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.