Alona weinecki Studer on the subantarctic islands, not Alona rectangula Sars (Chydoridae, Cladocera)1

Abstract

Since Harding in 1967 named the Alona on Signy Island (South Orkneys) Alona rectangula, this binomen has persisted there and has been extended to other island groups in the Scotia Arc. Elsewhere among the subantarctic islands the binomen Alona weinecki has been used, based on a description in 1878 and a redescription in 1914. A detailed redescription of A. rectangula from Norway and Denmark permitted comparison with available populations of A. weinecki from Signy and South Georgia and of the synonymous taxa Alona bukobensis var. subantarctica and Alona subantarctica. The two taxa, although related, are completely different. Alona rectangula sens. str. does not occur on the subantarctic islands at all.Whether the A. weinecki populations on these islands are the same species is presently unknown. Specimens on Kerguelen Island are reported to be larger than those on the Scotia Arc islands, and among the latter populations there are differences in structure of at least the labrum. The islands in the Scotia Arc, except the Falklands, were completely ice covered during the last glacial age, with documented advances and retreats of the glaciers during the Holocene until 2000 b.p. Conditions on at least some other subantarctic islands were probably comparable. If the Cladocera were eliminated by glacier advances, the islands must have been recolonized during the Holocene from a refugic population, possibly in the Falklands. Strong westerly winds and migration of sea birds might have spread the taxon. If the populations are different, however, then the taxa must have survived on their respective islands during the glacial ages. Large populations containing ephippial females and the three instars of males are needed to resolve this uncertainty.Lectotypes of A. rectangula Sars, A. bukobensis var. subantarctica Ekman, and A. subantarctica Ekman have been selected and are on deposit in the Zoological Museums in Oslo and Stockholm.