Perkinsus-Karlssoni N-Sp (Apicomplexa) In Bay Scallops Argopecten-Irradians

In May 1989 a previously unidentified apicomplexan parasite was found in broodstock bay scallops Argopecten irradians from the east coast of Canada. Host tissue responses ranged from focal hemocytic encapsulation to the formation of abscesses containing necrotic hemocytes, parasites and ceroid. Bay scallops held for 6 mo in closed-system, artificial seawater at 22 to 24-degrees-C produced detectable infections much earlier than specimens grown in estuarine waters. Additionally, the lesions in experimentally held bay scallops showed less phagocytosis and encapsulation than those found in field-sampled bay scallops. Transmission of the parasite appears to be direct, having occurred during the brief exposure of the spat generation to infected broodstock. The parasite was undetectable in bay scallops under 7 to 10 mm in length (i.e. < 6 mo old) and only appeared in tissue sections after the scallops had been held for 6 wk in warm water. Thioglycollate culture of tissues from infected scallops yielded zoosporangia which stained blue-black with Lugol's iodine. This test is diagnostic for Perkinsus species; however, the concentration of zoosporangia in the bay scallop tissues did not attain the levels found in previously described Perkinsus species. The species herein described is a new species based on both morphological and epidemiological differences from previously described species. The name proposed is Perkinsus karlssoni after John D. Karlsson, who has provided extensive studies of bay scallop reproductive biology as well as parasites. Biflagellate zoospores released from the Lugol-positive zoosporangia when transferred to seawater survived over 6 wk at 4-degrees-C, but were unable to tolerate either full-salinity seawater (3.3%) or salinities under 1.4%.