There often are tiny bits of plastic in the fish and shellfish we eat. Scientists are racing to figure out what that means for our health.

In a laboratory at Columbia University’s Lamont-Doherty Earth Observatory, in Palisades, New York, Debra Lee Magadini positions a slide under a microscope and flicks on an ultraviolet light. Scrutinizing the liquefied digestive tract of a shrimp she bought at a fish market, she makes a tsk-ing sound. After examining every millimeter of the slide, she blurts, “This shrimp is fiber city!” Inside its gut, seven squiggles of plastic, dyed with Nile red stain, fluoresce.

All over the world, researchers like Magadini are staring through microscopes at tiny pieces of plastic—fibers, fragments, or microbeads—that have made their way into marine and freshwater species, both wild caught and farmed. Scientists have found microplastics in 114 aquatic species, and more than half of those end up on our dinner plates.