1. Physical and chemical parameters were measured in the Japanese oyster Crassostrea gigas to investigate whether the first step of urine formation in the lamellibranchs could be an ultrafiltration and to give a quantitative description. 2. The effective filtration pressure was not constant, but a function of time, oscillating between 31.7 mmH2O and −3.8mmH2O. During the filtration, a separation of proteins took place: the protein concentration in the haemolymph was 17 μmoll−1 and the average molecular weight was 141 000. In the filtrate, the protein concentration was 2 μmoll−1 and the average molecular weight was 45 000. The marker substance inulin, applied via the gills, appeared successively within the haemolymph and the pericardial fluid. These findings establish the idea that the pericardial fluid is formed by ultrafiltration from the haemolymph. 3. The rate of filtration was found to be 0.4 μg−1 min−1 by quantitative analysis of the transport of the inulin. The coefficient of filtration was 4.5×10−6 mls−1 cm−2 mmHg−1.
Long standing water-gauge data recorded at the lower course of the Magdalena river (Colombia), show every 6 to 7 year extreme high and low water levels. These rhythmical changes could be proved highly significant with Student,s t-test. Furthermore, these oscillations of waterlevels could be correlated with long-term changes of rainfalls in the upper hydrographic basin of *) El primer autor quiere dedicar el presente trabajo y expresar sus sinceros agradecimientos a todas entidades y personas en Colombia, que por su ayuda facilitaron su labor como Director del Instituto Colombo-Alemán, durante nueve años. the Magdalena. The extreme high waters cause cyclical floodings of the Ciénaga Grande de Santa Marta, a brackish water lagoon communicated with the Magdalena, and thus the cyclical extinction of the oyster Crassostrea rhizophorae, which is of considerable economic importance.
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