Morphological and phylogenetic studies of marine Myxobolus spp. from mullet in Ichkeul Lake, Tunisia

Six species of the genus Myxobolus (Myxozoa) from the marine environment were collected from two species of mullet (Mugil cephalus and Liza ramada) in Ichkeul Lake, Tunisia. Four of these species were described previously (Myxobolus bizerti, Myxobolus ichkeulensis, Myxobolus spinacurvatura, and Myxobolus episquamalis) and two (Myxobolus exiguus and Myxobolus muelleri) are redescribed. The small subunit ribosomal (18S rDNA) sequences of these six myxozoans were obtained and compared to traditional criteria used in the identification and taxonomy of myxozoan species (such as spore morphology, host specificity, and tissue tropism). A distance analysis of 1,600-1,700 base pairs of the 18S rDNA of the six species indicates that they formed a monophyletic group separate from Myxobolus spp. found as parasites of freshwater fish. The sequence analyses also confirm that these morphologically different Myxobolus spp. that infect mullet represent different species. Lastly, M, exiguus and M. muelleri, which were found in the same host, exhibit clear differences in spore morphology but sequencing of two different regions of the 18S rDNA show they are closely related. These results demonstrate the utility of DNA sequence data in providing more detailed relationships among the Myxobolus species based upon existing morphological taxonomic approaches. We suggest that future descriptions of Myxobolus spp. provide both careful spore descriptions as part of the traditional criteria but also 18S rDNA sequence data that will aid in situations where morphological details may be absent or misleading.