Infection in Nature.
The Glochidia are ripe from the end of November to the end of January. The adults have been found to frequent the stony beds of several small lakes in West Canterbury, notably Lake Sarah, by the Midland Railway line, at an altitude of about 1800ft. Collections made at the end of November with a tow-net trailed slowly about six inches from the bottom, contained a large number of Glochidia. It is difficult to say whether the larvae were actually floating or were ejected by the parents as a result of the disturbance caused by the passage of the net through the water. It is probable that the latter is the explanation since, in such quiet water, organisms such as they with their relatively heavy shells would fairly readily sink after being ejected. Curtis and Lefevre (l.c., p. 98) have referred to the inability of Glochidia to propel themselves in spite of their flapping valves. They also (p. 101) point out that hooked Glochidia respond actively to tactile stimuli by closing up immediately on being touched. This applies equally well to the larvae of Diplodon.
Natural infections have been found in the young forms of the fishes, Galaxias brevipennis Günth. and Gobiomorphus gobioides Cuv. & Val. In the former case, the hosts were about 4.5 cm. long, and carried the parasites on the pectoral fins. In the latter case, the fish were about 1 cm. in length. The Glochidia had taken hold in various places, on the pectoral fins, on the upper lip clasping the
maxillary region, on the roof of the mouth, on the snout, in fact any place appeared suitable for attachment. A number of young Gobiomorphus was taken in Lake Mason, North Canterbury, at the edge of the water, and most of the specimens were infected. It is the habit of the small post-larval Gobiomorphus rather to keep at the bottom seeking food among the stones and vegetation, so that the heavy infections which were found, as many as six on the head and pectoral fins of an animal less than 1 cm. in length, is not to be wondered at. The post-larval Galaxias also tend to keep near the edge of the lake or stream, this depending on many conditions, one being the rate of flow which may drive them down or near to the edge. In lakes they are more often found, at the stage of 4 or 5 cm., swimming near the surface feeding on free floating particles rather than upon the more benthic material. The relatively light infections, three Glochidia at most, found on the young Galaxias, may be accounted for largely by the consideration of this habit of feeding nearer the surface.