Crustacean Eyes
(^) Some species in the “lower” orders of crustacea have very simple eyes, basically a
retina with no associated focusing apparatus. Among copepods, however, some
groups have lenses formed from cuticular chitin that do focus light on small retinal
elements. Those copepods are not found at substantial depths. The deep-sea ostracod
Gigantocypris has a parabolic tapeta surrounding its retina and focusing light upon it.
This likely provides good sensitivity with almost no image capability, and retrieves
minimal information about source location. Amphipods, mysids, euphausiids, and
shrimp have complex compound eyes, and deep-sea species have modifications
similar to those of nocturnal moths. In amphipods at all depths, vision is often a
dominant sense, the ommatidia (facets) forming nearly transparent domes surrounding
their heads. The ommatidial light guides narrow down into a cluster of minimized
retinal elements (rhabdomes) that must be pigmented to function. Like fish,
crustacean λmax values are in the blue portion of the spectrum, mostly from 460 to 500
nm, but in some species another pigment absorbs close to 400 nm (Warrant & Locket
2004), useful for sunlight down to perhaps 250 m, and possibly useful for some
bioluminescence. The common offset of the main pigment’s λmax toward green
wavelengths suggests adaptation to see bioluminescence.
(^) Specializations for vision in dim light parallel those in fish, sacrificing resolution
for sensitivity (Warrant & Locket 2004). In near-surface euphausiids and shrimp, the
ommatidia (cuticular lens, light-guiding crystalline cone, and retinal rhabdome) are
surrounded by pigmented masking cells that prevent light leakage between them,
giving each an image from a narrow angular field (Fig. 12.11 – left). This is an
“apposition” eye, meaning there is no unshielded space between the light-directing
structures and the rhabdome. The images integrate in the brain as a segmented scene
that has extreme sensitivity to motion (including very short integration times), as
elements in the scene shift from one facet’s rhabdome to the next. Try catching a
dragonfly with a butterfly net on a warm day.
Fig. 12.11 Left: diagrammatic section of an apposition compound eye: short, pigment-
cell-shielded crystalline cone cell of each ommatidium in direct contact with the
rhabdome; typical of near-surface (and terrestrial) compound eyes. Right:
superposition compound eye: crystalline cones more extended; pigment cells absent
or clear; light from many lenses accesses each rhabdome. Abbreviations: CC –
crystalline cone, P – pigment cell, R – rhabdome.
(^) (After Warrant & Locket 2004.)