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 6 to 35 °C (43 to 95 °F), very low oxygen levels (down to 1 mg/L), and heavily polluted ecosystems.

Colors are more intense in males during the reproductive season, as they become dark olive-green on the back, steel-blue on the sides with about 15 silvery bars, and yellow or orange-yellow on the underside; the dorsal fin is mottled and a small eyespot may be present near the rear edge.

[15] The two species may overlap in their choice of habitat, but in general the banded killifish is more commonly found in freshwater, which is not the case for the mummichog.

It is present on Sable Island, 175 km (109 mi) southeast of the closest point of mainland Nova Scotia in the Atlantic Ocean.

[citation needed] Introduced populations have become established on the Atlantic coast of Portugal and southwestern Spain, starting in the 1970s[17][18][19] and some have now reached the western Mediterranean basin.

[22] The mummichog is a common fish in coastal habitats such as salt marshes, muddy creeks, tidal channels, brackish estuaries, eelgrass or cordgrass beds, and sheltered shorelines.

A few landlocked populations may exist in freshwater lakes close to shore, for example on Digby Neck, Nova Scotia.

This is partly achieved by varying the isoenzyme of the lactate dehydrogenase (Ldh-B) enzyme expressed in warm or cold waters.

[27] Based on genetic studies, the enzymes serum esterase (SERE) and malate dehydrogenase (MDH) also appear to play an important role in mummichog temperature control.

[33] One study[34] has looked at the genomic variation exhibited by mummichogs populations living in Newark Bay, New Bedford Harbor, and the Elizabeth River (Virginia) (in some areas heavily polluted with polychlorinated biphenyls and creosote, a complex mixture containing dioxin-like chemicals) and has found that about 20% of their genes were modified as compared to populations living in clean sites.

[citation needed] During cold winter months in the northern parts of their range, mummichogs move to upstream tidal pools, where they burrow into the mud at depths up to 20 cm (7.9 in) to overwinter.

[37] In the laboratory, mummichogs have yielded clear examples of free-running circadian rhythms, in both body colour[38] and swimming activity.

Evidence of free-running semi-lunar rhythms have also been obtained in mummichogs: in constant laboratory conditions, egg production peaked every 14.8 days for up to 5 months.

[13] The eggs adhere to plants, algal mats, empty mussel shells, sand, or mud at sites that are reached by water only at high spring tides.

[13] Mummichogs are hosts to a parasitic fluke, Homalometron pallidum, which has a complex lifecycle involving the aquatic snail, Ecrobia truncata.

[26] They are the most popular baitfish species in the Northeast of America and traditionally when used as bait they were lip hooked and then dressed with a piece of squid.

[53][54] With the successful sequencing and assembly of the full killifish genome,[55] they serve as a premier scientific model for studying biochemical and physiological responses to varying environmental conditions.

[56] Their remarkable ability to tolerate various extremes of temperature and salinity has made them popular subjects in scientific studies of toxicology.

[58] Killifish eggs are used in developmental studies and when teaching embryology because the eyes, the beating heart, and the different stages of ontogenesis can be easily examined.

In the absence of gravity the fish at first exhibited an unusual swimming behavior: they constantly pitched forward and therefore described tight circles.