In 1956, people in Minamata, Japan, began to exhibit severe neurological disorders and birth defects
This problem was eventually attributed to methylmercury, a water-insoluble and breakdown-resistant chemical formed from mercury dumped into the adjacent bay by a local factory. Which of the following types of seafood would have the highest levels of methylmercury, and therefore cause the severest effects?
A) Krill (small crustaceans that eat tiny microscopic algae)
B) Tuna that feed on smaller fish that feed on krill that feed on algae
C) Sea urchins that feed on kelp
D) Kelp (large, multicellular, photosynthetic protists)
E) Mussels that feed on tiny microscopic algae

Methyl Mercury is a persistent substance which is not easily biodegradable, when released to the water body it is ingested, and can build up in living organisms, or bioaccumulate.

This build up in living organisms, inflicts a spiraling levels of harm as the food chain ascends, for example on higher order species such as predatory fish and fish eating birds and mammals — through a process know as "biomagnification".

The most affected is the species highest in the order. The tuna fish eats smaller fishes and is therefore most affected through biomagnification of mercury bioaccumulated food it ate.

Thus the Tuna fish have the highest levels of methylmercury, and therefore feels the severest effect.