The Ecology of Neoechinorhynchus rutili (Muller)

Abstract

The life cycle, distribution, and abundance of Neoechinorhynchus rutili infecting a single population of three-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 parasite. 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. Neoechinorhynchus rutili is a characteristic and ubiquitous parasite of freshand brackishwater fishes in the Northern Hemisphere. It has a circumpolar distribution, having been recorded from a wide range of definitive hosts throughout Europe, Asia, and North America (Van Cleave and Lynch, 1950). Although details of life history are sparse, it has been generally accepted, after Villot (1885), that the aquatic larva of Sialis niger functions as an intermediate host for this parasite. Adult worms occur in the gut, posterior to the stomach, of the definitive host. Following copulation, eggs are retained by the female worm until the shelled-embryo or acanthor stage is reached (Meyer, 1931). At this stage, the embryos are liberated by the parent female and pass to the exterior in the feces of the fish host. Merritt and Pratt (1964) described the course of further development, during which the parasite passes through acanthella and juvenile stages before becoming infective to the definitive host. The present work is concerned with the patterns of infection attained by N. rutili in its two hosts and represents part of a broader investigation into the ecology of host-parasite relationships in an isolated population of freshwater fish. MATERIALS AND METHODS The fish population selected was of the threespined stickleback, Gasterosteus aculeatus L. This species was chosen because of its wide distribution, its generally high level of parasitization for a freshwater fish, and the ease with which it can be Received for publication 18 January 1967. sampled. In addition, the fish frequently attains a high population density and matures within its first year. A small, isolated population of sticklebacks was located in a pond at Monkton near Gateshead, County Durham. The pond, 1,200 m2 in area, possessed no inlet or outlet streams and, although originally intended as an industrial reservoir, had for some years prior to the investigation supported a stable, natural fauna and flora. No fish species other than G. aculeatus was recorded in the pond. Besides being infected with N. rutili, sticklebacks were also parasitized by the tapeworm Proteocephalus filicollis (Rudolphi) and the fluke Phyllodistomum folium Braun. The fish population was investigated by means of successive monthly samples during the 2-year period January 1960 to January 1962. Samples were taken by means of a small, lightweight beam trawl (beam 1.5 m) covered with a net of 10 mm stretched mesh. Laboratory examination of fishes was undertaken as soon as possible after capture and was always complete within 12 hr of netting. The fish were killed by pithing, opened by a midventral incision, and the whole of the gut removed for microscopic examination in Ringer's solution. The position, number, degree of development, and sex of all worms was recorded. In addition, the lengths of worms were noted after relaxation in cold tapwater. Distinct stages are difficult to identify in the postjuvenile development of N. rutili, being restricted to a gradual increase in size of the parasite and in the size and functioning of its gonads. Without histological examination, it is impossible to detect the onset of maturation. As a practical alternative, size was taken to be indicative of development and three stages were recognized. These were described as immature, mature, and gravid and consisted respectively of worms with length less than 2.0 mm, 2.0 to 3.5 mm, and more than 3.5 mm. Maximum length of the immature worms was set at 2.0 mm since, during the examination of larval stages of the parasite, several juveniles were recorded with a length of 1.9 mm.