As to the mud oyster, he thinks very highly of it, and regrets that it has been so ignored by our oyster culturists. He is quite sure that if our mud oyster were cultivated and educated as it is now in Europe, it would be brought to the same perfection as the European and American oyster. It has been said of our mud oyster that it will not keep, and will not carry; but the same was said of its European representative until its cultivators came to discover that by a gradual process of raising it could be educated to keep quite long enough for all commercial purposes.
To come to the real point on which Dr. Cox considers that all oyster culture has failed in Australian waters. It is an established fact that the drift oyster and also the mud oyster require a diatomatic food for their existence. These two varieties of oysters no doubt consume other forms of food, but living diatoms constitute by far the greatest part. On the contrary, the rock oyster does not appear to need the diatomatic nutriment to any extent, and is fed chiefly by larval forms of marine life. Thus, knowing that the drift and mud oysters require different food from the rock oyster, it is easy to see why our oyster culturists have failed in establishing new beds of oysters in various places. For the whole purport of Dr. Cox's paper may be summarised into expressing his belief that sufficient attention has not been devoted to the replenishment of our natural beds, with their own kind.
In former days, when our drift and mud oysters were in their prime, there were many pools of naturally preserved fresh water - in fact, often very extensive lakes on the banks of many of the estuaries and inlets running up into our rivers and creeks. Now, these reservoirs appear to have been constantly supplied by subaqueous springs of fresh water, and in consequence the supply of diatomatic food was abundant. It was abundant, because, as it is well known, diatomatic life depends for its existence, to a great extent, on the presence of fresh water. These collections of fresh water no longer exist, so that the diatomatic food supply is not forthcoming to maintain the drift and the mud oyster. But there are other additional causes for the disappearance of these latter. The surrounding ground has been cleared for agricultural purposes, and the earth, broken up by ploughing, has been washed into these estuaries, and has suffocated, as it were, the oysters in their natural position. Again, the water which flows over the oysters is continually being disturbed by the different steamers passing up and down. The stirred-up mud they create gets into the gills, and destroys the oysters. From the preceding it will be seen that Dr. Cox is of opinion that the loss of diatomatic food is one of the principal causes in diminishing the supply of drift and mud oysters, and in addition he believes that this decrease has been also brought about by muddy water. Indeed, fairly clear water is absolutely necessary for their existence. On the contrary, water loaded with any sediment interferes with the functions of the oyster so much as to destroy it. In this way floods are considered to be beneficial, and even almost necessary, to proper oyster development; for they clear out the accumulations of mud, silt, and marine vegetable growth, thus giving the beds every chance. And further, Mr. Thomas Whitelegge, of the Australian Museum, has made some investigations into what is known as the "worm disease," due to the polydora ciliata. It was commonly supposed that it was not the worm itself which was fatal, but that by boring through the shell it afforded entrance for the fine mud, which quickly destroyed the oyster. From the result of his researches, however, Mr. "Whitelegge believes that the young worm simply swims into the open oyster, and that it immediately begins to construct a tube and collect a large quantity of mud. The worms appear to have the power of collecting a large quantity of mud in a very short time. The mud is covered over at once by the oyster with a thin layer of shelly matter, thus enveloping the worm, together with its mud. After this, one of two things happens : if the oyster be healthy, it envelops the worm and mud so quickly as to dispose of the intruder for good; but, on the other hand, if the oyster be unhealthy, or already infested, the shelly deposition is far slower, as a consequence of which the worm gains the ascendency, and the oyster succumbs.
In Victoria, too, the oyster fisheries are in a most unsatisfactory condition. According to Mr. Saville Kent, the author of The Great Barrier Reef of Australia and formerly Commissioner of Fisheries in several of the Australian colonies, and who is qualified to speak on these matters, the destruction of the oyster there has been brought about by sedimentary deposits, by parasitic growths, such as sponges, mussels, ascidians, and sea-weed; by the attacks of the dog-whelk and other natural enemies; and by their continual removal by human agency. He points out that there are the remains of magnificent natural beds in different parts, but that they are on the verge of ruin through neglect on the one hand and the invasion of poachers on the other. In short, he very plainly shows that unless active measures be taken for their general resuscitation and development, Victoria will have to look elsewhere for her oyster supply.