Hiding Out
(^) Many features of mid-water organisms imply that being very close to invisible is
important at all levels where there is some light, say more than 10−11 W m−2 (∼ 1100
m at mid-day in clear tropical water), i.e. very dim illumination indeed. The limit of
human vision is of the order of 10−9 W m−2. At a maximum of about 800 m in clearest
tropical waters, the “sky” is visible as a small, blue circle to a human eye looking
through a dorsal window in a submarine. Daytime irradiance just below the sea
surface is approximately 10^3 W m−2. For light at wavelengths near 475 nm, the pure
seawater diffuse absorbance is ∼0.017 m−1, so illumination is reduced at least 10-fold
every 135 m. The absorbance spectrum of very clear ocean waters is quite flat from
400 to 500 nm, but absorbance is considerably greater at longer and shorter
wavelengths, so the only light not absorbed above the mesopelagic is blue. Thus, the
mid-water lighting is dim and blue only. Sundry observations suggest that deep-sea
fish with large eyes, near-perfect tapeta behind the retina (reflective surfaces returning
photons not absorbed initially for a second retinal pass); up to five layers of
pigmented and long retinal rod cells; and specialized, concentrated retinal pigments,
should have useful vision even with about 100-fold less light than required for human
vision. Owls, for which behavioral experiments are possible, see effectively in light
that dim. In mid-water, the array of visual adaptations for using the residual
downwelling and bioluminescent light is complex in single species and also variable
and distinctive among families of fish, crustaceans, and squid. Since vision is useful
throughout this thick ocean layer, survival for most animals requires means to avoid
being seen and then eaten. Adaptations of camouflage and vision to deep-sea
illumination have been thoroughly reviewed by Warrant and Locket (2004). We will
consider camouflage first.
Color
(^) Most mid-water fishes, especially below ∼650 m, are black, and the larger crustacea
are dark red. Both black and red appear black where all light is blue. There is a
gradient, however, with depth. In levels around 200–400 m, many shrimp are “half-
red”, that is, red anteriorly to cover luminous food in the foregut, and nearly
transparent ventrally and posteriorly. Fish in the upper mesopelagic are dark dorsally,
silvered on the sides, and bear photophores ventrally. Blackness or redness over the
entire body is typical in the deeper mesopelagic. In the bathypelagic, below about
1200 m, many pelagic fish are gray-brown or chalky.