Physiological Adaptations To Aestivation And Osmotic Regulation Of The African Lungfish, Protopterus Annectens (Owen) Of Anambra River, Nigeria

ABSTRACT

Physiological adaptations to aestivation and osmotic regulation of the African lungfish, Protopterus annectens (Owen, 1839) (Lepidosirenidae) of Anambra River, Nigeria (Lat. 50 42‟ & 70 40‟N and Long. 60 30‟ & 70 42‟E) were investigated. Some physico-chemical parameters of Anambra River and aestivation habits of P. annectens in the river and its flood plains were also determined, as well as the seasonal changes in gonadosomatic and hepatosomatic indices, condition factor, fecundity and sex ratio. P. annectens lives an amphibious type of life. During the wet season (April to October) it is diurnal and lives in the swampy, oxygen poor, shallow and weed covered edges of the river. During the dry season (November to March), it aestivates in the mud. Seasonal analysis of some of the physico-chemical parameters that prevail in Anambra River revealed higher dissolved oxygen, air, surface water and soil temperatures, salinity and ionic concentrations in the dry than wet seasons. The mean burrow length, burrow width and width of chamber base were 37.5cm±10.5cm, 5.8cm±3.5cm and 11.0cm±7.1cm respectively. Analysis of soil sample collected from aestivation spots revealed 82.4% ± 6.2 clay, 15.2% ± 4.7 sand and 2.3% ± 2.4 silt. One year of laboratory aestivation led to 22.1% to 30.1% loss of body weights, polycythaemia, thrombocythaemia, hyperhaemoglobinaemia, hyperglycaemia and elevation of haematocrit, osmolality, urea, uric acid, creatinine and ionic concentrations as well as reduced hepatosomatic indices and blood pH values. There was leucocytosis within the first and second months of aestivation, after which the rate of leucopoiesis was slowed down so that the leucocyte number came down to its normal aquatic (non-aestivating) values but without leucopenia. P. annectens was able to tolerate seawater up to a maximum of 10.5‰ due to the possession of chloride secretory cells in the gills which actively transport Na+ and Cl- from the blood into the surrounding seawater media, an indication that it might be possible to culture P. xxvii annectens in brackish water. Some physiological adjustments of P. annectens to brackish water regimes mainly included increased haematopoiesis and buccal respiratory rates. Mean gonadosomatic indices, fecundity and condition factor were low in dry season with values of 2.2098±0.3, 18±60 and 0.5±0.1 respectively. These values were high in wet season with 4.0685±0.4, 1361±248 and 1.5±0.6 respectively. This indicates that the fish thrives better in wet season, the latter also being its spawning season. The hepatosomatic indices were high in dry season (2.2314±0.3) and low in wet season (1.6180±0.2). There was a high positive correlation between fecundity and total length (r=0.9286) and between fecundity and body weight (r=9843). When compared to other catfishes, P. annectens had a relatively low fecundity (mean F = 801). This is attributed to several months of aestivation without food or water. The sex ratio during the wet season was between 1:3 and 1:4 (male/female) but during the dry season the sex ratio of dug out aestivated specimens was 1:1.2 (male/female). This suggests that a great percentage of the females especially the late spawners probably moved into deeper areas of the water as the water was fast receding (during the dry season) in order to avoid desiccation of the future fry that might be unable to aestivate. As P. annectens grew larger in size (or weight) the size of the gills decreased while that of the lungs increased. In the laboratory, P. annectens was nocturnal and preferred to stay at the bottom of tank or aquarium. There was a linear negative correlation between the rate of survival of P. annectens and volume of water in laboratory tank (r=-0.9261). This is probably because it is an obligatory air breather that inhales air from the nostrils and mouth simultaneously which goes into the lungs to supplement the amount of dissolved oxygen it receives from water via the gills. This study has established that the biological indices of the African lungfish, P. annectens were elevated during the wet season as a physiological adaptation to tide over the hydro-climatic features of the dry season aestivation period.