Echinorhynchus salmonis Müller, 1784 (Acanthocephala: Echinorhynchidae) from the Bothnian Bay, Baltic Sea: morphological variability and radial asymmetry of proboscis hooks.

Echinorhynchus salmonis is a common parasite of salmoniform and other fishes, occurring in fresh and brackish waters throughout the Holarctic. Presented here is the first analysis of the morphometric and meristic variation in a Palaearctic population of E. salmonis, collected from whitefish Coregonus lavaretus L. and smelt Osmerus eperlaus (L.) from the Bothnian Bay, northern Baltic Sea. Morphological data were compared with published descriptions of congeneric taxa. Nearctic populations of salmonid echinorhynchids considered by some to represent a distinct species, E. coregoni Linkins in Van Cleave, 1919, did not show any morphological divergence from Palaearctic populations, indicating that the name E. coregoni should be suppressed. Similarly, E. alpinus Linstow, 1901 has been considered to be a junior synonym of E. salmonis. However, E. alpinus should be regarded as a valid species, because it has a longer and more elongate body. The armature of the acanthocephalan proboscis typically displays radial symmetry. However, in E. salmonis the hooks on the dorsal surface of the proboscis are smaller than those on the ventral surface. Size differences between dorsal and ventral hooks are most pronounced at the base of the proboscis. The systematic and functional significance of radial asymmetry of proboscis hooks is discussed.