Transparency
(^) As in surface layers, some mid-water organisms are transparent, for example the
leptocephalus larvae of eels. These have the appearance of a glass tobacco leaf, and
achieve near-transparency mostly by virtue of being thin. Their eyes, however,
because they require opaque, light-absorbing pigments and tapeta, cannot be
transparent. For such animals, seeing raises the risks of being seen. Deep-living
jellyfish, amphipods, some squid, and others are likewise nearly transparent. This is
effective if it sufficiently reduces the visual contrast of the animal against the
background. Organic molecules absorb very little more light than water, but both
proteins and lipids scatter light differently due to their effects on refractive index. In
some transparent tissues, the effects of scattering are reduced by submicroscopic
“bumps” in membrane surfaces smaller than the wavelengths of visible light (Johnson
2001). They generate destructive interferences that reduce the overall effects of
scattering, a complex subject in physical optics. Contrast, C (negative by convention),
for an eye looking at some tissue very close in can be lowered to about C = –9% (=
91% transparency) by adding water to the tissue and evolving specialized
arrangements of lipid and protein structures. However, effective contrast fades
exponentially with the distance of a detecting eye from the object. Detectable contrast
for deep-sea fish and invertebrates is unknown; it can be small (C ∼ –1%) for near-
surface fish (Johnson 2001). Nevertheless, nearly transparent animals at mesopelagic
depths are effectively invisible.