Small-sized euryhaline fish as intermediate hosts of the digenetic trematodeCryptocotyle concavum

Cercariae of the trematodeCryptocotyle concavum, which encyst in skin and/or kidney of sticklebacks and gobies, were studied in the Schlei Fjord (western Baltic Sea). Mean incidence of dermal cysts was 48 % inGasterosteus aculeatus and 37 % inPungitius pungitius. No cysts were found in the kidneys of sticklebacks. While 97 % ofPomatoschistus microps had encysted metacercariae in the kidneys, only 2 % had cysts in the skin.Pomatoschistus minutus, however, showed hardly any cyst infestation of either skin or kidney. InP. microps the intensity of infestation by metacercariae was frequently more than 50 cysts; in contrast, sticklebacks rarely exhibited more than 5 dermal cysts. Infested fish were larger than 10 mm in total length, the incidence rate increasing with growth. Parasitic infestation depends on ambient salinity:C. concavum was not found at salinities below 4 . In contrast to the high incidence in fish, the first hosts — the snailsHydrobia stagnalis andH. neglecta — showed remarkably low infection rates (3 to 5 %). The findings reported are related to the distribution ofC. concavum, the mode of life of infested fish, the feeding habits of the final hosts and the infestation ofP. microps by other parasites. Evidently,P. microps represents an optimal second host forC. concavum.