TERRESTRIAL OPTICS IN AN AQUATIC EYE - THE SANDLANCE, LIMNICHTHYTES FASCIATUS (CREEDIIDAE, TELEOSTEI)

The sandlance, Limnichthyes fasciatus (Creediidae, Teleostei), behaves like a marine chameleon, with independent movements of its turret-lik e eyes, highly-effective camouflage and rapid strikes for isolated, mo bile prey at close quarters. The optical system has a fixed circular p upil, a deep pit fovea and a flattened lens unlike any other teleost l ens so far described. The convex, laminated structure of the cornea is also unparalleled in a teleost which suggests that the cornea may pla y a refractive role that might compensate for the reduced power of the flattened lens. This suggestion has been supported in the present inv estigation by four independent sets of observations:- i. Purkinje imag es formed underwater by the cornea; ii. Measurements of the magnificat ion of intra-corneal iridophores viewed through the corneal lenticle; iii. Measurements of the magnification produced by the dissected corne al lenticle and lens when viewed over a grating; iv. Ray tracing exper iments comparing the degree of refraction produced by the lens and by the corneal lenticle. All experimental observations confirm that the c ornea of the sandlance has a significant refractive role, with a power of approximately 200 D compared with a power of 550 D for the lens. T his is the first report of a significant refractive role played by the cornea in a teleost. The optical system of lens plus cornea, in combi nation with a deep pit fovea, may be more suitable for the detection a nd visual localisation of small, moving, underwater prey than the conv entional wide-field spherical lens system of other teleosts. The evolu tionary convergence of this marine optical system and lifestyle with t hose of the chameleon is remarkable, given the constraints imposed by underwater optics.