Detection and Quantification of an Ecologically Important Marine Pathogen, Paramoeba invadens

Abstract

A pathogenic amoeba, Paramoeba invadens, causes re-current mass mortalities of sea urchins (Strongylocentrotus droebachiensis) in coastal Nova Scotia, with major ecological consequences for the structure of the rocky subtidal ecosystem. Outbreaks of this disease (termed paramoebiasis) are sporadic, and due to a lack of reliable monitoring tools, the source population and epizootiology of this species is poorly understood. I developed and validated a PCR-based assay to detect and/or quantify P. invadens in sea urchin tissue, sediment, and seawater. Primers specific to P. invadens were designed to the nuclear SSU rRNA gene and were used in PCR and qPCR analyses to detect and quantify abundances of P. invadens during, following, and in absence of a natural disease outbreak. Using these methods, P. invadens was reliably detected in sea urchin tissue and a comparison of pathogen load (cells mg-1 tissue) in “asymptomatic” and “symptomatic” sea urchins indicated a minimum load of ~ 1 cell mg-1 tissue for observing overt signs of paramoebiasis in sea urchins. Paramoeba invadens was detected for the first time in sediment during and following an outbreak of disease. It also was detected in seawater collected in fall 2015 in the absence of sea urchin mortality or a strong storm event, when consequently P. invadens was not expected to be present. Abundance (cells l-1) of P. invadens in seawater appeared to be associated with peaks in seawater temperature, but was low overall (range 0 – 9.74 cells l-1), and possibly below the level required to encounter and infect small, localized populations of sea urchins. Evidence of a possible annual presence of P. invadens in Nova Scotia may shed new light on mechanisms of introduction, spread, and persistence of P. invadens along this coast and the role of large-scale meteorological events in these processes.