Mollusks (or Shellfish)
Examples: clams, mussels, oysters
Mollusks are important prey for many animals, including otters. If otters eat oil-covered mollusks, then they are ingesting oil, which can harm their organs. Also, because most mollusks are filter feeders, they might have very high levels of dangerous chemicals in them, making eating them even more dangerous to humans and wildlife.
When ingested, oil usually kills them outright. If they survive, oil can cause other problems, like weakening their protective shells.