Brill et al.: Effects of rapid decompression and exposure to bright light on visual function in Sebastes melanops and Hippoglossus stenolepis 431 
Table 1 
Mean (and 95% confidence bands) intensities (log candela/meter 2 ) of broad spectrum white light required to produce electro- 
retinogram (ERG) responses 50% of the maximal observed responses in black rockfish ( Sebastes melanops) and Pacific halibut 
(Hippoglossus stenolepis). The 50% response points were calculated to explore day-night differences, species-specific light sen- 
sitivities, and treatment effects. The mean 50% response point in control Pacific halibut was more than an order of magnitude 
lower than that of black rockfish, indicating the significantly greater light sensitivity of the former. There was no effect of rapid 
decompression on the 50% response point of black rockfish, whereas 15 minutes of exposure to simulated sunlight increased the 
50% response points of Pacific halibut, clearly indicating a diminished light sensitivity in the retina. 
Day 
Night 
Black rockfish, control 
Black rockfish, rapid decompression 
Pacific halibut, control 
Pacific halibut, light exposed 
2.1 (2. 1-2.2), n=l 
2.3 (2. 2-2. 3), n = 6 
0.09 (0.01-0.19), n = 6 
0.56 (0.47-0.63), n = 5 
2.0 (1. 8-2.1), n=l 
1.9 (1. 8-2.0), n = Q 
-0.07 (-0.16 -0.05), n = 5 
0.78(0.61-0.91), n=5 
Statistical procedures 
To explore species differences and treatment effects on 
retinal light sensitivity, the Weibull four-parameter sig- 
moid function contained within SigmaPlot for Windows 
version 10 (Systat Software, San Jose, CA) were fitted 
to the mean V-log I response data. This function was 
chosen because ERG responses generally exhibit a sig- 
moid response to increasing light intensities (Kobayashi, 
1962). Light intensities required to produce responses 
that were 50% of that needed to produce a maximal 
response (and their 95% confidence bands) were taken 
from the predicted values produced by the curve-fitting 
program. 
Differences in the mean flicker fusion frequency data 
collected during the day and night were tested with 
paired-7 tests (SigmaStat 3.1, Systat Software, San 
Jose, CA) because they represented “before-and-after” 
trials on the same animal. In those instances where 
no differences were found, day and night data were 
combined. Differences in the flicker fusion frequency 
between control animals and those subjected to rapid 
decompression (black rockfish) or simulated sunlight 
(Pacific halibut), were likewise treated with 7-tests. 
In instances where no treatment effects were present, 
data were combined. The Mann-Whitney rank sum test 
was used to test for species differences because the 
combined data sets were not normally distributed. In 
all instances P<0.05 was taken to indicate significant 
differences. 
To explore the specific effects of bright light expo- 
sure on retinal function in Pacific halibut, the vita- 
min Al rhodopsin absorbance templates developed 
by Stavenga et al. (1993) were fitted to the normal- 
ized spectral sensitivity data assuming the pres- 
ence of two visual photopigments. Unknown model 
parameters (photopigment absorption maxima and 
their weighting proportions) were estimated by using 
maximum likelihood within the software package R 
(vers. 2.7.0, R Foundation for Statistical Computing, 
Vienna, Austria). 
Results 
Effects of rapid decompression 
Five of eight black rockfish exposed to simulated cap- 
ture showed severe exophthalmia immediately after 
rapid decompression, and one fish in this condition also 
displayed corneal emphysemas. All eight fish showed 
other external signs of barotraumas, such as gas bubbles 
under the branchiostegal membrane, and seven of the 
eight showed esophageal eversion. None showed any 
gas bubbles within the vitreous humor. Exophthalmia 
and other signs of barotrauma disappeared in all fish 
immediately after recompression. There were no obvi- 
ous external anatomical abnormalities or evidence of 
disease in any of the fish at the time they were used in 
an experiment. 
Responses to increasing light intensities 
The amplitude of ERG responses of both black rock- 
fish and Pacific halibut increased with increasing light 
intensities (Fig. 2) and produced the expected sigmoid 
V-log I response curves (Kobayashi, 1962). Based on 
the overlap of mean 50% response points and the 95% 
confidence bands (Table 1), there were no significant 
day-night differences in the light sensitivity of either 
species within treatments with the possible exception 
of black rockfish exposed to rapid decompression. The 
mean 50% response points in the control Pacific halibut 
were more than an order of magnitude lower than those 
of the black rockfish (Table 1), indicating a significantly 
greater light sensitivity in the former. 
There was no effect of rapid decompression on the 
50% response point for black rockfish. In contrast, the 
responsiveness of Pacific halibut retinas was signifi- 
cantly diminished by exposure to bright light. During 
both day and night recordings, the V-log I response 
curves for Pacific halibut exposed to simulated sun- 
light were clearly right-shifted compared to controls 
(Fig. 2). The mean 50% response points for treated 
