Marine aggregates as reservoirs for thraustochytrids such as the bivalve parasite, quahog parasite unknown (QPX)

Marine aggregates (eg, marine snow, organic detritus, and flocs) may enhance the transmission of shellfish diseases by at least two mechanisms: (1) as reservoirs, if aggregates concentrate pathogens within their matrix; and (2) as vectors, if pathogen-laden, marine aggregates are more readily captured and retained by the gills of suspension-feeding bivalves. Northern hard clams (= quahog; Mercenaria mercenaria) from the northeast coast of North America have suffered severe mortalities from a bivalve parasite known as quahog parasite unknown (QPX). QPX is a facultative parasite but little is known regarding the ecology of QPX. Morphologic and molecular analyses have characterized it as a thraustochytrid. Thraustochytrids are single-celled, eukaryotic, fungal-like, marine protists associated with decaying vegetation, shells, and marine aggregates. The focus of this research is: if QPX is a thraustochytrid, and thraustochytrids are linked to marine aggregates then, are marine aggregates one of the reservoirs for QPX? Detection strategies for QPX-enriched marine aggregates, including fluorescent stains and in situ hybridization, will be presented along with endoscopic video illustrating hard clams filtering marine aggregates. Hard clams ingest some, but not all, aggregates. Those that are not ingested are collected at the base of the incurrent siphon until rejected as pseudofeces. Because this is also the site of QPX-filled nodules in infected clams from Massachusetts, we hypothesize marine aggregates facilitate infection.