ERG-determined spectral and absolute sensitivities in relation to age and size in the halfcrab Petrolisthes elongatus (Crustacea; Decapoda; Anomura).

The electroretinogram (ERG) waveform of Petrolisthes elongatus is biphasic and transient and does not change with age (monitored as carapace length and weight of the individual). Though there was a very slight decrease in sensitivity with age (R2 = 0.008), spectral peaks and curves remained virtually unchanged. A shift of 2.2 log units in threshold sensitivity, however, was observed in relation to the different adaptational states. The greatest difference, not unexpectedly, occurred between dark-adapted eyes, tested at night, and light-adapted eyes tested during the day at a background illumination of 250 1x. The gradient of the V/LogI curve (n) in light-adapted animals increased, which means a smaller increase in intensity is required to produce an increase in response compared to a dark-adapted animal, but there was no difference in n with regard to the different age-groups. Also, no significant difference was noticed between dark-adapted eyes tested during day and night. It is concluded that the intensity range over which vision in P. elongatus is maximally sensitive to contrast (the dynamic range) is 2.5-3.0 log units. Since there is apparently little or no alteration in the photic environment and the behaviour of P. elongatus as it ages, there is no advantage in fundamentally altering the parameters of the absolute and spectral sensitivity function with age. Our observations suggest that the eyes are able to adjust within 30 min to changing ambient light levels irrespective of whether or not a circadian activity rhythm operates.