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Parasitic Worms in a Fish

The whitish spots on this fish are individual parasitic trematode worms.
(Hans Hillewaert)

The whitish spots on this fish are individual parasitic trematode worms. Trematodes have complicated life cycles that usually involve multiple hosts -- often starting in a snail and then moving on to other hosts, such as fish, birds, and mammals (including humans). They may have even lived in dinosaurs! Read more about parasites in marine organisms