The Sensitivity of Barnacles and Their Larvae to Copper and Mercury

Abstract

ABSTRACT Experiments on the toxic effects of copper and of mercury on various stages in the life history, nauplii, cyprids, metamorphosing cyprids and adults of acorn barnacles are described. Most of the work has been carried out on the appropriate stages of Balanus balanoides, but it has been supplemented, wherever possible, by parallel experiments using B. crenatus. Cupric sulphate and mercuric chloride solutions were used as sources of the poisons, and the actual concentration of poison present was determined at the end of each experiment. The period of exposure to the poison was usually 6 hr. A sharp decrease in sensitivity occurs, both in B. balanoides and B. crenatus, between the last naupliar stage and the cyprid. The relative sensitivity of the cyprid of B. balanoides to copper and to mercury is similar to that of the sixth stage nauplius, but the cyprid of B. crenatus is particularly insensitive to copper, but is more sensitive to mercury than that of B. balanoides. Cyprids of B. balanoides only settle in the laboratory some 4 or 5 days after being taken in the plankton and during this period the sensitivity to copper and mercury increases. Settlement can be prevented by very low concentrations of copper and mercury, even though there are no obvious lethal effects. The cyprid of B. crenatus settles more readily in the laboratory, and little change in sensitivity seems to occur during the short interval between catching and settlement. Metamorphosis of the cyprid of either B. balanoides or B. crenatus cannot be prevented by the concentrations of copper (up to 7 mg./l.) possible in sea water. This means that another sharp change in sensitivity occurs after the cyprid of the former species has settled but, because of the low sensitivity of the free-swimming cyprid of B. crenatus, such a change has not been detected for this species. The young barnacle of both species immediately after metamorphosis is much more sensitive to copper than the metamorphosing cyprid. For B. balanoides the sensitivity, both to this poison and to mercury does not change significantly as the barnacle grows, but a small specimen of B. crenatus (less than one month old) is appreciably less sensitive, both to copper and to mercury, than slightly older individuals. Copper and mercury appear to be roughly equi-toxic to the adult of B. balanoides ; the adult B. crenatus is slightly more sensitive to copper and distinctly less sensitive to mercury than that of B. balanoides. In view of these differences in sensitivity of the various stages in the life history of two closely related species, the results obtained with one species should not be held to apply to other species of barnacles. A number of other experiments were carried out using the free-swimming cyprids of B. balanoides. Diluted sea water, though it has no toxic effect in itself over short exposure periods, markedly reduces the toxicity of both copper and mercury. Hypertonic sea water also reduces the toxicity of copper. The sensitivity of these larvae to either copper or mercury is not affected by the presence of sodium oleate. Exposure of these larvae to a wide range of copper concentrations in artificial sea water reveals certain anomalies which may be important in explaining the results obtained when they are exposed to mixtures of copper and mercury in natural sea water. The toxic effect of mercury seems to be similar in either natural or artificial sea water.