The tapetum lucidum of gars (Lepisosteidae) and its role as a reflector

Abstract

Eyes of four species of gars were studied, namely, alligator (Lepisosteus spatula Lacépède), spotted (L. oculatus (Winchell)), shortnose (L. platostomus Rafinesque), and longnqse (L. osseus (L.)). The retina is duplex, a yellow tapetum lucidum is present, rods and retinal pigment move, and the tapetum is uncovered in dim light. The visual pigment is a porphyropsin, λmax 523, and the optical density of the pigment in the retina is 0.22. Transmission through the retina is low at short wavelengths and rises steeply above 460 nm. Reflectance from the tapetum is diminished in the blue, and rises progressively at longer wavelengths to reach a maximum (70%) at about 650 nm. Underwater illumination was measured in waters where gars occur. In lake waters maximal transmissivity occurred at 375 nm and the transmission per meter ranged from 76 to 62%. In rivers and bays, transmission was maximal at long wavelengths, ≥600 nm, and the transmission per meter ranged from 37% to 0.001%; these waters sometimes were stained, and were fairly turbid to very muddy. The yellow tapetum is an efficient reflector of underwater light, the greater part of the energy of which occurs at long wavelengths. It is calculated that the tapetum increases light absorption in the retina by 30%. In muddy rivers inhabited by gars, scotopic vision involving the tapetum probably operates during the daytime at depths of 2 m or less.