Oysters and Copper

Abstract

I AM interested to hear of Mr. Lowe's case, where he considers the oysters from near Swansea owe their colour to a very considerable amount of copper. As I stated in the concluding paragraph of my last letter (p. 293), "It is evident that there are several distinct kinds of greenness in oysters." Amongst these I cited Dr. Thorpe's recent demonstration of notable amounts of copper in oysters from Falmouth; so it can scarcely be said that I " appear to doubt the occurrence of copper." Dr. Charles Kohn has kindly re-investigated the matter for me lately, and has determined the amounts of both copper and iron present in various kinds of oysters by electrolytic methods. He finds the green Marennes oyster contains about 0.4 mgrme. (say.006 grains) of copper, which agrees pretty closely with the figures given by previous writers. This seems to be the normal amount present in all oysters, white or green, and due to the hæmocyanin of the blood. Dr. Thorpe, however, finds that the green Falmouth oysters have, on the average, each.023 grains of copper, which falls to the normal amount (.006) on re-laying in another locality, and which is "obviously caused by the mechanical retention of cupriferous particles" (Thorpe, NATURE, p. 107). If Dr. Thorpe means by this that copper mud is entangled in the water and food passages of the oyster, is it not possible that, although the oyster is green, and copper is present, the colour may be due—as in most green oysters—to another cause? This mere entanglement (more or less accidental) of copper-bearing material in the passages of the oyster may also be the explanation of the extraordinarily high figure reported by Mr. Lowe—a figure (.04 grammes) as large, I may remark, as that of the total ash in the case of some of my oysters investigated by Dr. Kohn.