Type of fossil that forms with an organism decays leaving a cavity behind

What kind of seafood would you expect to have the highest levels of mercury and why? kelp, because they use the sun to photosynthesize on the bottom trophic level. oysters, because they filter feed on plankton on a lower trophic level. small fish, because they eat plankton on a higher trophic level. sharks, because they feed on organisms on a higher trophic level.

Would really appreciate it, i have an idea but not completely sure

Sweating and breathing is an example of differentiation specialization maintaining homeostasis metabolism