ECOLOGY OF NEOECHINORHYNCHUS RUTILI (MULLER)

The life cycle, distribu-tion, and abundance of Neoechinorhynchus rutili infecting a single population of 3-spined sticklebacks, Gasterosteus aculeatus was studied over a period of 2 years. Fish stomach content analysis and the experimental infection of food organisms showed the ostracods Cypria ophthalmica and Candona Candida to be intermediate hosts of the para-site. The aquatic larvae of the alder fly, Sialis, did not function in the life cycle of the parasite. Variations in the occurrence of Acanthocephala were correlated with variations in the ingestion of ostracods by sticklebacks. An increase in infection was observed with increasing age of fish hosts. It is suggested that this is due partly to variations in fish feeding habits and partly to the method of attachment of the parasite. Records of incidence, burden, and degree of development show that the parasite experiences an annual maturation cycle but does not exhibit a cycle of incidence. It is postulated that the maturation cycle is initiated by annual temperature changes in the external environment. The occurrence of an annual incidence cycle, however, is masked by overall fluctuations in the density of the parasite population.