Adaptation of Dactylogyrus extensus (Mueller and Van Cleave, 1932) to Ecological Conditions of Artificial Ponds in Israel

Abstract

The resistance of Dactylogyrus extensus Mueller and Van Cleave, 1932, infesting carp (Cyprinus carpio L.) reared in artificial ponds in Israel, to normal summer temperatures of 24 to 28 C was demonstrated under experimentally controlled conditions. At these relatively high temperatures egg hatching was unimpaired. The parasite was also found resistant to prolonged (up to 6 days) lowered oxygen tension and survived on fingerling carp kept in water of salinity up to 9,000 mg Cl/liter (1.485% NaCi). Carp were introduced into Israel 30 years ago from eastern Europe where the optimum temperature for D. extensus ranges from 13 to 17 C and is susceptible to lowered oxygen tension. The behavior of D. extensus under different temperature and oxygen tensions in Israel may indicate a relatively rapid effect of natural selection and the establishment of a strain of D. extensus well adjusted to local ecological conditions. The tolerance to high salinity, however, is probably not the result of a new local adaptation since it is inherent in D. extensus in eastern Europe. Recent studies on the ecology of Dactylogyrus extensus Mueller and Van Cleave, 1932 (syn. D. solidus Achmerov, 1948), a gill parasite of wild and cultured carp, Cyprinus carpio, in USSR, showed that the optimal temperature for reproduction is 17 C (Bauer and Nikolskaya, 1954). Higher temperature (20 to 25 C) is a limiting factor for population growth of this parasite since egg hatching decreases with increase of temperature beyond 17 C. Iziumova (1958) showed that a fall in the amount of dissolved oxygen in water significantly reduced the population of D. extensus. Prost (1958, 1959) noted the natural tolerance of this trematode to high salinities. D. extensus found on the gills of carp in Israel was morphologically identical with material described from USSR (Paperna, 1959). As the environmental conditions in this country are different from those in USSR, it was thought to be important to examine the ecological characteristics of the local population. Received for publication 23 April 1963. * Supported by a grant from the Israel Fish Breeders Association. t This paper is part of a doctoral thesis carried out in the Department of Parasitology, Hebrew University, Hadassah Medical School, Jerusalem, under the direction of Professor G. Witenberg. MATERIAL AND METHODS From 1960-1962 carp ponds throughout Israel were surveyed seasonally for the presence of gill parasites. Observations were also made on groups of 20 to 100 young carp (20 to 60 mm in total length) from infected ponds and maintained in several aquaria of various sizes (from 30 to 200 liters) under controlled conditions. Some aquaria were kept at a constant temperature of 28 C while others were kept at room temperature of 21 C (in early spring) and 27 C to 30 C (during summer). Oxygen deficiency was produced by maintaining ten fish in small covered jars of 1 to 2 liters capacity; similar jars uncovered and well aerated served as control. Additional observations were carried out on groups of 20 to 50 fishes kept in larger aquaria (100 to 200 liters) covered and nonaerated. The amount of dissolved oxygen in the water was examined by the Winkler method. Salinity (in mg chlorine/liter) was measured by Moore's method. Student's t test was employed in calculating the significance of data obtained from the experiments on the resistance of D. extensus to different temperatures.