Thermal control of rod outer segment length and shedding in a fish, Fundulus zebrinus

The effects of temperature on rod outer segment (ROS) length and membrane shedding were studied in a cyprinodont fish, Fundulus zebrinus . After 30 days in 14L/10D cyclic light and 17°C, ROS length averaged 41·2 μ m. Fish were then exposed to 7, 17 or 27°C for 10 and 25 days before being sampled 5 hr before and 1–4 hr after light onset. In 7°C ROS shortened to 83·5% of initial controls within 10 days, then only 4·1% further, to 79·4% by day 25 (34·4, 32·7 μ m). ROS length did not change significantly in fish remaining at 17°C (39·7 and 40·7 μ m at day 10 and 25) or in fish moved to 27°C (41·7 and 41·6 μ m). Phagosomes were most numerous in 7°C and least numerous in 17°C, but varied in overall size among the three temperatures. Phagosomes were generally smaller in 7°C, and larger in 17 and 27°C, with the largest phagosomes being more common after light onset. After light onset at day 25, the estimated volume per phagosome was 1·14, 4·73 and 5·75 μ m 3 in 7, 17 and 27°C. Total phagosome volume per 100 μ m RPE at 27°C was generally double that at 17°C. Apparently, in F. zebrinus , the number of disks shed from ROS is adjusted during thermal acclimation to stabilize ROS length.