An Analysis of the Trematode Genus Allocreadium Looss with the Description of Allocreadium neotenicum sp. nov. from Water Beetles

Abstract

A major problem in the systematics of the Digenea concerns relationships among genera that have at one time or another been placed in the family Allocreadiidae. Cable (1956), reviewing this problem in detail, pointed out that if these genera constitute a natural group, the cercariae have undergone considerable divergence, whereas if the complex is polyphyletic, the adult stages have experienced convergence, in either case to a degree not approached in other groups as well known as the allocreadioids. Even within the genus Allocreadium 2 cercarial types have been reported, an ophthalmoxiphidiocercaria from sphaeriid clams by Looss (1894) and Dollfus (1949) for A. isoporumn, and a cercaria of the lepocreadiid type from prosobranch snails by Seitner (1951) for A. ictaluri. Because Allocreadium is the type genus of its family, a critical analysis of its features is an essential step toward the solution of the allocreadioid problem. Such a review was prompted by studies on a species of Allocreadiumn which attains full sexual maturity in water beetles. Moreover, this form has a well developed stylet and intact eyespots, apparently retained from the cercarial stage (Peters, 1955). The name Allocreadium neotenicum sp. nov. is proposed for it. Crawford (1940) reported Allocreadium sp. from water beetles in Colorado but did not describe his specimens sufficiently to name them. Material was obtained near Douglas Lake, Michigan, and was studied alive or was fixed with no pressure in 60-65? C Gilson's fluid. Semichon's carmine was used to stain whole mounts and Harris's haematoxylin for both whole mounts and sectioned material. All measurements are in millimeters, based on whole mounts of 24 egg-bearing specimens.