Mucochytrium quahogii, also known as QPX or Quahog Parasite Unknown, is the causative agent of QPX disease in the hard clam (Mercenaria mercenaria). Host-pathogen-environment interactions between M. quahogii, the hard clam, and temperature were explored in a microcosm experiment. Hard clams were housed in individual tanks with sterile seawater under two temperature regimes: low (13 °C) temperature, which is thought to be optimal for QPX disease development, and high (20 °C) temperature, which has been shown to promote "healing" of QPX-infected clams. Hard clam tissue, pallial fluid, seawater, and shell biofilms were collected and assayed for M. quahogii. The release of M. quahogii from naturally infected live hard clams into seawater was detected only in the low temperature treatment, suggesting that temperature influences the release of potentially infectious cells. M. quahogii was commonly found in hard clam pallial fluid, even after 9 weeks in the lab, suggesting pallial fluid is a stable reservoir of M. quahogii within its primary host and that M. quahogii is not a transient component of the hard clam microbiota. Overall, results support a host-specific relationship and that M. quahogii is a commensal member of the hard clam microbiota, supporting its classification as an opportunistic pathogen.
Keywords: Mercenaria mercenaria; QPX disease; Quahog Parasite Unknown; commensal; host-pathogen-environment interactions; labyrinthulomycetes; thraustochytrid.