Effects of pH of Culture Water on the Growth of the Japanese Pearl Oyster

Abstract

The present study has been done to make clear the effects of pH of culture water on the growth of the Japanese pearl oyster, Pinctada fucata (GOULD). Thirty five one-year-old oysters were equally divided into 7 groups, which were reared in separate recirculating tanks (Fig. 1) for 40 days. All the groups were given rice powder as food at a rate of 40mg per oyster per day, and 15l out of 40l of culture water in each tank was replaced with fresh sea-water once a day. The pH of water of the experimental tanks was first adjusted to 6 different levels from 7.3 to 8.1 by adding necessary amounts of hydrocloric acid or sodium borate to them and to keep the pH values constant calculated amounts of either chemical was added after every partial change of water, Chemical analysis of culture water and measurement of under-water weight of the animals were made seven times at five or seven-day intervals during the rearing period.
The results obtained are summarized as follows:
1) Average pH values of culture water throughout the rearing period were 7.36, 7.48, 7.66, 7.78, 7.89 and 8.07 in the 6 experimental tanks and 8.04 in a control (Fig. 2). As to the alkalinity of culture water of the tanks was in the same order as pH (Fig. 4). No remarkable differences were observed on the other chemical conditions of culture water among the 7 tanks except for the two adjusted to pH 7.36 and 7.48 on the average (Figs. 5-7).
2) The oyster in pH 8.07 on the average and control tanks showed a rapid growth represented with respective straight lines throughout the rearing period (Fig. 10). Among the other oyster groups such a relation was found that decrease in oyster weight was in a direct proportion to pH. The greater part of the animals reared in pH 7.36 and 7.48 on the average died during the first half of rearing period (Figs. 10 and 11). The relation between the weekly mean value for pH and the daily increase of under-water weight of oysters differed with the stage of rearing (Fig. 14). The weight increases in a certain degree at higher pH, about 7.8 or over, and decreases remarkably at lower pH during the first 19 days. While after 20 days the relation is linear, being represented with the following expression.
G(mg) 29.589 pH-234.794
Where G and pH show the weekly mean of daily increase of under-water weight and that of pH after 20 days. The limiting pH for growth is estimated from the expression to be pH 7.935.
3) From the significance test of the differences among the oyster groups in the dry weight of shell and that of meat at the end of rearing (Fig. 12), and the relationships between the appearance rate of etching and dissolving features on the surface of nacreous layer of the shell and the pH (Fig. 13), it may be concluded that the decrease in under-water weight of the oysters reared in low pH is attributed mainly to the dissolving of the shell rather than the decrease of the meat weight.