Mucochytrium quahogii S. Geraci-Yee & B. Allam, in Geraci-Yee, Brianik, Rubina, Collier & Allam, Protist 171(no. 125793): 3 (2021)

Species name registrations: 102596 (PhycoBank)

MycoBank number: MB 557969; Index Fungorum number: IF 557969; Facesoffungi number: FoF 12589;

Etymology: quahogii = Latinized version of the Native American name for hard clams (quahog)

Type host: Hard clam (Mercenaria mercenaria)

Holotype: Ex-type strain NY0313808BC7 (abbreviated 8BC7) has been cryogenically preserved and deposited in the American Type Culture Collection (ATCC) as deposit number TSD-50 and is available for purchase.

Holotype locality: Raritan Bay, Staten Island, New York, USA (40.501867, -74.1869)

Paratypes: Strains LI1, MA17, 20AC1, MA (ATCC 50749), VA

Diagnosis: Vegetative cells (thalli) 2-10 µm, and sporangia (sori) 10-120 µm, are globose and suspended in mucopolysaccharide secretion. Thalli do not attach to pollen grains and lack a rhizoidal ectoplasmic net. Two sporulation pathways exist, both of which produce non-motile spores (immature vegetative cells) and motile zoospores. In the first pathway daughter cells (endospores and zoospores) are produced internally, then released via fragmentation of the sporangial wall (sporangium type 1). In the second pathway, dissolution of the cell wall is followed by rapid budding of the mature sporangium, resulting in successive equal or unequal division of the multinucleate protoplasm, releasing both zoospores and non-motile spores concurrently (sporangium type 2). Daughter cells of this pathway often remain connected by cytoplasmic strands. Zoospores and type 2 sporangia are only observed in cultures washed free of mucus. Biflagellate zoospores are ovoid, 1-2 x 2-3 µm (width length) with flagella of unequal length, typical of thraustochytrids. Cyst stages are present, characterized by a thickened cell wall that may be either stratified or unstratified, and ranging in size from 30 to 70 µm. Short-lived protoplast stages are observed and only present when leaving the thickened cell wall of the cyst stage. Motility of protoplasts or an amoeboid stage were not observed. The species is cryopreserved in a metabolically inactive state by deep-freezing (reference provided below).

In the hard clam host, only the sporangium type 1 pathway, or endosporulation, is observed. The disease is marked by a massive inflammatory response often associated with the necrosis of infected tissues, resulting in poor clam condition and subsequent mortality. Gross signs of the disease include swollen, retracted, yellow/tan mantle edges, and yellow/tan inflammatory nodules (blisters or pustules 0.25 to 5 mm in diameter) along the mantle, accompanied with increased amounts of mucus. Primary areas of QPX infection are the mantle, gill, and siphon, with the visceral mass (including

gonads and kidneys), muscle (foot and adductor), and connective tissues involved in advanced cases. Histological examination of infected clams shows vegetative cells often surrounded by translucent halos, which are filled with parasite mucus in living clams that is washed away during the staining pro- cess. These halos are free of host cells and likely formed as QPX digests clam tissue extracellularly or as host cells are physically displaced by QPX mucus production.

GenBank Accession Numbers: 18S rRNA: MT484273.