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|Title:||The brackish water clam Rangia cuneata as indicator of ecological effects of salinity changes in coastal waters|
|Authors:||Texas A & M University. Department of Biology.|
Hopkins, Sewell Hepburn, 1906-
Anderson, Jack W.
|Publisher:||Hydraulics Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Contract report (U.S. Army Engineer Waterways Experiment Station) ; H-73-1.|
Abstract: In the search for a laboratory test that could be used to determine when salinity changes were favorable or unfavorable, salinities in the range from 0 to 38 ppt were tested on adult Rangia clams for effects on survival; regulation of internal salinity; intake, use and release of amino acids; respiration; glycogen use under aerobic and anaerobic conditions; feeding rate; ciliary activity; uptake of glucose; glycogen storage and "index of condition" in natural environments through a seasonal cycle; carbohydrate-digesting enzymes; and reproduction. It was determined that Rangia cuneata has a system of compensating reactions that allows it to adjust to changes in salinity over the range frcm 0 to 38 ppt and over the temperature range from 10 to 35°C without harm. It was concluded from these and further studies that the key to the welfare of a Rangia population is not the physiology of the adult individuals, but reproduction and recruitment. The keys to the use of Rangia cuneata as an indicator were found to be two facts : (1.) a change in salinity, either up from near 0 or down from 15 ppt and above, is necessary to induce spawning; (2.) the embryos and early larvae can survive only in salinities between 2 and 10 (or 15) ppt. On the basis of laboratory and field studies, the model proposed for Rangia in estuaries has the population consisting of: (1.) a central subpopulation in the most favorable breeding zone where the salinities between 2 and 10 ppt and the changes in salinity necessary for reproduction occur in most years; (2.) a low-salinity population upstream that is made up of one, two, or three year-classes resulting from larvae that were carried up the estuary, set, and survived in infrequent favorable (high-salinity) years; and (3.) a similar subpopulation of one or a few year-classes downbay that set in years of freshets. Since a change in salinity, not just a favorable level, is required for reproduction, perfect stabilization of salinity at any level will result in dying out of the population in 15-20 years when old clams reach the limits of their life span. To use Rangia as an indicator of salinity climate, a large number of clams of all sizes are collected at random and measured for construction of a histogram that will reveal the number of modal peaks, which represent year-classes. If there are several such peaks, especially if there is a peak in the lengths below 30 mm, the population is in good shape and can feed many fishes, crustaceans, wild ducks, and other desirable clam-eaters. If there is only one size class, the population is in danger unless there is a nearby breeding population to replenish it whenever conditions permit. When Rangia dies out, there will inevitably be a decrease in the number of fish, crustaceans (crabs and shrimp), and birds that the brackish-water area can support. NOTE: This file is large. Allow your browser several minutes to download the file.
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