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We describe the effects of a recessive mutation on visual behavior, the electroretinogram (ERG), and photoreceptor structure in zebrafish. At 6 d post-fertilization (dpf), no optokinetic reflex could be elicited in no optokinetic response c (nrc) mutant animals under any test condition. The animals exhibited ERG responses at 5-7 dpf that were markedly abnormal and could be categorized into two groups. The first showed an initial negative a-wave followed by a delayed positive b-wave of small amplitude. Often a second ERG-like response was recorded after the initial b-wave. The second group showed only a large negative a-wave; an initial b-wave was not evident. In most recordings additional oscillatory waves varying in number, amplitude, and time course were observed. Multiple responses at the cessation of long-duration flashes were also observed. Light and electron microscopy revealed that the cone photoreceptor pedicles of nrc fish were highly abnormal. Although the appropriate number of synaptic ribbons formed in these terminals, they "floated" in the terminal, unassociated with postsynaptic processes or arciform densities. The few processes invaginating the nrc pedicles resembled those of horizontal cells. Invaginating bipolar cell processes were rare, but basal contacts were observed on pedicle surfaces. The severity of the mutation did not change between 6 and 8 dpf, showing that there is neither a delay in development nor a degeneration of the terminals; rather, nrc pedicle development appears arrested. Bipolar cell terminals in the inner plexiform layer made normal ribbon synapses; thus, the mutation appears to affect only the outer retina.

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Sexual communication between male and female fireflies involves the visual detection of species-specific bioluminescent signals. Firefly species vary spectrally in both their emitted light and in the sensitivity of the eye, depending on the time when each is active. Tuning of spectral sensitivity in three firefly species that occupy different photic niches was investigated using light and electron microscopy, microspectrophotometry, and intracellular recording to characterize the location and spectral absorption of the screening pigments that filter incoming light, the visual pigments that receive this filtered light, and the visual spectral sensitivity. Twilight-active species had similar pink screening pigments, but the visual pigment of Photinus pyralis peaked near 545 nm, while that of P. scintillans had a lambdamax near 557 nm. The night-active Photuris versicolo, had a yellow screening pigment that was uniquely localized, while its visual pigment was similar to that of P. pyralis. These results show that both screening and visual pigments vary among species. Modeling of spectral tuning indicates that the combination of screening and visual pigments found in the retina of each species provides the best possible match of sensitivity to bioluminescent emission. This combination also produced model sensitivity spectra that closely resemble sensitivities measured either with electroretinographic or intracellular techniques. Vision in both species of Photinus appears to be evolutionarily tuned for maximum discrimination of conspecific signals from spectrally broader backgrounds. Ph. versicolor, on the other hand, appears to have a visual system that offers a compromise between maximum sensitivity to, and maximum discrimination of, their signals.

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Changes in visual sensitivity as a function of time-of-day were studied in the firefly Photuris versicolor. The electroretinograms (ERGs) elicited by photic stimuli of constant intensity and duration were recorded over long periods (18-52 h) while the insect was in constant darkness. The amplitudes of the ERGs elicited by the standard flash were small and remained constant during the daytime hours and increased on average sixfold during the evening hours, reaching a maximum at the onset of darkness and remaining at that high level until the morning hours. The amplitude of the standard response again was reduced in the morning hours to a daytime low value which again persisted until the following evening (Fig. 1). In addition, a series of flashes of varying intensity were administered periodically and logV/logI curves as a function of time-of-day were obtained. Compared to the daytime logV/logI curves, the nighttime curves were shifted about 4 log units towards the left on the intensity axis (Fig. 2). This period of high visual sensitivity was correlated with the species' flashing activity period, which occurs during the night hours (Fig. 1).

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The electroretinographic visual spectral sensitivity functions in day-active fireflies Lucidota luteicollis and Lucidota atra show a broad green sensitivity and a shoulder in the near-ultraviolet region of the spectrum as is commonly found among day-active insects. The nomogram for P530 visual pigment matches the spectral sensitivity curves in the green. The adult L. luteicollis retains its larval bioluminescent light organ which has a peak emission at 562 nm. The lambda max of the ERG spectral sensitivity does not match the bioluminescent peak as it does in twilight- and dark-active fireflies. Some relevant behavioural observations with respect to mating are presented.

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1. ERG S(lambda) were determined in dark-adapted intact preparations of 6 North American firefly species (Photinus collustrans, marginellus, pyralis, macdermotti, scintillans and Bicellonycha wickershamorum) which restrict their flashing activity to twilight hours. The curves possess narrow (1/2 bandwidth = 50-60 nm) peaks in the yellow (560-580 nm) and a shoulder in the violet (370-420 nm), with a marked attenuation (1.4-2.2 log units) of sensitivity in the green (480-530 nm) region of the spectrum (Fig. 1). Two additional species (Photuris potomaca and frontalis) which initiate flashing at twilight and continue on late into the night (twi-night) possess broad sensitivity maxima around 560 nm (Fig. 3). 2. Selective adaptation experiments isolated near-UV and yellow in P. scintillans (Fig. 2). In the dorsal frontal region of the compound eyes in P. frontalis, high sensitivity existed only in the short wavelength region (near-UV and blue) with a maximum in the blue (lambda max 435 nm) (Fig. 4). 3. The in situ MSP absorption spectrum of the screening pigments was determined in preparations of firefly retina. a) Two kinds of dark brown granules were found in the clear zone region. These granules absorb all across the spectrum with a gradual increase in optical density in the shorter wavelength region in P. pyralis (Fig. 5). b) Besides dark granules, pink-to-red colored screening pigments were present in the vicinity of the rhabdoms. The absorption spectra of these pigments determined in five species were narrow (1/2 bandwidth = 50-80 nm) with species-specific differences in their peak absorption in the green at 525 nm, 510 nm, 512 nm and 517 nm in P. scintillans, macdermotti, collustrans and pyralis, respectively (Fig. 6). A similar pigment was found in P. marginellus with a lambda max at 512 nm (Fig. 7). In all cases, transmission increased both at long and short wavelengths, but more sharply in the long wavelength region (Figs. 6 and 7). Hence each twilight-restricted species has its own unique colored screening pigment. A yellow pigment whose absorption spectrum differed from those found in genus Photinus was found in twi-night active Photuris potomaca (lambda max 461 nm) and night-active P. versicolor (lambda max 456 nm). The transmission of the Photuris pigment increased sharply only in the long wave-length region (Fig. 8).(ABSTRACT TRUNCATED AT 400 WORDS)

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Dark-active North American fireflies emit green bioluminescence and dusk-active species emit yellow, in general. Yellow light and yellow visual spectral sensitivity may be adaptations to increase the signal-to-noise (that is, foliage-reflected ambient light) ratio for sexual signaling during twilight. The peaks of the electroretinogram visual spectral sensitivities of four species tested, two dark- and two dusk-active, correspond with the peak of their bioluminescent emissions.

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Electrical responses (ERG) to light flashes of various wavelengths and energies were obtained from the dorsal median ocellus and lateral compound eye of Limulus under dark and chromatic light adaptation. Spectral mechanisms were studied by analyzing (a) response waveforms, e.g. response area, rise, and fall times as functions of amplitude, (b) slopes of amplitude-energy functions, and (c) spectral sensitivity functions obtained by the criterion amplitude method. The data for a single spectral mechanism in the lateral eye are (a) response waveforms independent of wavelength, (b) same slope for response-energy functions at all wavelengths, (c) a spectral sensitivity function with a single maximum near 520 mmicro, and (d) spectral sensitivity invariance in chromatic adaptation experiments. The data for two spectral mechanisms in the median ocellus are (a) two waveform characteristics depending on wavelength, (b) slopes of response-energy functions steeper for short than for long wavelengths, (c) two spectral sensitivity peaks (360 and 530-535 mmicro) when dark-adapted, and (d) selective depression of either spectral sensitivity peak by appropriate chromatic adaptation. The ocellus is 200-320 times more sensitive to UV than to visible light. Both UV and green spectral sensitivity curves agree with Dartnall's nomogram. The hypothesis is favored that the ocellus contains two visual pigments each in a different type of receptor, rather than (a) various absorption bands of a single visual pigment, (b) single visual pigment and a chromatic mask, or (c) fluorescence. With long duration light stimuli a steady-state level followed the transient peak in the ERG from both types of eyes.

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