Re: Why are some fish transparent?

Diane,
Sorry about the late response, I was having some major problems with my 
email accounts.  As to your question “Why are some fish transparent?” here 
is my answer.

Fish have colors mostly for several reasons:
1) To help them catch food
2) To help them avoid being eaten by other things
3) To attract mates or to identify similar fish species
4) Diet affected colors

Some fish are transparent because it really helps them to prevent being 
eaten.  Some fish are transparent throughout their entire lives (glass 
catfish, glassfish, small minnows), and others are only transparent during 
their early juvenile stages but then grow out of it.  Being transparent 
helps because no matter what the fish is around, the background color 
shows through, and for all practical purposes, the fish is (are) 
invisible.  Some predatory fish trigger to the presence of food, not by 
seeing the prey fish, but by sensing or seeing the shadow of the prey fish 
passing above it.  A clear fish may appear as a refraction of the water, 
and not cast a distinct shadow.  This is particularly effective if the 
fish remains still, or just moves slowly.  

This clear cryptic coloration allows the fish to not only survive, but to 
devote more of its energy into growing; rather than would a similar sized 
but brightly colored and very fast fish which must put a lot of energy 
into both chasing prey and evading predators.  Very few predatory fish are 
able to maintain the “clear” look past juvenile stages.  As they grow more 
than a few inches they get more colored.  One exception is the 
leptocephalus larvae of eels (not really fishes) which can get several 
feet long and still be clear.  These are usually only found in the 
Sargasso Sea.

An interesting thing about the true “clear” fishes (and shrimps).  If you 
have a regular fish, and feed it some food with a lot of pigment in it, 
the fish will gradually take on that color.  This is the same as flamingos 
getting their pink color from shrimp that they eat.  If you do the same 
with a “clear” fish, no matter how much you feed them a colored food, they 
will not take on that color.  (At least I was never able to do this in the 
pet stores).  Some well-meaning, but not so animal friendly people have 
taken this a bit further with a fish you may see in your local pet store.  
This is the Painted Glass Fish.  This is really a glass fish that someone 
has taken out and painted with a paintbrush.  The paint sticks to the 
scales for a few weeks until it wears off or the scales are replaced, and 
eventually the fish turns clear again.

Most creatures that have evolved to live in dark caves are white, rather 
than transparent.  You see, in a dark cave, it does not matter what color 
the fish are, there is no light to see them.  It would not matter that a 
fish was pink with polka dots, it would have no better or worse chance of 
being eaten or finding food than one which was transparent. So to put it 
short, A)in a dark cave, being transparent would not help, B) in a dim 
cave it might help, but their predator fish would have evolved their eyes 
to see them in the dimmer light, and would probably still find the 
transparent fish, and C)being transparent only helps when there is light.
  
In areas of low light, this allows them to blend in with their normally 
white or cream background.  The transparent fish probably evolved in more 
brightly lit waters, or where the background rocks, bacteria, or algas 
cast a phosphoresent glow.  A good test is to see if the transparent fish 
have eyes.  
True cave-evolved creatures typically do not.  There is no light to see, 
so why waste the energy on eyes? I bet the fish you mention have eyes, and 
therefore are either relatively new to the caves, or actually leave the 
caves during some time of their lives.

Any good icthiology book will have these details if you want more 
information.

I hope I was able to help!!
Justin Strynchuk
B.S.Aquaculture Technology, A.S. Environmental Technology
Florida Institute of Technology