Early development of eye and retina in lanternfish larvae

The morphological characteristics of the eyes and the retinae of lanternfish larvae of Lampanyctus crocodilus, Benthosema glaciale, and Myctophum punctatum were analyzed in pre-flexion, flexion, and post-flexion stages. Pre-flexion larvae of L. crocodilus, the species with the shallowest depth distribution, had spherical eyes located antero-laterally on a strongly laterally-compressed head, suggesting a forward binocular visual field. B. glaciale and M. punctatum larvae live deeper in the water column and had eyes elongated in the dorsal-ventral plane. The eyes of B. glaciale were prominent, projecting slightly outward from a laterally-compressed head, suggesting a strongly laterally-directed visual field. M. punctaum had stalked elongated eyes projecting from a dorso-ventrally flattened head. The eyes can be freely rotated allowing lateral, anterior and dorsally-directed vision. A prominent choroidal gland was situated beneath the ventral portion of the eye in M. punctatum and B. glaciale, while a smaller gland was present in the dorsal and ventral portions of the eye of L. crocodilus. In pre-flexion stage larvae, the retina of all three species was differentiated with numerous rod photoreceptors in the peripheral retinal areas and fewer cone photoreceptors mainly distributed in the central retina. This distribution suggests concomitant enhancement of scotopic sensitivity in the vertical visual plane and improved photopic acuity in the lateral and forward visual directions. The concurrent development of cones and rods, as observed in the pre-flexion stage of myctophid larvae, is consistent with meeting the special demands of visual planktivory in sub-surface waters. During larval development a gradual increase of ROS length was also accompanied by a progressive loss of cones that were almost totally absent in post-flexion larvae. This can be interpreted as an adaptive response to an impending deep mesopelagic adult life.