WORLDWIDE DISTRIBUTION AND ECOLOGY OF EURYCERCUS AND SAYCIA (CLADOCERA)1

Abstract

The two species of Eurycercus are largely allopatric in the Northern Hemisphere, glacialis tending to be peripheral in distribution in the far north, occurring only as far south as 51‐52° N in the outer Aleutians, Labrador, and the northern Kuriles. There are vigorous disjunct populations of unknown history in oligotrophic heath and dune localities in Denmark and the Netherlands. The center of distribution is Greenland–Iceland. Eurycercus lamellatus is widely distributed over much of continental North America south to Mexico City and over much of Eurasia south to Algeria and southern Tibet. It also is known from farther north than glacialis. It is the only species occupying the large mountain masses of the Northern Hemisphere, where glacialis might be expected to occur. The center of distribution seems to be western Europe. Co‐occurrence of the two species in the same water bodies is known only for a few scattered lakes in Denmark, northern Sweden, Iceland, and Alaska.Eurycercus lamellatus also occurs as disjunct populations in Argentina and South Africa, but not at comparable latitudes in Australia and New Zealand. Here, Saycia cooki occurs, which might be considered an ecological equivalent of E. lamellatus except that it is confined to temporary water bodies.The finding of three males of Saycia from Australia—the first males ever recorded— stimulated a detailed study of the morphology of the species and a comparison with Eurycercus. Retention of the two genera in separate subfamilies seems justified. The male of Saycia is unique among chydorid Cladocera in having a plumose sexual seta on the antennule and in having the postabdominal claw spatulate at the tip. The sperm ducts open near the tip of the postabdomen, rather than at the base as in Eurycercus, the latter condition being unique among the chydorids.Saycia from New Zealand differs from the population in Australia chiefly in having more preanal teeth on the postabdomen and in being higher and having a shorter posterior margin relative to length of the animal. All these characters change with size. These regressions are presented and their significance discussed. The New Zealand population has been designated a new geographic subspecies, pending the eventual recovery of males.The number of postabdominal teeth in E. lamellatus increases with increasing size, making it likely that the species described as E. polyodontus merely represents individuals from the upper end of this regression. The number of teeth in E. glacialis is smaller than in lamellatus and does not increase with size of animal. The variability of tooth number within each species is so great that the number 100 proposed in the past cannot be used to separate the two species with any reliability. Similarly, the subspecies of Saycia from New Zealand has a larger tooth number, which shows a marked increase with size of animal, than that from Australia.