Hi all,
as Apistogramma do not to my knowledge have lung-like organs yet they are capable of living in CO2 rich water containing lots of leaf litter sediment.
I think
Apistogramma are still relatively sensitive to high CO2 levels, but there are a lot of other SA fish from black waters that have adaptations to air breathing (
Arapaimids, Gymnotids, Erythrinids). The leaf litter is a bit of a red herring, it is persistent due to the toxic compounds contained in many of the leaves, very low nutrients levels and strongly acidic water, all of which slow microbial action. If you put the same leaves in more nutrient rich, oxygenated alkaline water they will decay much, much quicker. It is the same sort of effect you see when you get peat bogs, the carbon builds up, because the rate of decay is limited by the lack of oxygen and the other essential ions for microbial growth. As soon as you drain the bog and fertilise the peat it oxidises away as CO2. "Holme Fen post" <
Natural England - Holme Fen (SSSI and NNR)>.
The reason that relatively fish are adapted to conditions of low oxygen is to do with the genetic cost of any "unsual" adaptation. A good example of this is in the
Loricariid and
Callichthyidae catfishes, they form a monophyletic clade (within the Loricarioidea), and some members of each family have a special gas exchange organ in the intestine that allows them to take oxygen from gulped air, meaning that it is almost certainly a conserved feature from their common ancestor.
The fish that can gulp air are all from slow moving swampy waters, where even if low oxygen/high CO2 is a very occasional factor it will still be conserved, because even very short periods of low oxygen will winnow out all those fish who don't carry a combination of genes that allows efficient oxygen uptake from the gulped air.
In the big rivers things are very different, rheophilic Loricariids like
Chaetosoma, Pseudolithoxus and
Hypancistrus ssp. will never experience periods of low oxygen, and that means that there will be random genetic drift in the air breathing genes, in exactly the same way that cave fish all eventually become blind and albino. In cave fish there is a cost to producing eyes and pigment once they don't increase survivorship, and in the rheophilic fish natural selection will now work on those genes that give the fish an advantage faster flowing water, and they become stream-lined, have large pectoral fins etc., and their gill area will reduce to the minimum required, because that fish will be, on average, slightly fitter than one with luxury extra gill area that it doesn't need. This cost is mentioned in the paper that Clive linked earlier.
The out-come of all of this is that a fish like a
Chaetosoma spp. from the cool fast flowing streams of the Andes piedmont, will suffer respiratory distress even in relatively well oxygenated water, where they will gasp air at the surface, although it no longer has the ability to extract oxygen from it.
cheers Darrel