Deep-sea UV fish vision? was UV

["joe b." <joe-b@xxxxxxxxxxxx>]

In message <199609300216.WAA06691@xxxxxxxxxxxxxxxxx>, Dave
<gannet@xxxxxxxxxx> writes
>The shortest wavelength shown is about 380nm, and they list a depth 
>of 15m.  For 760nm they show about 8m.  In general, the tranmission 
>curve is more-or-less bell-shaped and peaks at about 425nm, with a 
>depth of 250m or more.  Light towards the red end of the spectrum 
>falls off much more rapidly than blue light, hence the fact that 
>underwater scenes of any depth appear blue to our eyes.
>It would seem likely that fishes that live at shallow depths have 
>near-human spectral sensitivity, but those who live at great depths 
>are possibly blue-adapted.  Pure speculation on my part.  I do note 
>that marine reef fishes and animals are the most exotically colored 
>and patterned species on earth - they wouldn't expend the effort if 
>it wasn't doing them some good, hence I conclude they can see it.

There's something going on here that doesn't quite make sense to me and
it's been bugging me so please forgive semi-off-topic reply. If I recall
correctly many deeper sea fish also show up (in the artificial lighting
brought down by humans) with an incredible variety of colours (I don't
mean the species that life in darkness right down low, but those still
substantially deeper than your regular coral reef fish). But if the only
colour of light that normally travels to these depths is blue, and
precious little of it, what is the point of these colourings? If the
deeper sea fish only see what is illuminated by the blue light that
makes it to such depths, then everything will look blue anyway! There is
no point evolving with colouring that isn't visible. Therefore it must
be visible. But in blue light it isn't visible, it will be in blue
monochrome. Unless these fish are viewing by some sort of illumination
that no-one knows about. Could they have enhanced ultra-violet vision
beyond the human spectrum in the same way as dogs can hear frequencies
too high for human ears- or something along those lines? Can someone
help me out here? (I promise to put any resulting deep-sea infrared
Ektachrome / UV fluorescence photos on the IR gallery first...)  
-- 
joe b.

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End of INFRARED-PHOTOGRAPHY Digest 79
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