BARATARIA, La. — To assess how heavy a blow the BP oil spill has dealt the Gulf of Mexico, researchers are closely watching a staple of the seafood industry and primary indicator of the ecosystem’s health: the blue crab.
Weeks ago, before engineers pumped in mud and cement to plug the gusher, scientists began finding specks of oil in crab larvae plucked from waters across the Gulf coast.
The government said last week that three-quarters of the spilled oil has been removed or naturally dissipated from the water. But the crab larvae discovery was an ominous sign that crude had already infiltrated the Gulf’s vast food web — and could affect it for years to come.
“It would suggest the oil has reached a position where it can start moving up the food chain instead of just hanging in the water,” said Bob Thomas, a biologist at Loyola University in New Orleans. “Something likely will eat those oiled larvae … and then that animal will be eaten by something bigger and so on.”
Tiny creatures might take in such low amounts of oil that they could survive, Thomas said. But those at the top of the chain, such as dolphins and tuna, could get fatal “megadoses.”
Marine biologists routinely gather shellfish for study. Since the spill began, many of the crab larvae collected have had the distinctive orange oil droplets, said Harriet Perry, a biologist with the University of Southern Mississippi’s Gulf Coast Research Laboratory.
“In my 42 years of studying crabs I’ve never seen this,” Perry said.
She wouldn’t estimate how much of the crab larvae are contaminated overall, but said about 40 percent of the area they are known to inhabit has been affected by oil from the spill.
While fish can metabolize dispersant and oil, crabs may accumulate the hydrocarbons, which could harm their ability to reproduce, Perry said in an earlier interview with Science magazine.
She told the magazine there are two encouraging signs for the wild larvae — they are alive when collected and may lose oil droplets when they molt.
Tulane University researchers are investigating whether the splotches also contain toxic chemical dispersants that were spread to break up the oil but have reached no conclusions, biologist Caz Taylor said.
If large numbers of blue crab larvae are tainted, their population is virtually certain to take a hit over the next year and perhaps longer, scientists say. The spawning season occurs between April and October, but the peak months are in July and August.
How large the die-off would be is unclear, Perry said. An estimated 207 million gallons of oil have spewed into the Gulf since an April 20 drilling rig explosion triggered the spill, and thousands of gallons of dispersant chemicals have been dumped.
Scientists will be focusing on crabs because they’re a “keystone species” that play a crucial role in the food web as both predator and prey, Perry said.
In this undated file photo provided by University of Southern Mississippi and Tulane University Gulf Coast Research Laboratory, small oil droplets are visible trapped inside the shell of an immature blue crab collected near Grand Isle, La. by researchers from the University of Southern Mississippi and Tulane University. Researchers wondering how badly the Gulf of Mexico will suffer from the oil spill are paying close attention to the blue crab. (AP Photo/USM Gulf Coast Research Laboratory)