WHAT METAZOAN PARASITES TELL US ABOUT THE EVOLUTION OF AMERICAN AND EUROPEAN EELS

Two hypotheses have been forwarded to explain the divergence of American (Anguilla rostrata) and European (Anguilla anguilla) eels from a common ancestor. The "oceanic" hypothesis proposes that eels formerly inhabited either North America or Europe. Divergence occurred on the spawning grounds in the Sargasso Sea, with new oceanic currents arising during the Pleistocene carrying leptocephalid larvae to the new continent. The "vicariant" hypothesis proposes that an ancestral eel population inhabited the North Atlantic regions, including Greenland, Iceland, and Scandinavia. This population was forced southwards by the Pleistocene glaciation and separated into European and American components. Examination of the freshwater metazoan parasite fauna specific to eels on both continents forces rejection of the "oceanic" hypothesis. Six parasites specific to eels (Gyrodactylus anguillae, Pseudodactylogyrus anguillae, Bothriocephalus claviceps, Proteocephalus macrocephalus, Daniconema anguillae, Paraquimperia tenerrima) infect eels in freshwaters of both Europe and North America. This transcontinental distribution of freshwater parasites specific to eels is incompatible with an oceanic separation of eels on the spawning grounds, because adult eels die after spawning and leptocephalid larvae cannot acquire these parasites at sea. The disjunct distribution of these parasites implies that at one time their ranges were continuous, thus supporting the "vicariant" speciation hypothesis. The American eel, Anguilla rostrata (LeSueur), and the European eel, Anguilla anguilla (L.), are sister species inhabiting separate continents. They differ only in the number of vertebrae, the frequency of certain enzymatic alleles, and the frequency of several restriction endonucleases in the mtDNA genotype (Avise et al. 1986, 1990). Although it has been established that these eels are distinct populations, some doubt exists as to whether they constitute distinct biological species (Williams and Koehn 1984). Although Anguillidae is an ancient family, the fossil record is virtually nonexistent (Cavender 1986). The earliest known Anguilla in North America dates from the late Pleistocene, while in Europe, to the Upper Miocene (Cavender 1986). These findings, together with the close relatedness of the sisterspecies Anguilla rostrata and Anguilla anguilla, suggest that the two species were recently separated. Both species grow in coastal, estuarine, and freshwater habitats, and then undergo a spawning migration to the southwestern Sargasso Sea, where they spawn in a largely overlapping area (McCleave et al. 1987). Leptocephalid larvae migrate to the continent of parental origin, partially by passive dispersal in oceanic currents. The nature of their sympatric spawning grounds has created difficulty in determining the origin of the two species. Two contrasting hypotheses have been forwarded recently to explain the divergence of these two anguilliforms from a common ancestral stock and to account for their present distributions (Avise et al. 1990). The "oceanic" hypothesis states that eels were native only to one side of the Atlantic, that is, Europe or North America (Avise et al. 1990). During the Pleistocene epoch, changes in ocean currents occurred that carried some eel larvae from the spawning grounds in the Sargasso Sea to the new continent. These eels matured in the new habitat and returned to the spawning grounds. However, as they migrated from a new direction, that of the newly colonized continent,