the scientists presented young fish with water from five neighbouring reefs of the Great Barrier Reef. "The larvae preferred the water from their genetically determined home lagoon to water samples from other reefs. They smelled the specific 'odour' of their reef and swam purposefully towards it," says the scientist.
However, this olfactory explanation is not enough. The odour of the reefs cannot spread indefinitely in the ocean. If the larvae are carried for kilometres by the currents, their sense of smell can no longer show them the way at some point.
So how do the fish find their way over long distances? The question is still unanswered. Gerlach is trying to solve the puzzle - together with the Oldenburg biologist and expert in migratory bird navigation, Prof Dr Henrik Mouritsen. The two Oldenburg researchers are asking themselves: Are similar mechanisms at work in fish as in the long-distance navigation of migratory birds?
This article - together with an interview with Gabriele Gerlach - also appears in the July issue of the university newspaper UNI-INFO.