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Field Performance and QPX Disease Progress in Cultured and Wild-Type Strains of Mercenaria mercenaria in New York Waters

ABSTRACT A field experiment was conducted to compare the performance of different hard clam (Mercenaria mercenaria) strains in local clamming waters of New York state. Experimental clams included a Mercenaria mercenaria notata seed obtained from a Florida broodstock, and 2 New York seed strains obtained from local hatcheries, including a cultured M. mercenaria notata strain and a first-generation “wild-type” strain. Quahog parasite unknown (QPX) was acquired by the Florida clams in less than 2 mo of a July deployment of grow-out cages. Prior field studies comparing susceptibility of northern and southern hard clam strains observed QPX acquisition after clams had overwintered in the field, raising the question that higher susceptibility observed in southern seed clams could be a result of poor adaptation to winter water temperatures. Our results show that the southern strain acquired QPX after the clams had only been exposed to the warmest period of water temperatures for this field site (22.3°C on average), thus excluding poor acclimation to winter temperatures as the main aggravating factor. In contrast, QPX was not observed until the second summer in the cultured New York (M. mercenaria notata) strain in which clam survival was high and infection prevalence remained minimal. The New York “wild-type” clams displayed good growth and did not acquire QPX at all, providing evidence for the potential utilization of local wild broodstocks to enhance the resistance of cultured strains. Histopathology observations offered further insights to infection dynamics, with early, light infections almost exclusively localized in mantle and gill tissues, clearly supporting the theory that these organs (predominately the mantle) are sites of acquisition for QPX infections.