MAINTENANCE OF 2 GENETIC ENTITIES BY HABITAT SELECTION

In the laboratory, the two species of copepods Lepeophtheirus thompson i and Lepeophtheirus europaensis, ectoparasites of flatfishes, can mee t and mate on at least one host species. In the wild however, these tw o species are found isolated on their sympatric hosts. Habitat selecti on theoretically represents a powerful enough mechanism to explain the maintenance of genetic heterogeneity in the wide sense. In this paper , the host colonization process is studied for both parasite species. It is shown that each parasite can develop and reach adult age on each host species. However, L. thompsoni is highly selective; it almost to tally refuses to colonize hosts other than its natural one. Lepeophthe irus europaensis, on the contrary, readily infests turbot and brill in single-host experiments, but strongly prefers the brill when it has a choice. It appears that these two genetic entities are sympatrically maintained due to strong habitat selection. Such a pattern could theor etically only occur in a soft-selection context (density dependence). This point is discussed with respect to the different patterns in host use found in the geographical distribution of these parasites.