Estimating species richness of myxozoan parasites (Phylum Myxozoa) in the Pacific Northwest

Background/Question/Methods

Myxozoans are microscopic, multicellular endoparasites that are best known from fishes. Most myxozoans likely have two spore stages and two obligate hosts - a myxospore stage that develops within a vertebrate host and an actinospore stage that develops within an invertebrate host. Life cycle counterparts have been matched for ~2% of known species. Our goals were to investigate the diversity of Pacific Northwest myxozoan taxa and identify conspecific stages from both vertebrate and invertebrate hosts.

Results/Conclusions

We found myxozoans in wild-caught and cage-exposed fish and annelids from 6 river basins, including 9 fish hatcheries. Nearly 500 infected fish yielded 62 myxospore taxa from 10 genera: Myxobolus, Myxidium/Zschokkella, Henneguya, Chloromyxum, Sphaerospora, Myxobilatus, Ceratomyxa, Parvicapsula, Unicauda and Acauda; most of these were novel records. Samples of 163 infected oligochaete and polychaete worms yielded 44 actinospore types from 5 morphological collective groups: Triactinomyxon, Antonactinomyxon, Tetractinomyxon, Siedleckiella and ERA (a term we introduced to accommodate collective groups Echinactinomyxon, Raabeia and Aurantiactinomyxon whose members appear to fall out on a morphological continuum of form). We found Triactinomyxon and ERA-type spores with unusual morphologies: hair-like, barb-like, or finger-like protrusions on their caudal processes. We discovered the largest known actinospores -  almost 1mm across -  which demonstrated that myxozoans can range across 3 orders of magnitude in size (10-1000μm). We sequenced the SSUrRNA genes of 53 myxospores and 45 actinospores, to resolve species from morphologically ambiguous taxonomic units and to identify conspecific life stages. Only 12/44 actinospore sequences matched myxospores from our dataset or GenBank. We used these data with a modified Lincoln-Peterson method to estimate that there are at least 122 myxozoan species in the study area, of which we encountered only about half. These data attest to the diversity and species richness of myxozoan parasites in the Pacific Northwest, and suggest that at least as many unknown species exist as those already encountered.