Defence reactions in and susceptibility of Australian and New Guinean freshwater crayfish to European-crayfish-plague fungus.
The deposition of melanin of the surface of the cell wall of the crayfish-plague fungus, Aphanomyces astaci, in infected cuticles of Australian crayfish seemed to be correlated with some degree of resistance to infection. The most obvious reactions towards the fungal structures present in the blood in vivo were their encapsulation by blood cells, many which then disintegrated. This was followed by melanization of the fungal surface. Crayfish blood inhibited completely hyphal growth in vitro and the disintegrating blood cells were apparently responsible for this effect as well as for the melanization. Phagocytosis of fungal structures was not observed. Melanization in vivo, and in vitro, due to a polyphenoloxidase released by the blood cells, was stimulated by the presence of the fungal cell wall surface. This seems to be a typical reaction of the crayfish in general. It was known earlier that European and Japanese freshwater crayfish are very susceptible to attack by the cray-fish-plague fungus but that North American cray-fish have a higher, although not complete, resistance and may be the natural hosts for the parasite. Nine species from different parts of Australia and from New Guinea were tested in aquaria and were all found to be susceptible, often very susceptible. Their susceptibility to infection was very similar to that of the European species, and the results support strongly the hypothesis that the parasite originated in North America and not in the Australian region or elsewhere. It would appear that man has not yet introduced the disease to the Australian region. Should this, however, occur, it will probably prove disastrous to the Australian and New Guinean freshwater crayfish fauna.