Two species of coastal flatfish (brill and turbot, Scophthalmidae) were analysed electrophoretically at 17 common enzymatic loci in samples taken from 1 1 sites representing the species ranges in Europe. Brill showed a mean heterozygosity ( H ) ofO.1 I while that ofturbot was 0.02. The virtual absence of genetic diversity in turbot is probably due to a very low evolutionary rate, and provides little evidence for population substructure even if various signs indicate the possibility of a hidden differentiation (presence of the taxon maoeticcc in the Black Sea and differentiation of a speciesspecific cestode parasite on either side of the Strait of Gibraltar). On the other hand, the weak geographic structure in brill seems to result from rapid recolonization following the last ice age.
The two monogenean species Metamicrocotyla cephalus and Microcotyle mugilis have specific microhabitats on the gills of Mugil cephalus on which they may or may not coexist. M. cephalus is found in sector 1 of the posterior hemibranch of arch I. M. mugilis is found on the filaments of sector 5 of the posterior hemibranch of arch I. The coexistence of these two monogenean species on the same fish does not induce a change in their respective distribution, except for a marked preference of M. cephalus for the left side and of M. mugilis for the right. This 15-month-long study shows that when the two species of monogeneans coexist the infection prevalence and intensity are higher than in cases of monospecific infection. The specific character of the microhabitat, the apparent absence of interspecific competition, and the existence of positive species interactions are discussed.
On the Atlantic Moroccan coast, two species of pleuronectiform fish coexist, wedge sole (Citharus linguatula) a temperate species, and spotted flounder (Dicohgoglossa cuneata) a tropical one. These two species are the definitive host for Bothriocephalus andresi and Acanthocephaloïdes propinquus, respectively. A negative binomial fits the dispersion of B. andresi in the population of C. linguatula. The fish become parasitized between their first and third year, while still immature, with the highest abundances in the third year. After the fish become ichthyopagous, the level of infection drops drastically. We postulate that copepods are the sole source of infection. The presence of B. andresi in both Atlantic and Mediterranean populations of C. linguatula shows that the parasite follows the host during its migration along the Mediterranean coasts.
The prevalence of A. propinquus remains above 50% as a consequence of the large numbers of amphipods included in the diet of D. cuneata, regardless of the age of the fish or the season. The amphipods, Phtisica marina and Pariambus typicus are an important food and represent potential intermediate hosts for A. propinquus. The population dynamics of A. propinquus on the Atlantic Moroccan coasts are similar to those observed in parasite populations of Gobius niger in the Gulf of Lion (French Mediterranean). The observation of B. andresi and A. propinquus in both Moroccan Atlantic and Mediterranean fishes highlights the problem of the evolution of these parasite populations during the colonization of the Mediterranean by the hosts.
The fine structure and differentiation of the embryonic envelopes before and after hatching in two bothriocephalid species of teleostean fish are described. Scanning electron microscopic observation of the egg surface confirmed the specific ornamentation of the capsule. Its thickness and topographic morphology reflect the physiocochemical characteristics of the environment. Beneath the eggshell, two syncytial layers were observed around the oncosphere. The outer envelope, visible during the preoncospheral phase, degenerated before the liberation of the coracidium. Glycoconjugate labelling confirmed the nutritive character of this layer. After hatching, the inner envelope was the only layer covering the oncosphere. The apical plasma membrane of the coracidial sheath bore cilia and numerous clublike microvilli. Beneath the cortical zone, the syncytial cytoplasm was vacuolated, resembling a honeycomb; the electron-dense zone, described from other pseudophyllideans, was not present. The basal membrane of the inner envelope, uniformly electron-dense, was assimilated by the oncospheral membrane. Comparison of the composition of the embryonic envelopes of pseudophyllideans with those of other tapeworm orders and trematodes revealed ontogenetic, structural and functional similarities between the embryotrophic layers. The physiological and ecological significance of the structure of the free-swimming coracidium is discussed. In this respect, the inner envelope around the oncosphere appears to be responsible for the inability of the larva to detect and invade the intermediate host.
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