The Japanese weatherloaeh (Misgurnus anguillicaudatus Cantor) can exchange gases both with water, via gills and skin, and with air, via the posterior region of the alimentary canal (intestine). Air breathing occurs by unidirectional ventilation of the alimentary canal with air taken in at the mouth and simultaneous expulsion of intestinal gas from the vent. Although the weatherloaeh is not an obligate air-breather, aerial gas exchange normally occurs even at 10°C in air-saturated water.
The alimentary canal was examined histologically to assess differences in capillary density and distribution and the diffusion distance for gases across those regions modified for aerial respiration. A respirometer system specifically designed for 2- to 3-g fish allowed continuous measurement of O2 and CO2 exchange via both aquatic and aerial routes at rest and at various ambient temperatures, and respiratory gas partial pressures. Air ventilation volumes, O2 and CO2 partial pressures of exhaled gas, O2 extraction, and O2 and CO2 exchange via the intestine were also determined, allowing the role of the intestine in total gas exchange in the weatherloaeh to be determined and compared with aerial gas exchange organs in other fishes.
The alimentary canal is divided into three zones, an anterior glandular portion separated by a spiral section from the posterior, respiratory zone which has the greatest capillary densities and shortest gas diffusion distances. At rest (20°C), the intestine takes up about 20% of total O2 but accounts for less than 3 % of total CO2 elimination (gas exchange ratio = 0.08 for intestine). O2 extraction averages 50%. Increasing temperature causes only slight increases in total metabolic rate (Q10 for MOO2= 1.5-1.8), but highly significant increases in intestinal gas exchange relative to total gas exchange develop as temperature rises. Intestinal gas exchange also rises with decreasing O2 availability. A strong hypoxic drive and weak hypercapnic drive exist for aerial ventilation of the intestine, but are reduced or absent for aquatic ventilation of the gills. In spite of having to function in respiration, absorption, secretion and buoyancy regulation, the potential effectiveness of intestinal gas exchange is shown to be similar to that of other structures used for aerial gas exchange in facultative air-breathing fish.