Competitive Exclusion of Dactylogyrus extensus by Dactylogyrus vastator (Trematoda, Monogenea) on the Gills of Reared Carp

Abstract

Carp (Cyprinus carpio Linn.) infested with D. vastator, Nybelin, 1924, are slightly or not at all infested with D. extensus (Mueller and Van Cleave, 1932). Carp infested with D. extensus were found to be free or nearly free from D. vastator. These observations were made on 796 carp in ponds and subjected to statistical analysis. Under laboratory conditions in carp infested with D. extensus and subsequently exposed to D. vastator the latter species replaces the former, i.e., the number of D. extensus on the gills falls to a minimum or to zero even when the original infection with D. extensus was heavy. Histological changes of gills infested with D. vastator result in the loss of D. extensus and, eventually, in heavy infestations of D. vastator itself. With subsequent healing, the gills again become a suitable habitat for D. extensus and D. vastator. Eventually a specific resistance to D. vastator eliminates this species from the dactylogyrid fauna of the gills, and the habitat remains suitable for D. extensus and other dactylogyrids. Antagonism between different species of parasites to which fish are naturally exposed has been discussed by Dogiel (1958) and Bauer (1959). In a review of the subject the above authors cited the following examples: an increase in infestation with Lernaea (Copepoda) is associated with diminished susceptibility to glochidia and vice versa (Wilson 1916). Infection with Eubothrium crassum (Cestoda) prevents infection with Triaenophorus nodulosus (Cestoda) in trout (Scheuring, 1923). There is also antagonism between Octobothrium melangi (Monogenea) and Clavella devastatrix (Copepoda) in Gadus merlangus (Leigh-Sharp, 1933) and between Phyllodistomum folicum (Digenea) and Myxidium lieberkiihni (Neosporidia) in the urinary bladder of Esox lucius (Petrushevski, 1940). Ivasik (1953) found that Dactylogyrus extensus and D. vastator were antagonists to Trichodina carassii (Ciliata), while the above two species of Dactylogyrus and D. anchoratus were found Received for publication 23 April 1963. * 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. t Supported by a grant from the Fish Breeders Association, Israel. 94 together on the gills of carp and apparently showed no mutual antagonism. The above observations were made during field studies. Little experimental work has been done on interspecific competition between parasites. The few experimental demonstrations of competitive relationships include those of Adler (1954) between Leishmania infantum and Plasmodium berghei, von Brand and Tobie (1960) working with species and races of Trypanosoma, and Holmes (1961) with Moniliformis dubius (Acanthocephala), and Hymenolepis diminuta (Cestoda). MATERIAL AND METHODS My observations are based mainly on an examination of 796 carp fingerlings collected in fish ponds infested with Dactylogyrus vastator and D. extensus during the spring of 1961-1962. The total length of the fingerlings was from 20 to 50 mm. Supplementary observations were carried out on fingerlings which were transferred to the laboratory and maintained in aquaria of various sizes at two constant temperatures (21 and 28 C). The parasites were counted on gills removed quickly from the living fish and immediately pressed between two glass plates. The results were subjected to the chi-square test.