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K. van Doorn, J.G. Sivak, M.M. Vijayan; Lens optical quality changes during induced salmonid parr–to–smolt metamorphosis. . Invest. Ophthalmol. Vis. Sci. 2004;45(13):2657.
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Purpose: Migratory salmonid fish undergo a significant physiological metamorphosis during their freshwater–to–saltwater migrations, and while several aspects of their sensory systems are known to be affected, little is known of the effects of this pre–adaptive metamorphosis on the optics of their ocular lens. Cataract formation is known to occur, especially upon exposure to saltwater, although the functional impact of the hormonally–effected physiological changes on the optical quality of their lenses was unknown (see Bjerkås et al., 2003, for a review and discussion of salmonid cataract research). In this study, we followed the progression of lens optical quality changes in rainbow trout (Oncorhynchus mykiss) undergoing thyroxine(T4)–induced metamorphosis. T4 is one of several hormones known to effect metamorphic changes during smoltification. Methods: 100 yearling rainbow trout parr weighing 24–38g at the start of the experiment were partitioned into treated and control groups, the former fed food pellets laced with 0.4 mg of T4 per gram of food and the latter fed sham–treated food pellets. Treatment lasted 6 weeks. Regular samplings (Ntreated=5–8, Ncontrol=4–5) during the treatment period and 6 days after cessation of the treatment involved assessing excised lens optical quality using the ScantoxTM In Vitro Lens Assay System, which measures lens back vertex distance variability. Results: ANOVAs showed no statistical difference between samples of the control group at different sampling times (F0.05(2), 4, 19 = 1.161, p = 0.359). As a result, data on control fish were pooled to increase the power of the analyses of treated groups against controls. A statistically significant decrease in lens optical quality was evident on the second day of treatment (F0.05(2), 1, 30 = 29.810, p < 0.000), and this decrease was sustained throughout the 6 weeks of treatment (at each sampling time, p ≤ 0.016), except for a sample taken on the 20th day (p = 0.082). Samples taken on the 6th day after cessation of the treatment showed no difference with the control pool (p = 0.438). Conclusions: These results indicate a link between a reduction of lens optical quality and T4–induced metamorphosis specifically, with recovery taking place upon removal of exogenous T4. This supports the notion that smoltification can bring an increase in the risk of cataractogenesis, as cataract formation is normally preceded by such a decrease in lens optical quality. Furthermore, these effects should be evident during the seaward migration prior to contact with seawater.
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