In mysid, shrimp, and euphausiid species of the deep sea, the masking cells remain
but lose pigment along the crystalline cones, forming a substantial clear space
intervening between the crystalline cones and the retinula cells containing the
rhabdomes (Fig. 12.11 – right). The crystalline cones act as refracting bodies that
guide light from a single direction, across the clear space to the rhabdome centered
under that direction. However, gathering of light from lenses of multiple facets
enhances sensitivity at the expense of resolution. Eyes with such clear spaces are
termed “superposition” eyes, which operate by several variant optical mechanisms
(Nilsson 1989) suggesting recurring evolution of the general pattern. Many shrimp
and mysids living in dim light have rhabdomes that are star-shaped in cross-section,
producing interdigitation of rhadomes and, thus, sharing of scattered light between
adjacent cells (Gaten et al. 1992), rather than the strict isolation of rhabdomes in near-
surface forms. Similarly to fish, signals from many rhabdomes may be gathered by
single nerve cells before processing by the brain – making clusters, rather than
individual rhabdomes, the units of the visual field. Each of these modifications
improves sensitivity at a cost to resolution. Tapeta at the bases of the rhabdomes are
common, raising the proportion of captured incident photons.
(^) Whitehill et al. (2009) have demonstrated a developmental sequence from
apposition-like to superposition eyes in the mesopelagic mysid Gnathophausia ingens
as the juveniles (life cycle discussed below) progressively inhabit deeper strata. The
first free-living stage, inhabiting depths 175–250 m, has an eye of superposition form,
but with almost no clear space and substantial sheathing cells around the crystalline
cones. The clear space (actually transparent extensions of the crystalline cone layer
that are not surrounded by pigment cells) opens proximally in instar 5 (>400 m), and
surrounds the entire rhabdome layer of instar 10 (400–900 m). The eye structure shifts
with stage to gain sensitivity, although all stages have interdigitating, stellate
rhabdomes, but there is no shift among stages in either visual pigment absorption
spectra or electrode-recorded spectra of sensitivity to flashes (Frank et al. 2009).
Spectral maxima (λmax) were all close to 500 nm. Frank et al. identified genes for two
rhodopsin pigments in G. ingens, but their absorption properties appear to be additive
to a smooth, unimodal spectrum (λmax ≈ 512 nm).