Host-dependent and seasonal variation in opisthaptoral hard parts of Gyrodactylus cf. arcuatus from three Pomatoschistus spp. and G. arcuatus from Gasterosteus aculeatus: a multivariate approach.

Gyrodactylus arcuatus-like monogeneans collected from the gills of 3 goby species, Pomatoschistus minutus, P. lozanoi and P. pictus, from the Voordelta Area (SW Netherlands) were compared on a morphometric basis with G. arcuatus from its type host, the three-spined stickleback, Gasterosteus aculeatus. Univariate and multivariate analysis was carried out on measurements of 17 characters of the opisthaptoral hard parts of 268 specimens. The contribution of host-dependent and seasonal factors to the observed morphological variation are discussed. Univariate statistics (ANOVA) were used to detect features that were useful in separating the gyrodactylids from the different host groups. Subsequent factor analysis and discriminant analysis, combining all variables, led to the separation of 3 distinct forms (possibly species) dependent on the host species harbouring them. G. arcuatus (sensu stricto) was only present on G. aculeatus, while P. minutus and P. lozanoi carried a common species, Gyrodactylus sp. 1, and P. pictus hosted Gyrodactylus sp. 2. Seasonal variation in size of the sclerities was demonstrated on the Gyrodactylus species from P. minutus and P. lozanoi, and should be taken into consideration when specimens are compared. The findings are discussed in view of the possibility of phylogenetic host specificity of the gyrodactylids occurring on goby species that live sympatrically in the Voordelta area.