Many species of fish, including even marines such as catfishes and apogonids, hold their eggs and fry in their mouths to protect them from predation.
However, the actual effect that mouthbrooding has upon parent fish has been studied very little.
According to a new study by two scientists from the University of Oslo, marine apogonids, or Cardinals, such as the Ghost cardinal, Apogon leptacanthus and the Fragile cardinal, A. fragilis, the mouthbrooding process causes severe respiratory problems for the parents.
In a paper published in the Proceedings of the Royal Society of London recently the scientists state that high oxygen levels are critical to the success of the mouthbrooding fish. When the water is very well-oxygenated, the cost of mouthbrooding is fairly low, because the fish conserve energy and use up less oxygen by swimming around less.
However, when the oxygen level drops, mouthbrooding male cardinals spit out their brood.
But not every species of cardinal is affected by lower oxygen levels in the same way. A fragilis, which has a larger brood than A. leptacanthus, spits out its eggs when the oxygen level is higher than it is for leptacanthus to jettison their brood.
The other snag for some cardinals is that fragilis, for example, wasn't able to breath any quicker to make up for the problems it was experiencing in breathing when brooding eggs in lower oxygen conditions.
According the study this means that there's a trade off between hypoxia tolerance and the amount of eggs in a clutch: "Apparently, A. fragilis has sacrificed hypoxia tolerance in favour of a large brood size to a greater extent than has A. leptacanthus."
The study may have important implications for those attempting to breed the threatened Banggai cardinal, Pterapogon kauderni, in their aquaria. Wild stocks of this species are becoming smaller as a result of overfishing.
For more information read the paper: Ostlund-Nilsson S, Nilsson GE. (2004) - Breathing with a mouth full of eggs: respiratory consequences of mouthbrooding in cardinalfish. Proc R Soc Lond B Biol Sci. 2004 May 22;271(1543):1015-22.