The common killifish of the eastern United States, Fundulus heteroclitus (Linn.), is widely employed as an experimental animal by zoologists, embryologists, and comparative physiologists. The late Dr. Myron Gordon8' points out that it is a hardy species of convenient size, readily caught in tidal pools but not usually bred in aquaria for purposes of raising the stock. Dr. Gordon warns against confusion with the striped killifish, Fundulus majalis, which is more delicate and will not stand up as well under aquarium conditions. During the winter, in the latitude of New Haven, Connecticut, F.heteroclitus passes into a state of sexual regression, hibernating in the mud. In spring it may be caught in large numbers in estuarine waters where it breeds in the reed beds during June and July. The young become sexually mature the following year, at a length of about one inch, and fish of approximately 3 inches in length are usually in their second year. Unlike F.majalis, F.heteroclitus is completely euryhaline, moving freely from sal
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