Seafood from the Gulf of Mexico is being put under the microscope like no other kind on the market, with fish, shrimp and other catches ground up to hunt for minute traces of oil — far more reassuring than that sniff test that made all the headlines. And while the dispersant that was dumped into the massive oil spill has consumers nervous, health regulators contend there's no evidence it builds up in seafood — although they're working to create a test for it, just in case. More Gulf waters are reopening to commercial hauls as tests show little hazard from oil, and Louisiana's fall shrimp season kicks off Monday. Yet it's too soon to know what safety testing will satisfy a public so skeptical of government reassurances that even local fishermen voice concern. Basic biology is key: Some species clear oil contamination out of their bodies far more rapidly than others. Fish are the fastest, oysters and crabs the slowest, and shrimp somewhere in between.