Abstract
When ambient temperature is decreased, mammals and birds (endotherms) shiver reflexively to maintain core temperature, whereas reptiles, amphibians or fishes (ectotherms) do not shiver. It is not known, however, when shivering capacity appears in vertebrates. Here we show that shivering capacity appears in newborn fishes. When newborn zebrafish (3 dpf, days postfertilization), reared at 28 oC, were released to a pool of low temperature (9-18 oC), these fishes shook their bodies vigorously and rhythmically for some seconds. Cooling-induced shaking disappeared in adult fishes. Because newborn fishes do not move forward despite shaking, shaking-released energy should be used for heat production. However, thermogenesis in water is not effective for thermoregulation. Therefore, shivering capacity observed in newborn fishes may be inhibited in adult fishes. When decerebrated, 3 dpf fishes still showed cooling-induced body shaking, indicating that this response is physiological reflex. In convulsion of mammals, uncontrollable electric activity in the brain induces seizure. In fishes, even when decerebrated, cooling-induced shaking occurs. Therefore, cooling-induced shaking of zebrafish is not convulsion. We conclude that shivering capacity appears in fishes, which is the origin of the endothermy in vertebrates. [J Physiol Sci. 2007;57 Suppl:S181]
