Vision in Near Darkness
(^) We emphasize vision because that must be uniquely adapted to mesopelagic
conditions, and because much can be learned about eyes from comparative
morphology and visual pigments. Of course, other sensory modes remain available.
Detection of relative water motion is mediated, as nearer the surface, by lateral lines
in fish, innervated setae in crustacea, tango-receptors in chaetognaths, etc. The
antennal water motion sensors of deep-living crustacea are particularly impressive,
with very thin antennae often extending to over three times the body length. In fish, at
least, olfaction appears to be relegated primarily to mate finding: it is highly
developed in males, some having elaborate external receptor pads on the head (e.g.
Cyclothone microdon), but almost absent in females that presumably secrete male-
attracting pheromones. Smell appears to be of minimal value for locating food in the
mesopelagic zone. Its value re-emerges just above the seafloor, where large smelly
meals – such as dead whales – stop falling, and where direction upstream can be
distinguished by reference to the bottom.
(^) Mesopelagic vision can be optimized for two quite different sorts of lighting:
diffuse background that is brightest from above but illuminates the “scene” from all
angles, and bioluminescent point-sources, often flashes, that greatly exceed the diffuse
background. Optimizing eyes for low intensities of each of these kinds of light has
different requirements (Warrant & Locket 2004), but some designs trade optimization
for either to retain some capacity for both types of source. The result is a remarkable
array of modifications of both the camera-like eyes of fish and squid and the
compound eyes of crustaceans. Images of both scenes, say a squid against background
lighting, and points of light, degrade rapidly in water compared to air, due to much
more intense scattering. Mid-water animals can only see to ranges of a few tens of
meters, but that is far enough to make vision critically useful.
Fish Eyes
(^) Most deep-sea fish have only “rod” photoreceptors with a single type of rhodopsin as
the visual pigment, and no cones. Pigment absorption maxima, λmax, are mostly
between 470 and 490 nm. However, detailed studies of pigments are revealing that
some have more than one receptor pigment, in at least one case retinal cones, and thus
potential for subtle color differentiation. Because the refractive index of water is
∼1.33, about the same as tissue fluids like the vitreous humor (not meaning “funny”!),
stronger lenses are required for camera-type eyes than in air, so fish and squid eyes
have spherical lenses. Their protein composition varies such that the refractive index
shifts from 1.33 at the surface to 1.52 at the center. The focal lengths of such lenses
are 2.5 times their radii (“Matthiesen’s ratio”), and the gradient of refraction corrects