Morphometric discrimination of Gyrodactylus salaris Malmberg (Monogenea) from species of Gyrodactylus parasitising British salmonids using novel parameters

Morphometric data were collected from scanning electron micrographs of released hamuli and marginal hooks collected from five species and three morphotypes of Gyrodactylus parasitising salmonids in the UK and Canada. Five new parameters for measurement from the hamulus are described: shaft‐point length, hamulus angle, hamulus width, shaft area and the root area; and three from the marginal hook: sickle aperture, toe length and the marginal hook area. Univariate statistics were used to select seven of nine useful metrical hamulus features and seven of eight marginal hook features. Of the hamulus features, the shaft‐point length discriminated all Gyrodactylus salaris populations from all other Gyrodactylus populations and the hamulus angle discriminated only the Swedish G. salaris populations from those gyrodactylids on British salmonids. The marginal hook features, the sickle length, sickle aperture, total length and shaft length of G. salaris were significantly larger than those of all other gyrodactylids included within the study. Of the British fauna, it was found that G. derjavini from Welsh salmon was significantly smaller than all other British collections of Gyrodactylus in this study, having smaller hamuli point lengths, total lengths and shaft lengths. Separate principal components analyses were performed on the variables selected for the hamuli and for the marginal hooks. The principal component analysis (PCA) analysis demonstrated that individual structures from either the hamulus or the marginal hook, significantly discriminate every specimen of G. salaris from all other gyrodactylid specimens included within the analysis. The discrimination of G. derjavini from G. truttae by this method, was however, not possible and the description of additional morphometric parameters may be necessary before this separation can be achieved.