The mummichog (Fundulus heteroclitus) is a small killifish found along the Atlantic coast of the United States and Canada. Also known as mummies, gudgeons, and mud minnows, these fish inhabit brackish and coastal waters including estuaries and salt marshes. The species is noted for its hardiness and ability to tolerate highly variable salinity, temperature fluctuations from 6 to 35 °C (43 to 95 °F), very low oxygen levels (down to 1 mg/L), and heavily polluted ecosystems.