The facultative air-breathing fish Pangasianodon hypophthalmus native 
to the Mekong river system is a very active fish that migrates >2000km 
from the delta to breeding grounds in Cambodia. P. hypophthalmushas 
very large plastic gills whose respiratory surface area can vary from 
being on par with the most active water-breathers, to a small surface 
with almost no exposed lamellae. Its ability to air-breathe and its very 
high growth rate, even in deeply hypoxic water have contributed to its 
importance in aquaculture making it the most cultured air-breathing 
fish in the world. The typical gas-exchange pattern for air-breathing 
fish in hypoxia is to excrete CO2 to the water while obtaining O2 from 
the air. Consequently, it is believed that some O2 taken up by airbreathing 
is lost at the gills to the hypoxic water. To understand how 
P. hypophthalmus can both lose O2 and simultaneously maintain a 
very high growth rate in hypoxia, we quantified the extent of the O2 
loss using a newly developed two-phase respirometer enabling the 
measurement of both O2 and CO2 gas-exchange in water and air. In 
severe hypoxia (31C°, PO2: ≈1.5mmHg) we found that P. hypophthalmus 
lost only 4.6% of aerial O2 uptake, while maintaining an aquatic CO2 
excretion of 91%. Furthermore, at an aquatic PO2 of 4.7mmHg this loss 
was reduced to 3.4%. Previous oxygen profile and telemetry studies 
showed that P. hypophthalmus stays in surface waters when the PO2 
is low, indicating that in the natural environment, P. hypophthalmus 
likely avoids all O2 loss.

The air-breathing fish Pangasianodon hypophthalmus with huge gills voids all CO2 to surrounding hypoxic water with minimal branchial O2 loss

The Open Animal Biology session covers the whole breadth of animal biology, and therefore will contain those talks and poster presentations which don’t fit into the specific Animal Biology sessions. As much as possible presentations will be organised together with similar subject groups/themes, each set of talks acting like its own small session. This is therefore an interesting and important part of the scientific programme, with high quality presentations from across the animal biological spectrum. The session is open to all animal biology research areas, and from speakers of all levels of experience. We particularly encourage presentations by post-grads, post-docs, and early career scientists. 
**Session Organiser:** Dr. Jack Thomson

A13 - Open Animal 

technical paper

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The air-breathing fish Pangasianodon hypophthalmus with huge gills voids all CO2 to surrounding hypoxic water with minimal branchial O2 loss

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