BARATARIA, La. (AP) — 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.”