An unnamed microcercous cercaria (Digenea: Monorchiidae), a parasite of Amiantis purpurata (Lamarck, 1818) (Bivalvia: Veneridae) and its corresponding metacercaria from the province of Buenos Aires and the Patagonian coast of the Southwest Atlantic Ocean, are described. The cercaria described in this paper differs from the three other monorchiid microcercous cercariae, i.e., Lasiotocus minutus (Manter, 1931), Lasiotocus elongatus (Manter, 1931), and Cercaria caribbea XXXVI Cable, 1956, mainly because of the extension of the excretory vesicle and the location of the ventral sucker. Cercariae artificially extracted from sporocysts encyst in a dish and form metacercariae enveloped by a gelatinous sac with two prolongations, which are used to adhere to the substratum. The monorchiid described in this paper has a life cycle similar to those of L. minutus and L. elongatus, although the adult stage of the present species is still unknown. Their larvae are similar in morphology and have venerid clams as their first hosts. The presence of a monorchiid larva is reported for the first time in the Southern Hemisphere. Its monthly prevalence rates, ranging from 0 to 25% (mean: 8.3%), are given from the Patagonian coast. The infection seems to cause castration as it was observed that during March through to May, when most gametes were produced in uninfected individuals, 81% of the infected individuals did not produce gametes.
Maritrema orensensis sp. n. (Digenea: Microphallidae) from Balneario Orcnsc, Buenos Aires province, Argentina, is described. Adults of M. orensensis were collected from the intestine of the kelp gull Larus dominicanus Lichtenstein, 1823 (Aves: Laridae) and differ from adults of other related species, principally, in the shorter intestinal caeca and the incomplete ring of vitelline follicles.
A total of 2,086 specimens of Engraulis anchoita Hubbs ct Marini (Pisces: Engrauiidae), from waters off Argentina and Uruguay were examined lor parasitic digeneans. Four species of digeneans were found, Parahemiurus merits (Linton, 1910) (adult) in the stomach; Lecithochirium microstomum Chandler, 1935 (non-ovigerous and ovigerous juveniles) in the swimbladder; Cardiocephaloides sp. (metacercaria) in the eyes and Rhipidocotyle sp. (metacercaria) in the musculature. These four species are recorded for the first time as parasites of E. anchoita.