Goussia aculeati from the three-spined stickleback (Gasterosteus aculeatus): field observations and ultrastructural features.

Goussia aculeati, a parasite of intestinal epithelial cells of the three-spined stickleback (Gasterosteus aculeatus) was found in 418 (47%) of 891 sticklebacks examined. The fish were obtained from 7 localities of waters in a natural resort area in North-West Germany. G. aculeati mainly infected juvenile fish (312 (53%) of 590 examined). During a 2-years observation period no impact of season on the infection rate was obvious. Ultrastructural studies on merogonic, gamogonic, and sporogonic developmental stages revealed that meronts gave rise to merozoites by ectomerogony; microgametes had one flagellum attached to the surface of the microgamete. Due to veil-like membraneous structures covering the sporocysts, this Goussia species was placed into the subgenus Plagula.