Diclidophora merlangi (Monogenea: Diclidophoridae) on Atlantic cod, Gadus morhua

Diclidophora merlangi (Kuhn, in Nordmann, 1832), as species specific to whiting, Merlangius merlangus (Linnaeus, 1758), is reported and described for the first time on the gills of Atlantic cod Gadus morhua (Linnaeus, 1758). Of the about 1,200 cod examined in this article, only 2 specimens of D. merlangi occurred on 2 fish, suggesting that they represented accidental infections. The 2 specimens showed specific traits of D. merlangi, but their size was considerably smaller than that of D. merlangi on whiting. Principal-component analyses showed that the morphology of the specimens on cod was more similar to that of D. merlangi than those of other congeneric species from the North Atlantic, additionally supporting their assignment to D. merlangi. Quantitative data documenting the potential reproductive consequences of D. merlangi occurring on cod is also provided. The testes and germaria of the specimens on cod were noticeably smaller than those of D. merlangi on whiting, but the number of testes was similar and spermatozoa were observed in the sperm duct. By contrast, the aspect and smaller size of the oocytes suggested that D. merlangi on cod could not produce viable ova, although 1 specimen exhibited an egg with normal sized and shaped capsule.