Because crabs don't filter feed and can't build up high concentrations of toxins, they're usually safe for humans to eat -- even when there is a red tide. Crab fisheries can still experience a slowdown in sales because of the negative stigma attached to red tides. Red tides can harm humans, because the toxins that shellfish can absorb can cause deadly paralysis in humans. Take every precaution when consuming seafood when a red tide has been discovered.