Pupil light reflex in the Atlantic brief squid, Lolliguncula brevis.

Coleoid behavioral ecology is highly visual and requires an eye capable of forming images in a variety of photic conditions. A variable pupil aperture is one feature that contributes to this visual flexibility in most coleoids, although pupil responses have yet to be quantitatively documented for squid. The pupil light reflex (PLR) of the Atlantic brief squid, Lolliguncula brevis, was analyzed by directly exposing one eye of individual squid to light stimuli of varying irradiance and imaging the reflex, while simultaneously recording from the opposite, indirectly stimulated eye to determine whether the constriction was consensual between eyes. A PLR was measured in L. brevis, with an asymmetrical constriction observed under increasing irradiance levels that was not consensual between eyes, although a response of some level was observed in both eyes. Response thresholds ranged between 12.56 and 12.66 log photons cm(-2) s(-1). The PLR was rapid and dependent upon the stimulus irradiance, achieving half-maximum constriction within 0.49-1.2 s. The spectral responsivity of the PLR was analyzed by measuring the magnitude of the reflex in the eye directly stimulated by light of equal quantal intensity at wavelengths from 410 to 632 nm. The responsivity curve showed a maximum at 500 nm, indicating the eye is especially well suited for vision at twilight. These results, when considered in the context of the ambient light characteristics, show that the PLR of L. brevis contributes to a dynamic visual system capable of adjusting to the highly variable composition of light in its estuarine habitat.