Small pieces of plastic in the ocean are harming worms that are food for many other species.

Pieces 5mm in size shouldn't be called "microplastics". Perhaps some pieces are small enough to deserve that name.

Once the worms have a low level of toxins, the animals that eat the worms will accumulate higher levels, and the animals that eat those will get even higher levels. The same thing happens with mercury, which is why tuna have so much mercury that they are dangerous for humans to eat.