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October 12, 2001

Billfish DNA

New molecular technology promises to answer the most challenging and important question of billfish biology -- where do they come from?

New molecular technology promises to answer the most challenging and important question of billfish biology -- where do they come from?
We know very little about when or where billfish spawn, says Dr. John Graves of the Virginia Institute of Marine Science. One obstacle is the difficulty in identifying billfish species from roe and larvae. Scientists have collected samples of billfish larvae but have not always been able to tell them apart based on the way they look, Graves said.

Billfish change markedly as they grow. For example, in extensive investigations of pelagic larvae in the Pacific, researchers mis-identified black marlin larvae as sailfish.

The problem is compounded in areas where many different types of billfish congregate. In the Florida Straits and around Hawaii, for example, the larvae from all of the local billfish species can be found together. Major features like fins and body shapes don't help identify species at these stages, so scientists are looking for answers in the molecular markers found in DNA.

Molecular markers provide definitive identification of larvae as well as information on spawning season, Graves said. Combining this knowledge with age information learned from certain bones and movement data from prevailing currents, "we can determine where spawning occurred," Graves said. Identifying spawning times and areas leads to identifying essential fish habitat.

Finding molecular markers to identify different billfish is still in its early stages, however. Several years ago Graves received a grant to find markers that differed between species. Jan McDowell, a doctoral student, took on the project and developed two markers that differentiate the different billfish species. These markers will provide a new tool to probe roe and larvae.

Once McDowell had the technique, she needed billfish specimens. VIMS turned to the University of Miami. "Stacy Luthy, a graduate student at the University of Miami, was trying to find physical characteristics to identify larval billfish. Working together they developed a method that allows them to make an identification with only the tissue from one eyeball of a 3-mm larva. As a result, the two have now identified hundreds of larvae," says Graves.
The next step is to use the molecular data to take another look at external features and develop a definitive guide to identifying baby billfish by the way they look. "This information will allow us to find out when and where billfish spawn -- information that is pretty essential for management," Graves said. "Without being able to identify the larvae, it was anyone's guess as to who was spawning when and where."