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O.S. Gustafsson, S.P. Collin, R.H. H. Kröger; Multifocal Lenses in Lampreys . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1210.
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Multifocal crystalline lenses compensating for the prismatic effect of simple lenses (chromatic aberration) are present in teleosts (bony fishes) and terrestrial vertebrates (Kröger et al. 1999 J. Comp. Physiol. A 184: 361–369), but not in cephalopods, which also have well–developed camera eyes. The aim of this study was to gain information on the evolutionary origin of multifocal lenses. The earliest vertebrates comprise the hagfishes and lampreys (agnathans or jawless fishes), which originated in the early Cambrian, approx. 540 million years ago (mya). Of the extant agnathans, hagfishes have rudimentary eyes and lampreys possess well–developed eyes as adults. Recent results from the southern hemisphere lamprey Geotria australis suggest that lampreys possess pentachromatic vision (Collin et al. 2003 Current Biology 13: R864–865) and therefore are confronted with the problem of chromatic aberration.
Three complementary optical methods were used; photorefractometry on living animals, schlieren photography and laser scanning on fresh excised lenses. Two southern hemisphere species of lamprey (Geotria australis and Mordacia praecox) and two northern hemisphere species of lamprey (Lampetra fluviatilis and Petromyzon marinus) were studied. Scanning electron microscopy (SEM) was also used to study the structure of lens fiber cells in L. fluviatilis.
All four species have almost spherical, multifocal lenses optically similar to the lenses of bony fishes. The flattened hexagonal lens fiber cells are covered with small protrusions possibly playing a role as an anti–reflex measure.
Given that agnathans, teleosts and terrestrial vertebrates all possess multifocal lenses, this optical specialization was most likely present in the ancestral vertebrates. The ability to create well–focused colour images in large eyes with high spatial resolution may be one of the reasons for the evolutionary success of vertebrates.
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