Epizootiology of Quahog Parasite Unknown disease in northern quahogs Mercenaria mercenaria

Journal of Shellfisheries Research, August, 2007 by M. Maille Lyons, Roxanna Smolowitz, Marta Gomez-Chiarri, J. Evan Ward

ABSTRACT The economically important marine bivalve mollusc, Mercenaria mercenaria, (commonly called a northern quahog or hard clam), has endured considerable mortalities caused by a thraustochytrid pathogen called Quahog Parasite X (QPX). Data on the percent prevalence of QPX infections were compiled from published reports along with our data to describe the epizootiology of QPX disease. QPX infections occurred in clams collected from both cultured beds and wild populations, but a higher percentage of QPX cases (76.5%) were from cultured clam beds. In addition, samples from cultured beds had a significantly higher prevalence (29.2 [ o r -] 27.2%) of QPX infections compared with samples from wild populations (9.6 [ or -] 9.6%). The highest prevalence of QPX infections occurred in clams from samples with an intermediate size range (shell lengths 20-55 mm). QPX infections occurred in both male and female clams, but infection prevalence does not appear to be correlated with sex or sex ratios. The geographical range of QPX-related clam mortalities was Atlantic Canada to the Eastern Shore of Virginia, USA. Only marginally significant differences were detected between the prevalence of QPX at different locations. There were no latitudinal gradients in QPX prevalence or frequencies, suggesting local factors were important in determining its distribution. Although QPX infections occurred throughout the year, no seasonal trends in the prevalence or frequencies of QPX were discernable. This summary of information available on QPX disease highlights the need for more thorough data collection regarding factors believed to be associated with its presence and severity in hard clams.

KEY WORDS: Quahog Parasite X (QPX), thraustochytrid, Mercenaria mercenaria, clam, northern quahog

INTRODUCTION

The commercially important bivalve, Mercenaria mercenaria, (northern quahog or hard clam), has suffered catastrophic mortalities from eastern Canada to coastal Virginia because of a pathogen called Quahog Parasite X (QPX; Whyte et al. 1994, Smolowitz et al. 1998, Ragone-Calvo et al. 1998). QPX is a small (4-120 [micro]m), spherical protist, characterized as a thraustochytrid (Whyte et al. 1994, Maas et al. 1999, Ragan et al. 2000, Stokes et al. 2002). In culture (Kleinschuster et al. 1998, Brothers et al. 2000) and in tissue section (Smolowitz et al. 1998), QPX occurs in three life stages: (1) thalli that develop into (2) sporangia that rupture to release (3) endospores (immature thalli). The presence of a fourth stage, zoospores, has not been confirmed (Brothers et al. 2000). In laboratory experiments, cultures of QPX proliferated best at 24[degrees]C, with a pH 7-8 and a salinity of 28 ppt and greater (Brothers et al. 2000).

The QPX organism has been documented in the environment. Positive results have been found for QPX in seawater, marine aggregates, sediments, and in association with invertebrates and macrophytes (Lyons et al. 2005, Lyons et al. 2006, Gast et al. 2006, Gast et al. submitted) using real time PCR (Lyons et al. 2006) and nested PCR followed by DGGE (Gast et al. 2006). The ecology of QPX is not well known, but there is information regarding the ecology of other thraustochytrids (Raghukumar 2002). Thraustochytrids were first described by Sparrow (1936) from a decaying piece of algae. Since then, thraustochytrids have been documented in coastal and oceanic waters, in pelagic and benthic habitats, and on plant and animal substrates (Miller & Jones 1983, Raghukumar et al. 1990, Raghukumar & Schaumann 1993, Naganuma et al. 1998, Raghukumar & Raghukumar 1999, Santangelo et al. 2000). Several parasitic associations have been documented for thraustochytrids and molluscs including octopuses, squid, sea hares, abalone, and bivalves (Polglase 1980, McLean & Porter 1982, Jones & O'Dor 1983, Bower 1987, Azevedo & Corral 1997, Anderson et al. 2003). Benign associations have been described for thraustochytrids with salps, sea urchins, corals, hydroids, and sponges (Raghukumar 1988, Frank et al. 1994, Ilan et al. 1996, Raghukumar & Raghukumar 1999, Thorsen 1999).

The parasite was first designated QPX in a paper by Whyte et al. (1994), which described mass quahog mortalities (occurring since 1989) in a Canadian hatchery on Prince Edward Island. The authors found the parasite to be identical to an unnamed one that caused mass mortalities of wild quahogs in Canada in the early 1960s (Drinnan & Henderson 1963). From 1990-1998, Canadian researchers monitored cultured hard clam (quahog) beds and hatcheries in New Brunswick, Nova Scotia, and Prince Edward Island (Bacon et al. 1999, MacCallum & McGladdery 2000) and documented the QPX organism in samples of clams collected from all three Canadian Maritime Provinces.

The first report of an outbreak of QPX disease in the United States was published by Smolowitz et al. (1996) and described a 4-y history of chronic, severe mortalities in cultured hard clams in Provincetown and Duxbury on Cape Cod in Massachusetts. Dying clams were positive for an endosporulating protist similar to the one observed in the Canadian hatchery (Whyte et al. 1994). Smolowitz et al. (1998) detailed the gross- and histopathology by comparing observations of the Massachusetts QPX-like organism to the one described by Canadian researchers. Subsequently, United States researchers reviewed archived tissue sections from old cases and discovered that episodic dieoffs in Barnegat Bay, NJ as early as 1976 and high mortalities in the Mitchell River in Chatham, MA around 1992 were also caused by QPX (Smolowitz et al. 1998). Between 1995 and 1998, clam seed from South Carolina that were planted in New Jersey waters suffered significant mortalities from QPX (Ford et al. 2002). Likewise, clam seed from New Jersey that were planted in Massachusetts waters suffered markedly higher levels of QPX disease than clam seed originating from Massachusetts (Smolowitz, unpublished data). Since then, QPX has been detected in New Jersey in wild hard clams from Raritan and Sandy Hook Bays (Ragone-Calvo & Burreson 2002). In 1996, researchers in Virginia began surveying wild and cultured hard clams (Ragone-Calvo et al. 1997, Ragone-Calvo et al. 1998). Their study was the first to document the presence of QPX in cultured clams as far south as Quinby Inlet on the Atlantic side of Virginia's Eastern Shore. They did not detect QPX in samples of cultured clams from Chesapeake Bay or in any samples from wild populations (Ragone-Calvo et al. 1998). In 2001, QPX was confirmed in Barnstable Harbor, Barnstable, and Pleasant Bay, Orleans, both on Cape Cod in Massachusetts (Hickey et al. 2002). Also in 2001, the first large scale mortalities from QPX disease were observed at a seaside location in Virginia (Ragone-Calvo & Burreson 2002, Camara et al. 2004). In 2002, preliminary testing detected QPX in wild clams from Raritan Bay, NY (Dove et al. 2004). This prompted researchers in New York to survey over 600 clams from 21 sites to document infection prevalence within the bay (Dove et al. 2004). In 2004. QPX was reported in yet another Cape Cod embayment (Wellfleet, MA; Fraser 2004) and in cultured clams from Rhode Island (this report).