Reflectors in the light organ ofAnomalops(Anomalopidae, Teleostei)

Abstract

The suborbital light organ ofAnomalopscontains luminous bacteria which are housed in tubules perpendicular to the outer surface. It has two reflectors. One (external) is differentially transparent and lies in front of the tubules. The other (internal) is opaque, lies behind the tubules, and forms a concave backing to the organ. The internal reflector consists of a thick layer of cells containing stacks or packets of hexagonal crystals. The reflecting material is guanine. Reflectivity is high throughout most of the visible spectrum: about 70%. The internal reflector has a mat white appearance, and the diffuse reflectance comes about because the outer crystal stacks follow the curved bases of the tubules and have, therefore, diverse orientations. Stacks located in deep cells have the most varied orientations and may or may not contribute to the reflectivity. Oxygen diffusion from the organ may be reduced by the dense packing of crystals and thickness of this reflector. The external reflector lies at the lower ventral edge of the outer face of the organ and consists of a layer of cells containing a row of thin crystals lying oblique to the surface. It is a specular reflector which directs the luminescence outwards and upwards and dims light emission ventrally. In both reflectors the crystals form a system of quarter-wavelength films, reflecting light by constructive interference. The crystals are enclosed in membrane-bounded chambers which lie in crystal sacs; other cell organelles are sparse. These and other features of the light organ are compared with those of other luminescent fish.