Some aspects of the biology of Dactylogyrus vastator Nybelin, 1924 (Monogenea) a gill parasite of Cyprinus carpio L.

Abstract

Dactylogyrus vastator Nybelin, 1924 is a common, economically significant pathogenic monogenean parasite of European carp, Cyprinus carpio L. D. vastator attaches to the gills by means of an attachment organ, the opisthaptor, which carries two large hamuli, a connecting bar and fourteen peripheral marginal hooks. Experiments during the present study have shown that populations of D. vastator on young carp reach peaks of abundance at 12°C, 19°C and 22°C within 5,3 to 4 and 6 weeks, respectively, followed by a decline to a lower level. Parasite abundance was greatest at 19°C and lowest at 12°C. Principal component analysis was used to investigate the effects of temperature on sclerite measurements. It was shown that the basic length and internal root length of the hamuli are the major factors by which populations of the parasite reared under different temperature regimes can be discriminated. Parasites reared at 12°C were clearly separated from those reared at 14°C and 19°C. Scanning and transmission electron microscopic studies were carried out on D. vastator. The outer layer of the epidermis is a syncytial structure. Circular and longitudinal muscle is found beneath this outer layer. The muscle bands are not of uniform thickness. Epidermal secreting cell bodies are located below the muscle layer and communicate with the outer layer via ducts or channels. Possible. epidermal sensillae are unequally distributed over the worm's body. The parasite has four cephalic lobes each of which is provided with a cup-like opening at the border; the unicellular cephalic gland cells empty their contents into a collecting duct. D. vastator shows protandous gonadial development. The female reproductive system has an oval shaped ovary, uterus, ootype, accessory glands, whereas the male reproductive system has a single lobed testis located in the posterior region of the body. Clean sclerites prepared by using an ultrasonication technique were examined under the electron microscope. The hamuli of adult and immature dactylogyrids are divided into internal and external processes and a shaft which ends in a spike. Marginal hooks have a blade and spike. The adult and immature worms can be differentiated by the structure of the auxiliary sclerite. In mature specimens the outer and the inner surfaces of the auxiliary sclerite remain separate. The surface of the hamuli has an interlocking array of striations. The two hamuli are of unequal size in both adult and immature worms. The parasites are not randomly distributed over the gill apparatus. There were no significant differences between gill arches but parasites aggregated in certain areas of the gills, in particular the ventral proximal secondary filaments on both sides of the hemibranch are favoured. Smaller D. vastator which are found in higher numbers on fish presumably represent worms which have recently invaded the host. Larger worms are found in lower numbers. This may be due to competition or an age related mortality in which mature worms die off. D. vastator does not need to be attached to the host tissue to initiate egg laying. In-vitro oviposition was observed and described, however the egg laying rate varies with the environmental temperature. The first eggs that are produced in-vitro are of a large size but as time continues the size of the eggs becomes smaller. A severe hyperplastic tissue response was observed two weeks after the start of an experiment where fish infected with D. vastator were mixed with naive fish. Damage to the host gills caused by D. vastator was observed. Hyperplasia of gill tissue led ultimately to fusion of the secondary lamellae. Affected fish became lethargic and gulped air at the surface. Challenge experiments were carried out to investigate whether there is an acquired immunity by carp to D. vastator infections. The challenged fish had a significantly lower parasite burden compared to the naive fish. The infection causes a change in the blood proteins, as was clearly shown by comparison of infected and uninfected fish, the former having very visibly separable additional bands using gel electrophoresis.