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Isozyme analysis at 24 loci was carried out on anisakid nematodes of the Anisakis simplex complex, recovered from various intermediate/paratenic (squid, fish) and definitive (marine mammals) hosts from various parts of the world. A number of samples were found to belong to A. simplex sensu stricto and Anisakis pegreffii, widely extending the geographic ranges and the number of hosts of these 2 species. In addition, a new distinct gene pool was detected, showing different alleles with respect to A. simplex s. str and A. pegreffii at 5 diagnostic loci (99% level). Samples with this gene pool were assigned to a new species, provisionally labeled A. simplex C. Reproductive isolation between A. simplex C and the other 2 Anisakis species was directly assessed by the lack of hybrid and recombinant genotypes in mixed samples from sympatric areas, i.e., Pacific Canada for A. simplex C+A. simplex s. str., South Africa and New Zealand for A. simplex C+A. pegreffii, even when such samples were recovered from the same individual host. Similar levels of genetic divergence were observed among the three species (DNei from 0.36 to 0.45). At the intraspecific level, Canadian Pacific and Austral populations of A. simplex C were found to be genetically rather differentiated from one another (average DNei = 0.08), contrasting with the remarkable genetic homogeneity detected within both A. simplex s. str. and A. pegreffii (average DNei about 0.01). Accordingly, a lower amount of gene flow was estimated within A. simplex C (Nm = 1.6) than within the other 2 species (Nm = 5.4 and 17.7, respectively). Anisakis simplex C showed the highest average values of genetic variability with respect to both A. simplex s. str. and A. pegreffii, e.g., expected mean heterozygosity. Hr = 0.23, 0.16, and 0.11, respectively, in the 3 species. Data on geographic distribution and hosts of the 3 members so far detected in the A. simplex complex are given. Their ecological niche is markedly differentiated, with a low proportion of hosts shared. Intermediate and definitive hosts of A. simplex s. str. and A. pegreffii appear to belong to distinct food webs, benthodemersal, and pelagic, respectively; this would lead to different transmission pathways for the parasites.

The intramuscular phase of development of Kudoa thyrsites, the myxosporean associated with post-mortem myoliquefaction, or 'soft flesh syndrome', is described using histological preparations of the musculature of seawater netpen-reared Atlantic salmon Salmo salar. Hatchery-reared Atlantic salmon were naturally exposed to the infective stage whlle held in the expenmental seawater netpens of the Pacific Biological Station in Nanaimo, British Columbia, Canada. In fish exposed during the summer of 1995, K. thyrsites infections were first detected in the somatic musculature at 13 wk postexposure (p.e.) using only light microscopy. In the 1997 exposure, infections were first detected at 6 wk p.e. using a PCR test and at 9 wk p.e. using light microscopy. The earliest stage detected by histology was a small plasmodium containing 4 nuclei. No host response was observed that was directly related to the presence of intact plasmodia within muscle fibers. However, a response was associated with ruptured plasmodia, which was characterized by chronic, multlfocal inflammation between the muscle fibers.

The phylum Myxozoa has been considered to comprise two classes, Myxosporea Bütschli, 1881 (primarily of fishes) and Actinosporea Noble in Levine et al., 1980 (primarily of aquatic oligochaetes). About 10 years ago it was demonstrated that the life cycle of Myxobolus cerebralis Hofer, 1903 (Myxobolidae: Platysporina) of salmonid fishes requires transformation of the myxosporean into an actinosporean stage in the oligochaete worm Tubifex tubifex (Tubificidae), and that the stage infective to fish is the actinosporean spore. This type of two-host life cycle has now been demonstrated or strongly implicated for 14 myxosporean species, belonging to 6 genera in 4 families. In light of these findings, the taxonomy of the Myxozoa is revised. We propose the following: suppression of the newer class Actinosporea and the order Actinomyxidia Štolc, 1899; and suppression of all families in the Actinosporea except Tetractinomyxidae. This family and its one genus, Tetractinomyxon Ikeda, 1912, are transferred to the order Multivalvulida Shulman, 1959 (Myxosporea). We also propose that actinosporean generic names be treated as collective-group names, thus they do not compete in priority with myxosporean generic names. Triactinomyxon dubium Granata, 1924 and Triactinomyxon gyrosalmo Wolf and Markiw, 1984 are suppressed as junior synonyms of Myxobolus cerebralis. The myxosporean stage of no other previously named actinosporean has been identified. Other actinosporean species are therefore retained as species inquirendae until their myxosporean stages are identified. A revised description of the phylum Myxozoa is provided that includes our proposed taxonomic and nomenclatural changes.

Laboratory transmission studies were conducted on Myxobolus arcticus, a myxosporean that infects the brain of Pacific salmon (Oncorhynchus spp.). Attempts at transmission from fish to fish with spores of M. arcticus from sockeye salmon (Oncorhynchus nerka), including experiments with spores aged for up to 9 months in sterilized mud, were unsuccessful. Transmission was achieved when hatchery-reared (in well water) sockeye salmon fry were exposed to the oligochaete Stylodrilus heringianus collected from a lake where M. arcticus infections are common in this fish. All experimental fish exhibited the infection when examined 3 –4 months after exposure. Of 23 sockeye salmon fry exposed to triactinomyxon spores collected from naturally infected S. heringianus, 21 were found to be infected with M. arcticus spores when examined after 3 months. Thus, the life cycle of M. arcticus involves transformation into a triactinomyxon stage in S. heringianus. Alternate development of myxosporeans in aquatic oligochaetes has been established or implicated for nine other species of myxosporeans belonging to three families, but this is the first report of alternate development in a lumbriculid worm.