Fish Guts

Question
Hi Ron
Do you by any chance know why many fish species have black gut lining (including cichlids?)
I have heard two theories, one being that this raises temperature levels for speeding up digestion and the other being that it prevents photosynthesis in herbivorous fish which would otherwise result in high oxygen levels in the gut...
Thank you!
Ryan Hill

Answer
Hi Ryan,
 Excellent question. I wish I had an answer.  I was vaguely aware that some fish have a black lining but I have never looked at it systematically.  The latter hypothesis would be very testable, i.e., one could look at a selection of fish identified as either carnivorous versus herbivorous and see if the data fits the theory.  The first hypothesis is trickier to test and of course one would have to wonder why ALL fish would not want the benefits of faster digestion.  Hmm...

-- Ron
  rcoleman@cichlidresearch.com
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