During routine histological examination of oysters (Crassostrea virginica) from Great Bay, New Hampshire, a high prevalence and intensity of ciliate xenomas has been noted since 1997. Xenomas are hypertrophic lesions on the gills of bivalve molluscs caused by ciliates. Although not known to cause mortality in oysters, xenomas have not previously been reported at this high level of abundance. The objectives of this study were to characterize the xenomas, classify the ciliates, and gather baseline epizootiological data with correlations to environmental and biological parameters. Upon gross examination, xenomas appeared as white nodules located in the gill tissue, up to 3 mm in diameter, occasionally fusing into large masses along the gill filaments. Light microscopy of histological sections revealed xenomas located in the gill water tubes that often occupied the entire cross sectional area. Higher magnification revealed dual nuclei, eight kineties, and conjugation. Transmission electron microscopy revealed dual nuclei that vary in density, a maximum of twenty cilia in each kinety radiating from the oral apparatus to the posterior, and a 9+2 axoneme structure within the cilia. Sequencing of the 18S rRNA gene produced a unique sequence not present in GenBank. These oyster gill ciliates are generally listed as Sphenophrya dosiniae (Order Rhynchodida) and although there are no representatives of Rhynchodida in GenBank’s database, all similar matches were within the class Phyllopharyngea. Since 1997, xenoma prevalence has fluctuated with peaks in 2000, 2004, and 2011. Infected oysters generally contained 100, sharply contrasting the rare prevalence and low intensity reported elsewhere. Prevalence increased with oyster size, leveling off near 50% in oysters >60mm. Infection intensity peaked in 70-90mm oysters. Individually, oyster condition was not associated with xenoma intensity, but sites with oysters in higher condition generally had a greater prevalence and intensity of xenoma infections. Seasonal data indicated an infection cycle increasing from summer to fall, peaking at 55-65% in November and dropping to <10% by spring. The oyster population at Great Bay, NH warrants further examination to understand the mechanisms and conditions controlling xenoma formation, as well as the possible effects of a changing climate.M.S.Includes bibliographical referencesby Emily Scarpa McGur
