Endocrinological Studies on the Euryhaline Killifish, Fundulus Heteroclitus (Linn.) **

The common killifish of the eastern United States, Fundulus heteroclitus (Linn.), is widely employed as an experimental animal by zoologists, em-bryologists, 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.hetero-clitus 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 salt to fresh water. It may be kept indefinitely in either medium under laboratory conditions and is tolerant of standing water provided the tanks are well aerated and not overcrowded. Endocrinological studies on Fundulus heteroclitus were begun in the Bingham Oceanographic Laboratory in 1947. At that time a number of investigations, reviewed by Atz," had been concerned with the hormonal regulation of the reproductive cycle, Albert' had studied the thyroid and exophthalmos, and a substantial body of work, reviewed by Parker,' had been devoted to the chromatophore-regulating hormones. Dr. Ernest F. Thompson and I were interested in a different problem: the "year marks" on the scales of fishes. The formation of the annulus is correlated with the onset of spring growth, and we thought that it might be possible to induce an artificial off-season annulus by administration of growth hormone. Although our first experiment was, in this respect, unsuccessful it served to * Research Associate; Lecturer in Zoology.