| Autor: |
Elston, Ralph A., Cheney, Daniel P., MacDonald, Brian F., Suhrbier, Andrew D. |
| Zdroj: |
Journal of Shellfish Research; Dec2003, Vol. 22 Issue 3, p667-674, 8p, 5 Black and White Photographs, 1 Chart, 8 Graphs |
| Abstrakt: |
To determine under what conditions winter mortalities of the Manila clam (Venerupis philippinarum, A. Adams and Reeve, 1850) might be the result of excessive exposure to low salinities, a series of experiments was conducted. Clams were exposed to various concentrations of salinity to determine their physiologic lower limit of tolerance to salinity concentration, the duration they could withstand lethal or marginal low salinities through the mechanism of shell closure, and diagnostic structural changes in tissues indicative of low salinity exposure. Salinities of ≤10 parts per thousand (ppt) were not tolerated in long-term exposures of 13 groups of clams. This lethal low salinity was also confirmed by the exposure of clams with a resection of a portion of shell. A salinity of 12.5 ppt was considered marginal, and various proportions of the different populations were able to tolerate this salinity, while no significant mortality occurred at ≥15 ppt. Clams could withstand lethal low salinities of 5 ppt and 10 ppt for between 6 and 8 days, but all populations exposed to lethal low salinities for 14 days and then placed at high ambient salinity (∼31 ppt) showed a high cumulative mortality. Clams may not die until several days after exposure to lethal low salinity followed by placement in a recovery tank at their normally tolerated high salinity. We found no significant difference in the responses of several groups of clams to the marginal salinity of 12.5 ppt when exposed at temperatures of 6°C, 12°C, and 18°C. Histologic examination showed that the following sequential changes occurred in the digestive gland in clams exposed to 10 and 12.5 ppt for between 2 and 14 days: loss of granulation of the digestive tubular absorptive cells; swelling of these cells and occlusion of the tubular lumina; and finally the shedding of necrotic tubular epithelium into the digestive gland tubular lumina. [ABSTRACT FROM AUTHOR] |
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