Recruitment of Digenetic Trematodes Using Caged Snails

Abstract

Caged, laboratory-reared snails of two species (Physa gyrina and Lymnaea stagnalis appressa) recruited five species of digenetic trematodes in a natural stream habitat in Michigan from May through October, 1977. Temporal and spatial differences were observed in the infection rates of the two gastropod species. Many techniques have been devised to study the relationships between free-living stages of digenetic trematodes and their molluscan hosts. Successful measurement of the number of cercariae in field situations has been achieved (Rowan, 1957, 1965), but measurement of miracidial populations in nature has proven more difficult. Our study used caged, laboratory-reared snails to monitor the acquisition of trematode infections by snails in nature. This technique provides an indirect means to measure the low numbers of miracidia usually encountered in field situations. Application of this technique may provide information on seasonal periodicity of trematode infections and characterization of optimal sites of snail-miracidium interactions. Caged snails have been used by several workers to investigate miracidial dispersion of human schistosomes. McClelland (1965) designed small cylindrical cages made of perspex and mosquito netting to expose Bulinus nasutus snails to miracidia of Schistosoma haematobium. Webbe (1966) used caged Biomphalaria sudanica tanganyicensis to study the effects of water velocities on infection by S. mansoni, and Shiff (1968) investigated miracidial host-finding behavior over short distances using bulinid snails confined in small perspex cages. In a series of studies by Upatham (1972, 1973, 1976), caged B. glabrata were used to observe S. mansoni miracidial dispersion under laboratory and field conditions in St. Lucia. Upatham experimented with different cage designs and materials. Received 26 March 1981; revised 23 June 1981; accepted 5 October 1981. * Department of Zoology, Iowa State University, Ames, Iowa 50010. t Department of Biology, Hope College, Holland, Michigan 49423. An important difference between our study and all other work in this area is that in previous investigations miracidia were introduced into the habitat, usually in large numbers. No miracidia were artificially introduced in our study and therefore caged snails were exposed to numbers of miracidia present in natural habitats. MATERIALS AND METHODS This study was conducted from May through October, 1977 in a spring-fed stream running parallel to Highway U.S. 31 near Alanson in Emmet County, Michigan. The stream is located approximately 15 miles from the University of Michigan Biological Station and has been the focus of many studies involving digenetic trematodes. The stream is approximately 3 m wide and 21 cm deep and its bottom is covered by 0.5 m of mud. Water current in the stream was minimal, the average rate of flow having been measured at 0.02 m/second. Average daily water temperature varied from 10 to 11 C in May to 23 to 25 C in August. Cylindrical cages were placed at five sites, designated A through E, along a 304-m length of the stream. Dominant vegetation consisted of Chara, Nasturtium, Lemna, Potamogeton, and Typha. Stream fauna was characterized by large populations of aquatic gastropods, predominantly Lymnaea catascopium and Physa gyrina. Many other invertebrates, as well as several species of frogs, were observed in and around the stream. Redwinged blackbirds (Agelaius phoeniceus), common grackles (Quiscalus quiscula), and starlings (Sturnus vulgaris) used the trees near the stream as perching and nesting sites. Mammals frequenting the area included Microtus, Mus, Ondatra, Peromyscus, Rattus, and Zapus. An active muskrat den was located in the bank of the stream near Site D. Laboratory-reared P. gyrina and L. stagnalis were placed in cylinders constructed of 1.8-mm monofilament nylon mesh (Fig. 1). Cylinders measured 33 cm long by 12 cm in diameter and the open area of the mesh was approximately 60%. This allowed for uninterrupted flow of water. Seams were sewn with nylon thread and reinforced with a rubber-silicone aquarium sealer. Brass support frames were used to anchor cylinders in the stream bed. An alternate type of anchoring system was also