The Sensitivity of the Goldfish (Carassius auratus L.) to Point Heat Stimulation

By conditioned reflex training it was found that goldfish sense ambient temperature increments of about .1⚬C over their entire body surface. Even the fins bear warm sensitive nerve endings. The receptors apparently operate on the principle of areal summation with an instantaneous heat increment of 2⚬C as the lower limit to which a fish would react when an area of 2 sq. mm was stimulated with heated probes after it had been trained not to react to touch stimuli. An estimate of the total number of units involved in relative temperature discrimination suggested that several thousand units are involved; this number is of a comparable order of magnitude to that found in other vertebrates with a similarly fine or finer level of heat discriminatory capacity. This order of magnitude of unit receptors is found, be they collected in special organs or scattered over the entire body surface.