A hyperparasitic microsporidium Unikaryon richardi sp. n. from Probolocoryphe lanceolata (Microphallidae) metacercarial cysts infecting fiddler crabs from a Florida mangrove marsh.

Today, 36 species of hyperparasitic microsporidia are known to infect digenean hosts. Nine of those belong to the genus Unikaryon. Here, we report a new species of this genus recovered from encysted metacercaria of a trematode Probolocorphe lanceolata infecting fiddler crabs, Uca rapax, in Tampa Bay, Florida. Infected metacercarial cysts were heavily melanized and about twice as big as uninfected ones. Fresh spores liberated from cysts, measured 3.1-4.5 x 1.7-2.5 µm. In BLAST search, SSUrDNA sequence (GenBank Acc.# PV599985-86) similarity with the closest match, U. panopei, was 97.4% suggesting the new microsporidium belongs to the genus Unikaryon. Accordingly, on phylogenetic trees the novel sequence fell in a short-branched dichotomy with U. panopei. Ultrastructural study supported affiliation with Unikaryon as well. Mononucleate spores measured 2.7-3.3 x 1.3-1.5 µm on thin sections, resided in pairs in a thin-walled parasitophorous vacuole. Features of fine morphology, such as eccentrically positioned anchoring discs, large posterior vacuoles, bi-partite polaroplasts, structure and arrangement of polar filaments coiled in 6 rows, resembled Unikaryon spp., particularly U. panopei. However, larger sized spores, thinner and more fragile PV membrane, different structure of the spore envelope and other dissimilarities, allowed differentiation of the novel microsporidium form U. panopei, and other Unikaryon spp. We named a new species Unikaryon richardi, to commemorate our late friend and colleague, Dr. Richard Heard, an expert in marine crustaceans and their digenean parasites, who discovered this microsporidium. The paper provides new insights into Unikaryon spp. life cycles and expands the number of Unikaryon species with known SSUrDNA sequence and ultrastructure.