M5039 is an undescribed myxozoan parasite (class Myxosporea) assigned a provisional specimen identifier from a museum collection, representing a potentially new species within this parasitic cnidarian group. Myxozoans in general are obligate endoparasites of fish and other aquatic hosts, producing diagnostic spores with polar capsules homologous to cnidarian nematocysts. Genomic resources for undescribed myxozoan lineages are essential for understanding the full extent of parasitic cnidarian diversity and host-parasite molecular evolution.