Abstract

Ponyfishes emit light produced by symbiotic bacteria residing in an oesophageal diverticulum. A dorsal and, in some species, either a ventral or pair of lateral ‘shutters’ on the wall of this organ allow the duration and intensity of light emission to be controlled by the fish. By differential use of the shutters, the site on the body wall that is illuminated can also be varied. Four different light displays have been described from captive specimens of various species. We record a spectacular light display in schooling Leiognathus splendens in the field at Ambon, Indonesia. The display occurred at night. Fish emitted brief flashes of light at high frequency over extended periods. All the fish in a school synchronized their flashes, the pooled light markedly increasing underwater visibility for a human observer. Possible explanations for this display include spatial organization of the school, location of planktonic prey, courtship and reduction of attacks by predators.