Daphnia

Adamowicz, Sarah J., Hebert, Paul D. N. & Marinone, María Cristina, 2004, Species diversity and endemism in the Daphnia of Argentina: a genetic investigation, Zoological Journal of the Linnean Society 140 (2), pp. 171-205 : 191-192

DISPERSAL IN DAPHNIA

Among the 15 species identified in this study, four ( D. ambigua , D. ‘ pulicaria ’, D. similis , D. spinulata ) are either conspecific or very closely allied with North American species. A population of a fifth shared species, D. pulex , has been detected in Chile by a molecular survey of populations (S. J. Adamowicz, unpubl. data) and, recently, in the Argentine province of Neuquén, based on morphology (M. C. Marinone, pers. observ.). On the other hand, 11 species were found to be South American endemics. Additionally, three species known from morphological study are considered to be Andean endemics: Daph- ˆ

nia inca Korínek & Villalobos (2003) , Daphniopsis chilensis Hann (1986) , and D. marcahuasensis Valdivia Villar & Burger (1989) . Since members of the genus Daphniopsis are likely properly assigned ˆ

to Ctenodaphnia ( Hrbácek, 1987; Colbourne & Hebert, 1996), it would be desirable for future genetic studies on South American daphniids to include these taxa. Although further South American surveys are necessary to reveal additional narrowlydistributed species, it is likely that the majority of Daphnia species on this continent is now known either genetically or morphologically, as most major geographical regions harbouring suitable habitats have been studied. Thus, current evidence suggests that 14 of 19 species now known from South America (or 74%) are endemic to this continent.

North America is home to nearly twice as many Daphnia species as South America, as 34 taxa have been recorded ( Hebert, 1995). Part of this difference is attributable to the higher intensity of sampling in North America. However, the difference probably also reflects a real difference in species richness. The lower diversity of South America can be explained by the fact that much of this continent lies in the tropics, a setting where Daphnia diversity is low ( Fernando, 1980; Fernando et al., 1987; Fernando & Paggi, 1998). Interestingly, a similar proportion of North American species (66%) is endemic to this continent, while its other species are shared with South America or Eurasia. Australia, a much smaller continent, harbours approximately the same number of species as South America. About 21 Australian species are currently known (P. D. N. Hebert, unpubl. data), all belonging to the subgenus Ctenodaphnia . Five of these species have also been reported from Asia or Africa ( Hebert, 1978), although only one of these shared species, D. lumholtzi , has been genetically confirmed ( Havel et al., 2000). The remaining shared records are dubious, as the former morphospecies D. carinata s.l. has been shown to comprise a species complex of several species within Australia alone ( Hebert, 1977; Hebert & Wilson, 1994). Thus, Australia’s fauna contains a higher proportion (at least 76%, probably closer to 90–95%) of endemics than North or South America, reflecting the greater isolation of this continent.

Patterns of species richness and endemism cannot yet be compared between South America and Africa or the Eurasian landmass, as the daphniid faunas of the latter area have not been adequately characterized using genetic tools. Patterns of morphological similarity indicate that this would be an interesting endeavour. The nature of daphniid species distributions could be better understood by verifying the identity of those species nominally shared between South America and Africa: D. gessneri , D. laevis , D. ornithocephala , ˆ

D. pulex , D. pulicaria , and D. similis ( Korínek, 1999; 2002). Moreover, opposing biogeographical hypotheses could be tested using genetic data from species from different continents. It has been argued that the distributions of Daphnia clades reflect ancient vicariance events in Earth’s history, especially the break-up of Gondwanaland ( Benzie, 1987). Recent molecular studies have confirmed the pre-Gondwanan age of this genus ( Colbourne & Hebert, 1996; Schwenk et al., 2000), indicating that a phylogenetic study of the faunas of South America, Australia, and Africa might expose a Gondwanan signature. On the other hand, the present study revealed that intercontinental dispersal events do occur, suggesting that modern movements of taxa have also influenced clade distributions. Thus, an interesting avenue for future work would be to compare the roles of ancient geological events and recent bird-mediated dispersal in structuring contemporary biogeographical patterns in daphniids.