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Abstract
The world's wondrous array of coral reefs sets the stage for a diverse cast of fish to play out their lives in vivid colors, bizarre shapes, and oddball behaviors. Paradoxically, the balmy tropical waters that surround them - so appealing to human senses - are often likened to biological deserts. A coral reef is, in effect, an ecological oasis.
| The human genome? "It's all in silica." |
How all these myriad fish colonize and repopulate coral reefs, however, has long been a mystery because most reef fish make lousy parents. They simply spawn their young in a milky swirl of eggs and sperm on the reef edge and then abandon them to fate. Most of their larvae, each typically only 1 to 20 millimeters long, end up spending many weeks in open water. It was long thought by marine scientists that the vagaries of ocean currents mainly determined whether they would perish in the blue desert or live to reach an oasis and settle down.
But painstaking field observations and creative experiments by various researchers over the past two decades have revealed a very different picture. The larvae of at least some fish are far better equipped than we ever imagined to skew the survival odds in their favor. Not only can these tiny transparent voyagers sense where reefs are, but far from being passive drifters, they can perform remarkable feats of swimming and navigation to reach them.
| Larvae aren't as helpless as they look. |
Little is known about the biology of most larvae when they are in open water, but accumulating evidence suggests that at long range, some can steer a broad course by the sun; that others, at medium range, can detect and swim toward reef sounds; and that some have complex behaviors and use subtle sensory detection at close range to avoid predators and zero in on desirable habitats.
In turn, these findings have prompted important questions about the way tropical marine reserves are designed and managed, says Jeff Leis, a principal research scientist at the Australian Museum. "If fish larvae are passive particles in a current that flows the same way during the larvae's lifetime, then it is unlikely that a larva will settle on the same reef inhabited by its parents," Leis says. "On the other hand, if larvae are returning to their reef of origin or selectively choosing or avoiding some reefs, we must rethink our strategies for conservation of reef ecosystems and even fisheries management."
| Larvae clearly have a degree of control over their travels. |
One of the first assumptions to falter was that reef-fish larvae are randomly carried by ocean currents. Leis' group, using fine-mesh nets to capture larvae in open water near reefs, showed that different species leaving the same reef had different distributions. Larvae clearly had a degree of control over their travels. It is now known that they have considerable flexibility in their travels: some favor certain depths, certain directions, or travel at different speeds; some can even detect predators several meters away and take appropriate evasive action.
Ilona Stobutzki, then a student at Australia's James Cook University, recorded laboratory studies of prodigious swimming feats by some species. Larvae caught as they were about to settle onto a coral reef were placed in raceways through which seawater was pumped at the average local current speed around the Lizard Island Research Station, on Australia's Great Barrier Reef - a steady 13 centimeters a second.
| Surprisingly, the larvae can swim long distances. |
The results shocked disbelieving Northern Hemisphere scientists more used to relatively weak-swimming, temperate species such as cod and herring. "We now know that [cod and herring larvae] are pretty pathetic swimmers," says Stobutzki. Her largest reef-fish larvae were no more than two centimeters long and prowess varied from species to species, but she found that the best - surgeonfish larvae - could swim without food or rest for eight days straight, traveling the equivalent of almost 100 kilometers. "In nature, where they can rest and get food, they could certainly go a lot further." Even so, the surgeonfish's marathon performance was like a person swimming the English Channel from Dover to Calais and back 10 times nonstop.
Stobutzki then came up with another surprise. At night, she put captured damselfish larvae in a closed chamber in the center of a small cage placed in the sea at several locations about 50 meters away from a coral reef. To leave time to motor her boat away and not distract the larvae with its sound, she rigged the chamber door temporarily closed with a candy. When the candy dissolved, the chamber door opened and the larvae were free to swim out.
| Larvae swim toward a reef, even when released far away. |
Most of them consistently headed toward the edge of the cage nearest the reef, regardless of where it was in relation to the reef and prevailing currents (and, of course, without benefit of the sun to orient themselves). Leis' team has conducted other capture and release tests at various distances, and early results suggest that larvae of at least some species will swim toward a reef even when released 100, 500, or 1,000 meters from it. Subsequent field observations have shown beyond doubt that some larvae are equally impressive for speed.
Along with collaborators at the University of Perpignan, France, and the Australian Institute of Marine Science, Leis' team has directly observed larval behavior in the open ocean and in reef lagoons of tropical Australia and French Polynesia. Keeping a wary eye out for circling sharks, inquisitive marlin, and annoying sucker fish, the scientists captured larvae with light traps at night, then released them one at a time in open water during the day, with two scuba-diving observers at the ready.
| Divers worked in pairs to follow larvae. |
Because it is extremely hard to see, one diver did nothing but follow a larva and watch it like a hawk. "You can't even take your eye off them for a moment to check your air pressure gauge or they disappear," Leis says. The other diver recorded the direction, depth, and speed at which each released larva swam for about 10 minutes, over a typical distance of about 120 meters. Repeated observations were compiled to reveal a more comprehensive picture of how and where the larvae moved.
They found that some could travel up to 50 cm a second, or 20 of their own body lengths. That's faster than most ocean currents they encounter and is akin to a person swimming 100 meters freestyle in 2.5 seconds. Indeed, the divers struggled to keep up with some of their quarry. They also observed that in daytime damselfish larvae consistently swam toward the southeast until about 3 P.M., after which they swam southwest, strongly suggesting that they could orient themselves to sunlight. In short, there's now no doubt that many tropical fish larvae are fast and strong swimmers, that they can locate reefs at considerable distances, and that they can purposefully swim toward them in specific and variable ways using environmental cues.
| Sunlight can play a role. |
But what are those cues? Sunlight clearly plays a role in some cases, but how did those larvae in the cage experiments know how to head for a reef at night?
Leis and Stobutzki both suspect that the larvae can pick up and track the rich and complex output of sound emitted by a reef. Hydrophones can relay such sounds to human ears - a bit like the sizzle of a frying pan - from at least 300 meters away.
Researchers at Auckland University in New Zealand have confirmed that some temperate fish larvae can hear and discriminate between sounds. Captured triple-fin blenny larvae were tested for their reactions to a variety of natural and artificial sounds played to them in daytime in open water a kilometer offshore in New Zealand waters. They swam in all directions in the absence of sound and when random tones known to be in the audible range of fish were played.
| Some larvae use sound to navigate. |
But when recorded marine sounds with biological meaning were played, most of the blennies headed straight for them. "We can say now that some larvae can hear sound, but we can't say yet whether they can use it to navigate," says Leis. "There's some evidence as well that in laboratory conditions some species use chemical signals in the water to navigate at close range to a specific place. It is uncertain how these fish know how to find anemones - perhaps by a chemical signal or visual recognition. During our diving observations, we found, time and again, that when some larvae got close to a reef, say 10 or 15 meters away, they would pick out a patch of sand or a piece of coral and make a beeline for it."
Taken together, these findings dismiss the idea of passive larvae at the mercy of the currents, and demand new concepts of how reef systems operate and new models with which to design and manage tropical marine reserves. Protecting one reef habitat may not, as we once thought, be of much benefit to fish populations on other reefs nearby. Each reef may be repopulating itself, and trouble may be in store if fisheries and reserve managers assume otherwise.
| We may need a variety of reserve designs. |
A single large reserve, for example, may be less useful than a string of smaller ones with the same total area in each reef system, Leis says. As well, reef animals other than fish can have very different dispersal strategies, so different reserve designs may be needed to conserve and sustain different species.
That might be inconvenient, but the world's coral reefs are shrinking and their future is uncertain - like their remarkable little fish larvae, their fate is too important to be left to mere chance.
Bob Beale is a freelance science and environment writer based in Sydney, Australia. He has been a journalist for 20 years.
Image collage from NOAA Sea Grant Program images. Photographer: James P. McVey.
Coral Reef Disturbance and Resilience in a Human-Dominated Environment - reviews recent research on the role of disturbance in coral reefs' ecosystems. From Trends in Ecology & Evolution, 2000, 15:10:413-417. Full text available from BioMedNet.
Coral Bleaching: Transcending Spatial and Temporal Scales - a report on the International Coral Reef Symposium that was held in Bali, Indonesia, from October 23-27, 2000. From Trends in Ecology & Evolution, 2001, 16:3:119-121. Full text available from BioMedNet.
MPAs Receive Strong Endorsement - results of a three-year international study of marine protected areas. Trends in Ecology & Evolution, 2001, 16:5:226. Full text available from BioMedNet.
Relative Swimming Speeds in Reef Fish Larvae - a recent article from Marine Ecology Progress Series, 211:299-303. Full text in PDF format.
Population Dynamics of Coral Reef Fishes and the Relative Abundance of their Early Life History Stage - an example from French Polynesia.
Coral Reef Fish Ecology Online Course - includes larval ecology.
NOAA's Coral Reef - offers a wealth of information on coral reefs. From the National Oceanic and Atmospheric Administration.
ReefBase - provides access to data and information on coral reefs and associated shallow tropical habitats.
Reef Research Quarterly - a newsletter published by the Great Barrier Reef Marine Park Authority. Back issues available online.
Australian Institute of Marine Science - includes links to the CoralBase Project, Coral Reefs and Mangroves: Modelling and Management, Long-Term Monitoring of the Great Barrier Reef: Status Report, and Research: Reef Monitoring Index.
Regional-Scale Assembly Rules and Biodiversity of Coral Reefs, Episodic Fluctuations in Larval Supply, Reef Migrations, Bleaching Effects Stir the Air in Bali, Coral Reef Biodiversity: Habitat Size Matters, Assembly Rules for Coral Reefs, Connectivity of Marine Populations: Open or Closed? - recent articles from Science. Paid registration required for full text.
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