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 rele ased hamuli and marginal hooks collected from five species and three morpho types of Gyrodactylus parasitising salmonids in the UK and Canada. Five new parameters for measurement from the hamulus are described: shaft-point len gth, hamulus angle, hamulus width, shaft area and the root area; and three from the marginal hook: sickle aperture, toe length and the marginal hook a rea. Univariate statistics were used to select seven of nine useful metrica l hamulus features and seven of eight marginal hook features. Of the hamulu s features, the shaft-point length discriminated all Gyrodactylus salaris p opulations from all other Gyrodactylus populations and the hamulus angle di scriminated only the Swedish G. salaris populations from those gyrodactylid s on British salmonids. The marginal hook features, the sickle length, sick le 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 si gnificantly smaller than all other British collections of Gyrodactylus in t his study, having smaller hamuli point lengths, total lengths and shaft len gths. Separate principal components analyses were performed on the variable s selected for the hamuli and for the marginal hooks. The principal compone nt analysis (PCA) analysis demonstrated that individual structures from eit her the hamulus or the marginal hook, significantly discriminate every spec imen of G. salaris from all ocher gyrodactylid specimens included within th e analysis. The discrimination of G. derjavini from G. truttae by this meth od, was however, not possible and the description of additional morphometri c parameters may be necessary before this separation can be achieved.