The effects of salinity and temperature on the development and survival of fish parasites

In brackish water the variety of marine and freshwater parasite species is considerably reduced. The distribution in brackish water of most marine endoparasites is restricted by the salinity tolerance of their hosts, most of the parasite species are more tolerant than their hosts. The influence of salinity and temperature on nine species has been examined; first stage larvae of Contracaecum aduncum develop in 0-32‰ salinity; Cryptocotyle lingua proved to be infective at salinities down to 4‰. The greatest resistance was found in Anisakis larvae from herring Clupea harengus, which survived for more than half a year. Parasites in the fish intestines appear to be unaffected by changing water salinities, as the osmolarity in the intestines stays nearly constant. Marine ectoparasites (Acanthochondria depressa, Lepeophtheirus pectoralis) survive about three times longer than freshwater species (Piscicola geometra, Argulus foliaceus) when salinity is 16‰. High temperature increases the effects of adverse salinities on parasites. There is evidence that none of these ecto-parasitic species can develop within the range of 7-20‰ salinity

Some peculiarities of the endoparasites' evolution

Evolution of parasite - host systems should be treated as a specific kind of co-evolution. Although its course is in accordance with the general rules of organic world evolution, it has some peculiarities. One of them is retardation or acceleration of endoparasites' phylogenesis, as compared with the hosts' one. The second one is different speed of phylogenetic changes of endoparasites belonging to the same systematic group. These changes can be significantly limited, as well. Phylogenetic changes of euglenids - intestine parasites of Copepoda - are considerably limited, even in comparison with the scale of changes of such species which begin their parasitic life by infecting Copepoda eggs