Mucochytrium quahogii (=QPX) Is a Commensal, Opportunistic Pathogen of the Hard Clam (Mercenaria mercenaria): Evidence and Implications for QPX Disease Management

Abstract

Mucochytrium quahogii, commonly known as QPX (Quahog Parasite Unknown), is the causative agent of QPX disease in hard clams (Mercenaria mercenaria), but poor understanding of the relationship between host and pathogen has hindered effective management. To address this gap in knowledge, we conducted a two-year study quantifying the distribution and abundance of M. quahogii in hard clam tissue, pallial fluid, and the environment. M. quahogii was broadly distributed in clams and the environment, in areas with and without a known history of QPX disease. M. quahogii in clams was not strongly related to M. quahogii in the environment. M. quahogii was always present in either the tissue or pallial fluid of each clam, with an inverse relationship between the abundance in the two anatomical locations. This study suggests that the sediment–water interface and clam pallial fluid are environmental reservoirs of M. quahogii and that there is a host-specific relationship between M. quahogii and the hard clam, supporting its classification as a commensal, opportunistic pathogen. There appears to be minimal risk of spreading QPX disease to naïve clam populations because M. quahogii is already present and does not appear to be causing disease in hard clam populations in locations unfavorable for pathogenesis.

A Microcosm Experiment Reveals the Temperature-Sensitive Release of Mucochytrium quahogii (=QPX) from Hard Clams and Pallial Fluid as a Stable QPX Reservoir

“…From the 1 L of seawater removed, 400 mL to 1 L was filtered onto a 0.4 µm polycarbonate filter (or two filters if necessary, to meet the minimum of 400 mL) under low pressure (<5 in Hg) and the filter(s) was stored at −80 °C until DNA extraction. At the end of the experiment, seawater was collected (9-week sample), biofilms growing on the clam shell were swabbed (sterile cotton-tipped applicator), pallial fluid was collected, and QPX disease diagnostics were performed [ 16 , 29 ]. Hard clam mantle tissue homogenate samples were stored at −80 °C until extracted with the NucleoSpin Genomic DNA Tissue kit (Macherey-Nagel, Inc., Bethlehem, PA, USA) and assayed by qPCR [ 29 ].…”

“…QPX was assayed in pallial fluid using qPCR [ 29 ] (same assay as tissue samples) and in seawater and shell swabs using a nested qPCR (nqPCR) assay described previously [ 30 ]. For hard clam tissue and pallial fluid samples, weighted prevalence (WP) was determined based on the sum of QPX load rated on an intensity scale ( Supplementary Table S1 ) for each individual clam, divided by the number of clams assayed for each sampling event or cohort [ 16 , 29 ]. As previously established, for the qPCR assay (hard clam tissue and pallial fluid), samples that had a C q value and appropriate melt curve peak temperature (T m ) but were below the limit of detection (LOD) of the assay were considered positive and denoted “BLD” (below the LOD), since they were not truly negative but also could not be accurately quantified [ 16 , 29 ].…”

“…However, in addition to M. quahogii and the hard clam, other labyrinthulomycetes have been reported as opportunistic parasites in a variety of marine organisms [ 13 , 14 ]. Research into labyrinthulomycete pathogens has revealed their broad distribution in the marine environment, association with apparently healthy individuals, and opportunistic pathogenesis in response to changing host–microbe–environment interactions [ 15 , 16 , 17 , 18 ]. However, the question remains—are they avirulent commensals of their hosts or environmentally acquired pathogens?…”

“…If M. quahogii is an environmentally acquired pathogen, then it may gain access to the hard clam pallial tissues—where QPX infections appear to originate—during seawater pumping and filtering [ 2 , 4 , 19 , 20 , 21 ]. This requires the presence of infectious M. quahogii propagules in the environment, and indeed M. quahogii has been detected at low abundance but widespread throughout sediment, seawater, and other components of the hard clam habitat [ 16 , 20 , 22 , 23 ]. Whether M. quahogii lives independently in the environment, or is released at a sufficient rate from the hard clam (or other) hosts to maintain its observed distribution, is unknown.…”

“…In a recent survey of wild hard clams, it was determined that “healthy” hard clams (no detection of M. quahogii via quantitative PCR in hard clam tissue) contained M. quahogii in their pallial fluid [ 16 ]. Since M. quahogii was prevalent (73% of clams) and the dominant labyrinthulomycete (70% of clams contained only M. quahogii ) in hard clam pallial fluid, M. quahogii either represents a (1) common transient inhabitant from seawater or (2) a more permanent resident of the pallial fluid microbiota.…”