A new trematode metacercaria from the eyes of trout

Abstract

Several workers have reported the occurrence of eye metacercaria from various British freshwater fishes, and all have regarded their specimens as Diplostomum volvens. This, as described originally by Nordmann (1832) and reported later by various workers, has an oral sucker much smaller than the ventral, and its lateral adhesive organs are never in the form of cup-shaped structures. Moreover, its ventral sucker is also shown to be pre-equatorial in position. During an investigation on the trematodes of the brown trout, Salmo trutta L., at Edinburgh, I obtained, mostly from the vitreous humour of the eye, a metacercaria in which the oral sucker was much larger than the ventral, the lateral adhesive organs formed cup-shaped sucker-like structures and the ventral sucker was post-equatorial in position. It is, therefore, distinct from Diplostomum volvens. Furthermore, the present metacercaria on comparison comes nearer to some of the American and Australian species of Diplostomulum but differs from all these in several important features. It is, therefore, regarded as a new species and designated Diplostomulum truttæ n.sp. (Diplostomulum being the name given to eye metacercaria of which the adult has not yet been identified).