Geotimes - April 2003 - The Mighty, Tiny Larvae

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Coastal oceanography
The mighty, tiny larvae

Microscopic and with what seem like only minimal locomotive skills, some marine larvae are surprising scientists with their knack for staying close to home. A species by species investigation to identify dispersal patterns of marine fish and invertebrates has yielded an overall picture that coral reef and coastal critters have a far more complex relationship with their origin of birth than previously thought. These complexities are requiring marine scientists to revaluate the importance of physical oceanographic properties in the biological arena, even to the extent of the paleoceanographic environment.

Once considered drifters of the sea relegated to travel hundreds to thousands of kilometers with the dominant surface currents, larvae are now known to swim upstream, change their depth in the water column and interact with the dynamic currents that eddy along the shore. This shift in understanding has come about primarily because of increased efforts and technologies during the last few years to identify the genetic diversity between animal populations and to track larval dispersal patterns.

Scientists investigating larval dispersal of the Acorn barnacle collected adults from locations along the Oregon coast indicated at left. They found that nearshore features of oceanography, for example eddies, retain the larvae inshore — preventing the larvae from moving offshore and being transported by huge boundary currents such as the California Current. The Heceta Bank eddy shown at right is created when the southward California Current hits the Heceta Bank. Image by Erik Sotka, Hopkins Marine Station Stanford University.

“Until recently, larvae were thought to be weak swimmers and therefore dispersed passively by oceanic currents,” says Michael Taylor, a Ph.D. student at Louisiana State University and first author of a report in the Jan. 3 Science. The report provided genetic evidence that a species of Caribbean reef fish comes home after its 21-day larval stage in the island currents. Previous models had estimated the dispersal of the fish by basically “plunking down a passive particle and allowing it to drift” for 21 days, explains Robert Warner of the department of ecology, evolution and marine biology at the University of California at Santa Barbara. Adds Taylor, “These two factors, PLD [Pelagic Larval Duration, the time spent for each species in the larval stage] and ocean currents, became the foundation for fisheries management issues and the design of marine reserves.”

Warner heralded Taylor and his advisor Michael Hellberg’s work in both a written commentary in Science and at this year’s meeting of the American Association for the Advancement of Science (AAAS) in Denver in February. Warner works in collaboration with the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO), a consortium of universities from the West Coast. At the February meeting, PISCO researchers stressed the importance of understanding larval dispersal for improving fisheries and marine reserve management and discussed the technologies that are enabling that understanding.

Genetics, microchemical analyses of hard parts such as ear bones, or otoliths, and more sophisticated physical flow models of the ocean currents “suggest a sizable fraction of locally produced young might not disperse too far away,” Warner says. He explained that most scientists determining larval dispersal in the past relied on physical models of the ocean current that were large-scale, often 2-D models of mean flow.

Steve Palumbi of the Hopkins Marine Station at Stanford University is looking at the genetic differences of barnacles, work that implies gyres of ocean currents along the coast are helping to prevent barnacle larvae from washing out to sea. While marine scientists can still rely on the old models for the open ocean, he explains, near-shore water motion is much slower and more complicated. “Without a good idea of how near-shore waters are transported, larval durations have just been guesses.” Together with their colleagues at PISCO, Warner and Palumbi are working on creating a chemical road atlas for the Pacific Coast that will help map where in the ocean currents larvae species are dispersing.

Looking for a marine geologic trend, in 2000 Paul Barber of Boston University tested larval dispersal predictions and genetic connections among a species of shrimp in Indonesia. Barber concentrated on the paleo-significance of physical ocean properties. He found genetic patterns that appear to have origins during lowered sea levels in the Pleistocene that were maintained for at least 10,000 years following the rise of sea levels. “This result indicates that despite 10,000 years of modern oceanographic conditions, there is insufficient dispersal among these geographic regions in Indonesia to blur these historical genetic boundaries,” he says. So even when sea level allows for an interactive community of fish among different islands, history shows they stay put despite the currents.

Understanding that larvae play a more active role in the ocean processes and have perhaps an evolutionary history of staying close to home provides impetus for shoreline communities to protect local habitats. Adds Barber, “We really know so very little about marine larval dispersal. We’ve only just figured out that it doesn’t always happen the way that we thought.”

Christina Reed

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