The mummichog (Fundulus heteroclitus) is a small killifish found along the Atlantic coast of the United States and Canada. Also known as Atlantic killifish, 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 , very low oxygen levels (down to 1 mg/L), and heavily polluted ecosystems. As a result, the mummichog is a popular research subject in embryological, physiological, and toxicological studies. It is also the first fish ever sent to space, aboard Skylab in 1973. Taxonomy The genus name Fundulus comes from , meaning bottom, from the fish's habit of swimming near muddy bottoms. The species name heteroclitus means irregular or unusual. The type specimen was first described by Carl Linnaeus in 1766, from near Charleston, South Carolina. Other scientific names now considered synonyms for this species include Cobitis heteroclita, Fundulus fasciatus, Fundulus pisculentus, and Fundulus nigrofasciatus. The mummichog belongs to the order Cyprinodontiformes, and the family Fundulidae. There are two subspecies: F. h. heteroclitus (Linnaeus, 1766), in the south and F. h. macrolepidotus (Walbaum, 1792) in the north. As Fundulus mudfish this species was designated the type species of the genus Fundulus when he created the genus. The name mummichog is derived from a Narragansett term which means "going in crowds", which reflects the mummichog's strong shoaling tendency. Colloquial names include mummy, killie, kelley, chub, salt water minnow, mud minnow, mud dabbler, marsh minnow, brackish water chub, gudgeon, and common killifish. Some of these terms may lead to confusion: the term minnow should be reserved for species of the family Cyprinidae, the mudminnows are members of the family Umbridae, and the name gudgeon is used for various bottom-dwelling species of cyprinid, eleotrid, and ptereleotrid fishes, none of which belongs to the same family as the fundulid mummichog. Description The body of the mummichog is elongate but thick, with a deep caudal peduncle. Usual length is but a maximum length of up to is possible. The mouth is upturned and the lower jaw protrudes when the mouth is closed. Pectoral and tail fins are round. Mummichogs have 10-13 dorsal fin rays, 9-12 anal fin rays, 16-20 pectoral fin rays. Males have larger dorsal and anal fins than females. There is no lateral line on the body, but lateral line pores are present on the