Physiological, Pathological, and Defense Alterations in Manila Clams (Short-Neck Clams), Ruditapes philippinarum, Induced by Heterocapsa circularisquama

ABSTRACT In a laboratory study, we investigated the clearance rates (CRs), respiration rates (RRs), total hemocyte count (THC), pathological alterations, and mucocyte densities in the gills of the short-neck clam Ruditapes philippinarum when exposed to the toxic dinoflagellate Heterocapsa circularisquama. Within 2 h of exposure to H. circularisquama at a concentration of 5–103 cells/mL, both the CRs and the RRs were significantly decreased by 43–52% compared with the control CRs and 43–93% compared with the control RRs, respectively (ANOVA, Newman-Keuls, P < 0.05). Clams exposed to H. circularisquama at 103 cells/mL for 96 h showed a set of defensive and degenerative pathologies that were absent in control clams. Within 3 h, the gills exhibited cilia matting, followed within 24 h by heavy hemocytic infiltration in the connective tissue of plicae, distortion of filaments, hyperplasia, and fusion of adjacent filaments with matted cilia. Within 48 h, the gill plicae became contracted and showed multifocal ep...