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 °C to 35 °C (43 °F to 95 °F), very low oxygen levels (down to 1 mg/L), a wide variety of toxins, 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.
Etymology and taxonomy
Fundulus comes from fundus, meaning bottom, from the fish's habit of swimming near muddy bottoms. Heteroclitus means irregular or unusual. The type specimen was first described by Linneaus in 1766, from near Charleston, South Carolina. Past scientific names 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 southern half of the range, and F. h. macrolepidotus (Walbaum, 1792) in the northern half of the range. The approximate geographical division between the two lies in New Jersey, Delaware and Virginia. As opposed to their northern counterparts, the southern subspecies have eggs that lack filaments (adhesive chorionic fibrils) and they often deposit those eggs inside empty mussel shells. The two subspecies are also distinguished based on slight morphological and genomic differences.
The body of the mummichog is elongate but rather thick, with a deep caudal peduncle. Usual length is 75 to 90 mm (3 to 3.5 inches) but maximum lengths of 130 to 150 mm (5 to 6 inches) are possible. The mouth is upturned and the lower jaw protrudes when the mouth is closed. Pectoral and tail fins are round. The number of fin rays is: dorsal 10-13, anal 9-12, pectoral 16-20. 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 head. The colour is variable (and may even change in shade within the same individual when placed near different backgrounds)  but is generally olive-brown or olive-green. There can be vertical bars on the sides that are thin, wavy, and silvery. 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. Females tend to be paler, without bars, without the intense yellow on the belly, and their dorsal fin is uniformly coloured.
The mummichog is very similar to the banded killifish, Fundulus diaphanus, and indeed the two species have been known to interbreed. 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. The banded killifish tends to have thin dark bars on a light side, whereas in the mummichog the bars are thin and light on a dark side. Internally, the banded killifish has 4-7 gill rakers, as opposed to 8-12 in the mummichog.
This species ranges along the Atlantic coast of North America, from Gaspé Peninsula, Anticosti Island and Port au Port Bay in the north, to northeastern Florida in the south. It is present on Sable Island, 175 km (109 mi) southeast of the closest point of mainland Nova Scotia in the Atlantic Ocean. Introduced populations have established themselves on the Atlantic coast of Portugal and southwestern Spain, starting in the 1970s.    There may also be introduced populations in Hawaii and the Phillipines. As bait fish, mummichogs are sometimes released in freshwater habitats, where they can survive, and there has been reports of individuals in New Hampshire ponds, as well as the upper Ohio River and Beaver River (Pennsylvania).
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. It can be found within coastal rivers but seldom beyond the head of tide. A few landlocked populations may exist in freshwater lakes close to shore, for example on Digby Neck, Nova Scotia.
Mummichogs are omnivorous. Analyses of their stomach contents have found diatoms, amphipods and other crustaceans, molluscs, fish eggs (including their own species), very small fish, insect larvae, and bits of eelgrass.
Spawning takes place from spring through fall. In the southermost populations, up to eight spawnings are possible in a season. Spawning takes place most often at high tide and when the moon is new or full. Maximal spawning occurs when high spring tides coincide with night, though spawning during the day remains possible.
During courtship, males may pursue females, and females may attract males by turning on their sides near the bottom and flicking their tails. A male and female may swim together for a while, after which the male crowds the female against a rock or a plant and clasps her: the male's larger dorsal and anal fins curve around the female's body. Fingerlike projections that develop on the male's scales behind and below the dorsal fin may help the male maintain contact with the female. The pair quivers vigorously and eggs and sperm are released.
The eggs are pale yellow, about 2 mm in diameter, and strongly adhesive. During a spawning event, a female can deposit up to 740 eggs in separate clutches of 10 to 300 eggs at a time. 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. Eggs therefore develop while exposed to moist air, and they hatch when the next high spring tides reach them.
Most mummichogs become sexually mature when two years old, around 38 mm in length. Normal lifespan is four years.
Mummichog live in dense shoals that can include several hundred individuals.
During cold winter months in the northern parts of their range, mummichogs move to upstream tidal pools, where they burrow into the mud to depths up to 20 cm (8 inches) to overwinter. They can also bury themselves in mud if they are caught in a drying tidal pool between spring tides. Alternatively, they can travel short distances on land to get back to the sea.
This fish is well known for its ability to withstand a variety of environmental conditions. They can survive temperatures between 6 °C and 35 °C; even within the same tidal cycle they can tolerate rapid temperature changes from 15 °C to 30 °C. They are among the most euryhaline (tolerant of salinity changes) fishes. Mummichog larvae can grow in salinities ranging from 0.4 to 100 parts per thousand, the latter being about three times the normal salinity of seawater. Adult mummichogs tolerate low oxygen levels down to 1 mg/L, at which they resort to aquatic surface respiration (breathing in the surface layer of water, richer in oxygen because of contact with air) to survive.  They can even survive for a few hours in moist air outside of water, breathing air directly.
Populations have developed resistance to methylmercury, kepone, dioxins, polychlorinated biphenyl, and polyaromatic hydrocarbons. One study has looked at the genomic variation exhibited by mummichogs populations living in Newark Bay, New Bedford Harbor, and the Elizabeth River (Virginia) (heavily polluted with dioxins, polychlorinated biphenyls, and creosote, respectively) and has found that about 20% of their genes were modified as compared to populations living in clean sites.
Mummichogs are hosts to a parasitic fluke, Homalometron pallidum, which has a complex lifecycle which also involves the aquatic snail, Ecrobia truncata. Other parasite species repertoried in mummichogs include 10 various protozoans, eight trematodes, one nematode, two acanthocephalans, and two crustaceans.
Interest to humans
Mummichogs readily eat mosquito larvae and attempts have been made to use them as biocontrol agents of mosquito populations, though nothing concrete seems to have come out of these trials.
Mummichog eggs are often used in teaching embryology, because the eyes, beating heart, and various stages of development can easily be seen.
Mummichogs are relatively abundant and easily captured. Their remarkable ability to tolerate various extremes of temperature and salinity has made them popular subjects in scientific studies of toxicology and osmoregulation. They have also been the subject of many studies in endocrinology, stress biology, circadian rhythms and other aspects of physiology.
Mummichogs were the first fish sent to space. In 1973 a couple of them were flown in a plastic bag aquarium aboard Skylab, during the Skylab 3 mission. In the absence of gravity the fish at first exhibited an unusual swimming behavior: they constantly pitched forward and therefore described tight circles. However, by day 22 of the mission they swam normally. Fifty eggs at an advanced stage of development had also been taken on board, and 48 of them hatched during the flight. The hatchlings swam normally. More experiments with mummichogs in space followed as part of the Apollo-Soyuz Test Project and as part of a biological package aboard the Bion 3 satellite.
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, 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 Cyprinidae family, the mudminnows are members of the Umbridae family, 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.
Fundulus heteroclitus taken in the Chesapeake Bay
- Scott, W.B., and Crossman, E.J. 1973. Freshwater fishes of Canada. Bulletin 184 of the Fisheries Research Board of Canada, Ottawa.
- Morin, R.P. and Able, K.W. 1983. Patterns of geographic variation in the egg morphology of the fundulid fish, Fundulus heteroclitus. Copeia 1983: 726-740.
- Coad, B.W. 1995. Encyclopedia of Canadian Fishes. Canadian Museum of Nature, Ottawa, 928p.
- Taylor, M.H. 1986. Environmental and endocrine influences on reproduction of Fundulus heteroclitus. American Zoologist 26: 159-171.
- Able, K.W. and Felley, J.D. 1986. Geographical variation in Fundulus heteroclitus: tests for concordance between egg and adult morphologies. American Zoologist 26: 145-157.
- Brown, B.L. and Chapman, R.W. 1991. Gene flow and mitochondrial DNA variation in the killifish, Fundulus heteroclitus. Evolution 45: 1147-1161.
- Connolly, C.J. 1925. Adaptive changes in shades and color of Fundulus. Biological Bulletin (Woods Hole) 48: 56-77.
- Bagnara, J.T., and Hadley, M.E. 1973. Chromatophores and color change. Prentice-Hall, New Jersey.
- Hubbs, C.L., Walker, B.W., and Johnson, R.E. 1943. Hybridization in nature between species of American cyprinodont fishes. Contributions to the Laboratory of Vertebrate Biology of the University of Michigan 23: 21 p.
- Garside, E.T. 1969. Distribution of insular fishes of Sable Island, Nova Scotia. Journal of the Fisheries Research Board of Canada 26: 1390-1392.
- Hernando, J.A., 1975. Nuevas localidades de Valencia hispanica (Pisces: Ciprinodontidae) en el Suroeste de España. Doñana Acta Vertebrata 2: 265-267.
- Coelho, M., Gomes, J., and Ré, P.B. 1976. Valencia hispanica, a new fish to Portugal. Arquivos do Museu Bocage 6: 1-3.
- Gutiérrez-Estrada, J.C., Prenda, J., Oliva, F, and Fernandez-Delgado, C. 1998. Distribution and habitat preferences of the introduced mummichog Fundulus heteroclitus (Linneaus) in South-western Spain. Estuarine Coastal and Shelf Science 46: 827-835.
- Fish Base Mummichog distribution
- USGS Nonindigenous aquatic species database Mummichog occurrences
- Klawe, W.L. 1957. Common mummichog and newt in a lake on Digby Neck, Nova Scotia. Canadian Field-Naturalist 71: 154-155.
- Taylor, M.H., Leach, G.J., DiMichele, L., Levithan, W.H., and Jacob, W.F. 1979. Lunar spawning cycle in the mummichog, Fundulus Heteroclitus (Pisces: Cyprinodontidae). Copeia 1979: 291-297.
- Newman, H.H. 1907. Spawning behavior and sexual dimorphism in Fundulus heteroclitus and allied fish. Biological Bulletin (Woods Hole) 12: 314-345.
- Taylor, M.H., DiMichele, L., and Leach, G.J. 1977. Egg stranding in the life cycle of the mummichog Fundulus heteroclitus. Copeia: 1977: 397-399.
- Taylor, M.H., and DiMichele, L. 1983. Spawning site utilization in a Delaware population of Fundulus heteroclitus (Pisces: Cyprinodontidae). Copeia 1983: 719-725.
- Taylor, M.H. 1999. A suite of adaptations for intertidal spawning. American Zoologist 39: 313-320.
- Chidester, F.E. 1920. The behavior of Fundulus heteroclitus in the salt marshes of New Jersey. American Naturalist 54: 244-245.
- Raposa, K. 2003. Overwintering habitat selection by the mummichog, Fundulus Heteroclitus, in a Cape Cod (USA) salt marsh. Wetlands Ecology and Management 11: 175-182.
- Mast, S. O. 1915. The behavior of fundulus, with especial reference to overland escape from tide-pools and locomotion on land. Journal of Animal Behavior 5: 341-350. http://dx.doi.org/10.1037/h0075747
- Burnett, K.G., Bain, L.J., Baldwin, D.S., et al. 2007. Fundulus as the premier teleost model in environmental biology: Opportunities for new insights using genomics. Comparative Biochemistry and Physiology (D) 2: 257-266.
- Abraham, B.J. 1985. Species Profiles: Life histories and environmental requirements of coastal fishes and invertebrates (Mid-Atlantic)--mummichog and striped killifish. U.S. Fish and Wildlife Service Biological Reports 82 (11.40): 23 p. http://www.nwrc.usgs.gov/wdb/pub/species_profiles/82_11-040.pdf
- Whitehead, A. 2010. The evolutionary radiation of diverse osmotolerant physiologies in killifish (Fundulus sp.). Evolution 64: 2070-2085.
- Wannamaker, C.M., and Rice, J.A. 2000. Effects of hypoxia on movements and behavior of selected estuarine organisms from the southeastern United States. Journal of Experimental Marine Biology and Ecology 249: 145-163.
- Stierhoff, K.L., Targett, T.E., and Grecay, P.A. 2003. Hypoxia tolerance of the mummichog: the role of access to the water surface. Journal of Fish Biology 63: 580-592.
- Weis, J,S. 2002. Tolerance to environmental contaminants in the mummichog, Fundulus heteroclitus. Human and Ecological Risk Assessment 8: 933-953.
- Whitehead, A., Galvez, F., Zhang, S., Williams, L.M., and Oleksiak, M.F. 2011. Functional genomics of physiological plasticity and local adaptation in killifish. Journal of Heredity 102: 499-511. doi: 10.1093/jhered/esq077
- Stunkard, Horace W. (1964). "The morphology, life history and systematics of the digenetic trematode Homalometron pallidum Stafford 1904". The Biological Bulletin 126 (1): 163–173. doi:10.2307/1539426.
- Hoffman, G.L. 1967. Parasites of North American frshwater fishes. University of California Press, Berkeley, 486 pp.
- Kavaliers, M. 1980. Social groupings and circadian activity of the killifish, Fundulus heteroclitus. Biological Bulletin 158: 69-76.
- Kavaliers, M., and Abbott, F.S. 1977. Rhythmic colour change of the killifish, Fundulus heteroclitus. Canadian Journal of Zoology 55: 553-561.
- Reebs, S.G. (2009) Fish in space Retrieved 12 December 2014.
- von Baumgarten, R.J., Simmonds, R.C., Boyd, J.F., and Garriott, O.K. 1975. Effects of prolonged weightlessness on the swimming pattern of fish aboard Skylab 3. Aviation Space and Environmental Medicine 46: 902-906.
- Hoffman, R.B., Salinas, G.A., and Baky, A.A. 1977. Behavioral analyses of killifish exposed to weightlessness in the Apollo-Soyuz test project. Aviation Space and Environmental Medicine 48: 712-717.
- "Mummichog." Merriam-Webster.com. Merriam-Webster, n.d. Web. 6 Feb. 2014. <http://www.merriam-webster.com/dictionary/mummichog>