Morphology of the spores of Myxosoma cerebralis (Hofer, 1903) and M. cartilaginis (Hoffman, Putz, and Dunbar, 1965) [classical article].

The spore of Myxosoma cerebralis is redescribed and compared with that of a close relative, M. cartilaginis. M. cerebralis is smaller, with greater variability in size and shape. It possesses a mucous envelope whereas M. cartilaginis does not. The surface architecture of M. cerebralis, as seen with the scanning electron microscope, differs in possessing a conspicuous circumsutural groove, prominent polar filament pores, and a mucous envelope. Since a unanimous decision on the correct generic status of M. cerebralis has not been reached, we continue in this paper to call this species Myxosoma cerebralis, in spite of the views of Walliker (1968) and Lom (1969), who do not consider the absence of the iodinophilous vacuole as adequate reason to warrant generic separation of Myxobolus and Myxosoma and consider the latter a junior synonym of the former. Myxosoma cerebralis, one of the most notorious myxosporidean species, is the causative agent of whirling disease of salmonid fish. This parasite, discovered first in Germany, has been subsequently found in other European countries, the United States, far eastern USSR territories, and even South Africa (Hoffman, 1970). According to Shulman (1966) and Hoffman, Dunbar, and Bradford (1969) it has thus far been found in the following species: Oncorhynchus gorbuscha, 0. keta, 0. kisutch, 0. masu, 0. tshawytscha, Salmo gairdneri, S. salar, S. trutta, Salvelinus fontinalis, S. leucomaenis, S. malma, S. namaycush, and Thymallus thymallus. Quite recently, whirling disease has been reported to have Received for publication 27 April 1971. * Supported in great part by NSF Research Grant GB 7013X to Dr. John 0. Corliss, to whom the senior author is indebted for the use of his research facilities. t Present address: Institute of Parasitology, Czechoslovak Academy of Sciences, Prague, Czechoslovakia. spread through additional areas of the United States (Hnath, 1970; Yasutake, 1970). While Shulman (1966) supposes that M. cerebralis has long had an holarctic distribution, occurring in naturally infected water reservoirs or streams from which it is able to spread and infect cultured salmonids, it is generally assumed that the recent appearance of M. cerebralis infections in North America is due to the importation of infected fishes from Europe. The final solution of this problem would require, among other things, a complex comparative analysis of M. cerebralis from European, Asian, and American sources, including a detailed morphological evaluation. Such a morphological analysis would rely largely on the morphology of the spores and similar comparisons should also be made for other myxosporidean species. Curiously enough, no accurate description of spores of M. cerebralis is offered in the literature. Kudo (1920), in his now classical monograph, used spore drawings made by Plehn (1904). These rather simple drawings show a certain degree of spore variability, and depict the spores as having a protruding and narrow sutural valve. The more recent and authoritative monograph of Shulman (1966) again uses the drawings of Plehn, in addition to more recent but schematic drawings of the spores. In other publications on M. cerebralis, the spores are also represented very schematically. Everyone seems somehow to take the definition of