EFFECT OF CRUDE OIL POLLUTION ON OYSTERS 
207 
experimental oysters were kept in the extract only for a limited period of time, the 
result of the prolonged exposure remains to be determined. There was no indication 
in the present experiments of an increased tolerance in oysters due to repeated 
treatment. 
As it can be seen in tables 11, 12, 13, and 14, there was a large variation in the 
percentage of depression caused by a given concentration of the soluble fraction on 
individual oysters. Two explanations suggest themselves. First, there is a possibil- 
ity that in spite of the precautions taken in preparing the soluble fraction, the toxicities 
of individual samples were different. Second, the oysters used in the experiments 
may have different sensitivity and tolerance. The second explanation seems to be 
more plausible, for the wild oysters used in the experiments at Beaufort, coming 
from exposed flats, greatly varied in appearance, glycogen content, and other 
characteristics. 
From a large number of experiments with the water-soluble fraction the inference 
seems to be inevitable that crude oil discharged in the sea, regardless of whether it floats 
on the surface or, having been absorbed by mud particles, is deposited on the bottom, 
continuously yields water-soluble substances which narcotize the ciliated epithelium 
of the gills, thus reducing the rate of pumping of water and, therefore, materially 
decreasing the amount of food obtained by the organism. This should lead to gradual 
starvation and weakening of the oyster. The chemical nature of the substances and 
their concentration in the oil-polluted areas remains to be determined by future 
investigations. 
The effect of brine or so-called “bleed water”, which accompanies oil discharged 
by the wells and is usually dumped into the sea, has been studied by using the same 
technique as was employed in the experiments with oil. It has been found (table 6), 
that bleed waters of Lake Barre and Lake Pelto do not affect the muscular mechanism 
of the oyster in relatively high concentrations, provided the quantity present does 
not increase the salinity beyond the limits of tolerance. A 10-percent concentration 
of bleed water may exert a stimulating effect on the ciliated epithelium at least in some 
of the individuals. The depressing effect occurs at the concentration of 20 percent 
and higher. A 33-40 percent solution reduces the rate of pumping of water by the 
gills to 32.6 percent of its normal rate. The percentages of brine which cause this or 
greater depression are beyond any possibility of occurrence in nature. 
Experiments reported in the last section of the paper attempt to throw light on 
the possible effect of oil and bleed water on the production of the food of the oyster. 
It has been assumed that the results of the laboratory experiments with Nitzsc.hia, 
which occurs in the normal habitat of the oyster, and constitutes an important element 
in its diet, are applicable to other species of diatoms. It has been found that the 
presence of a heavy layer of oil on the surface of culture flasks inhibits the growth of 
Nitzschia (fig. 13) when oil remains on the surface for a week or longer. The soluble 
fraction of oil exerts a retarding effect on the growth of Nitzschia in concentration of 
25 percent and higher and when the extract is permitted to act over a considerable 
period of time. Low concentration may have a slightly stimulating effect. In many 
instances the addition of the oil extract stimulated the growth of bacteria, small 
numbers of which were always present in cultures, and caused the death of diatoms. 
Water-soluble substances obtained by dialysis through a collodion membrane also 
exerted a retarding effect on Nitzschia, both in natural sea water and in Miquel 
solution (table 22). 
