Frans Ollevier

Ectoparasites of the whitespotted rabbitfish, Siganus sutor (Valenciennes, 1835) off the Kenyan Coast: distribution within the host population and site selection on the gills.

Different populations of the whitespotted rabbitfish, Siganus sutor, were examined for ectoparasites: adults from the Mombasa area (sampled in December 1990) and different age classes (adult, subadult and juveniles) from Gazi Bay (sampled in December 1992 and August 1993). The most common gill parasites were: the monogeneans Pseudohaliotrema sp., Tetrancistrum sigani and Microcotyle mouwoi, the copepods Hatschekia sp., Pseudolepeophtheirus sp. and juvenile Caligidae, and prazina larvae of the isopod Gnathia sp. Adult siganids had a higher parasite load than subadults. Juvenile rabbitfish did not harbour any gill parasites. Temporal differences in the parasite load of subadult rabbitfish were observed for M. mouwoi (highest in the December samples) and for juvenile Caligidae (highest in August). The microhabitat of the 5 most common gill parasites was species specific. Most parasite species showed distinct site preferences with respect to both gill arches and gill sectors, within the gill arches. Niche breadth of the different gill parasite species was independent of the abundance of any of the other species present. However, niche breadths of M. mouwoi, Tetrancistrum sp. and Hatschekia sp. increased with their own abundance. This suggests that interspecific competition for space is low and that intraspecific factors could play an important role in the microhabitat choice of these gill parasites. The hypothesis that niche restriction leads to higher intraspecific contact and an enhancement of chances to mate was tested on 2 monogenean species, Pseudohaliotrema sp. and Tetrancistrum sigani. Their highly aggregated distribution over the gill filaments, leading to increased intraspecific contact, is consistent with the hypothesis.

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.

Some properties of the blood of European eel (Anguilla anguilla) and the possible relationship with Anguillicola crassus infestations

Abstract The haematocrit (Hct) and osmotic fragility of erythrocytes of eels with and without Anguillicola crassus specimens in the swimbladder were determined. No significant difference in mean Hct could be found, but erythrocytes of infested eels seemed to be more susceptible to lysis by a hypotonic solution of NaCl than those from uninfested eels. An approach (based on dietary modification) to the treatment of Anguillicola crassus infestations is discussed.