Nervous Control of Chromatophores in Teleost Fishes

Changes ot skin colour with temperature are remarkably consistent in reptiles; high temperatures cause paling of the skin while low temperatures induce darkening. The same rule generally applies to the Amphibia (Parker, 1948), although exceptions have frequently been noted. But in teleost fishes the responses are often ill-defined, unpredictable and even contradictory. There is a large literature of observations on these and other groups of animals, (listed in Pye, 1961). Von Frisch (191 ib) suggested that complex factors such as respiratory stress are involved when a fish is immersed in water of different temperatures. He therefore removed minnows (Phoxinus) from the water, giving them an oral supply of cool aquarium water to breathe, and restricted temperature changes to a separate jet of water playing on to the flank. Under these conditions a remarkably powerful and consistent reaction was elicited. An area of skin warmed to 35° C. showed maximum melanophore dispersion, appearing black, while cooling to 2° C. produced maximum aggregation to give a very pale skin colour. Both effects were extremely local in extent and could be produced simultaneously by playing jets on to opposite flanks or on to adjacent areas of the same flank. Both were completely reversible at the intermediate temperature of 15 C. Following surgical section of the chromatic motor tracts in the sympathetic chain anterior to vertebra 15 (Fig. 1) these responses disappeared in the anterior, chromatically paralysed region of the body and were occasionally replaced by weaker responses in the opposite direction, that is by paling at high temperatures and darkening at low temperatures. The posterior, chromatically intact, region was unaffected by the nerve section and continued to give the strong normal response. Smith (1928) repeated these experiments on the killifish (Fundulus heteroclitns) and was able to confirm them except for one point. Following section of the chromatic motor tract, the ' reversed' responses of Fundulus melanophores were always distinct and regular. Smith further showed that in the normal fish high temperatures below 28° C. produced no response, while temperatures above 29 C. produced a maximum response regardless of the application time. On the other hand reversed responses