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. Diet Mummichogs are agastric omnivores. Analyses of their gut contents have found diatoms, amphipods and other crustaceans, molluscs, fish eggs (including their own species), very small fish, insect larvae, and bits of eelgrass. Physiology This fish is well known for its ability to withstand a variety of environmental conditions. They can survive temperatures between ; even within the same tidal cycle they can tolerate rapid temperature changes from . They are able to survive this vast temperature range by altering their metabolic rates at high and low temperatures. This is partly achieved by varying the isoenzyme of the lactate dehydrogenase (Ldh-B) enzyme expressed in warm or cold waters. These two versions of the enzyme allow for faster catalytic function and metabolism depending on if the fish is in northern, colder waters or southern, warmer waters. Based on genetic studies, the enzymes serum esterase (SERE) and malate dehydrogenase (MDH) also appear to play an important role in mummichog temperature control. They are also among fish species most tolerant of salinity changes (euryhaline). 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) (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. Behavior Mummichogs live in dense shoals that can include several hundred individuals. During cold winter months in the northern parts of their range, mummichogs move to