The tapetum lucidum in the eyes of cusk-eels (Ophidiidae)

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Abstract
Eyes of cusk-eels Ophidion welshi and Lepophidium graellsi contain mostly short rods, arranged in five or six regular rows. In O. welshi, cones are single and double, whereas cones are lacking in L. graellsi. The pigment epithelium contains a lipid tapetum lucidum. In L. graellsi the tapetal spheres are 465 nm in diameter; in O. welshi most tapetal spheres have diameters of 187 nm; others, less frequent, are 86 nm. The tapetum of L. graellsi is white, that of O. welshi is blue; they reflect diffusely and reflectance at λmax is about 0.60. Increased tonicity causes the tapetum of O. welshi to turn darker blue and reduced tonicity causes it to become white. The lipid of the tapetum was examined by thin-layer and gas–liquid chromatography; it is a triglyceride containing mostly (88%) docosahexaenoic acid. The visual pigments are rhodopsins; L. graellsi λmax 494, O. welshi λmax 500 nm. Coastal waters where cusk-eels live were fairly turbid: maximal transmission of daylight occurred at 585 nm when the extinction coefficient was 0.6. Reference is made to the secretive and nocturnal habits of cusk-eels, and the functioning of their unusual eyes.

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