Research Article

Host-based identification is not supported by morphometrics in natural populations of Gyrodactylus salaris and G. thymalli (Platyhelminthes, Monogenea)

K. OLSTADa1 c1, A. P. SHINNa2, L. BACHMANNa1 and T. A. BAKKEa1

a1 Natural History Museum, Department of Zoology, University of Oslo, P.O. Box 1172, NO-0318 Oslo, Norway

a2 Institute of Aquaculture, University of Stirling, Stirling FK9 4LA, UK


Gyrodactylus salaris is a serious pest of wild pre-smolt Atlantic salmon (Salmo salar) in Norway. The closely related G. thymalli, originally described from grayling (Thymallus thymallus), is assumed harmless to both grayling and salmon. The 2 species are difficult to distinguish using traditional, morphometric methods or molecular approaches. The aim of this study was to explore whether there is a consistent pattern of morphometrical variation between G. salaris and G. thymalli and to analyse the morphometric variation in the context of ‘diagnostic realism’ (in natural populations). Specimens from the type-material for the 2 species are also included. In total, 27 point-to-point measurements from the opisthaptoral hard parts were used and analysed by digital image processing and uni- and multivariate morphometry. All populations most closely resembled its respective type material, as expected from host species, with the exception of G. thymalli from the Norwegian river Trysilelva. We, therefore, did not find clear support in the morphometrical variation among G. salaris and G. thymalli for an a priori species delineation based on host. The present study also indicates an urgent need for more detailed knowledge on the influence of environmental factors on the phenotype of gyrodactylid populations.

(Received May 15 2007)

(Revised June 25 2007)

(Accepted June 26 2007)

(Online publication August 16 2007)


c1 Corresponding author: Natural History Museum, Department of Zoology, University of Oslo, P.O. Box 1172, NO-0318 Oslo, Norway. Tel: +47 22 85 18 23. Fax: +47 22 85 18 37. E-mail: