Miniature genomes and endopolyploidy in cladoceran crustaceans

The haploid genome sizes (0.37 and 0.47 pg) of two members of the cladoceran crustacean genus Daphnia rank among the smallest known for Crustacea. An examination of cladoceran somatic tissues by scanning microdensitometry revealed abundant endopolyploidy in both species. Although cells in the labrum possessed the highest DNA content (1024C), endopolyploid cells (4–512C) were widely distributed throughout the body. Daphnia pulex and D. magna exhibited similar ploidy levels in most tissues, but differences between the two species were noted in the epidermis and labrum. The prevalence of polyploid nuclei suggests that endopolyploidy is an important process in organisms whose genomes have been miniaturized by nucleotypic selection.Key words: somatic polyploidy, genome size, Daphnia, ploidy shifts, macroevolution.

Secondary sex ratio of the cyclic parthenogen Daphnia magna (Crustacea: Cladocera) in the Canadian Arctic

Secondary sex ratios of the cyclic parthenogen Daphnia magna were studied in habitats near Churchill, Manitoba. Daphnia magna is believed to possess an environmentally mediated sex-determining system. Throughout the season females produced broods that were predominantly unisexual. The proportion of male offspring was low early in the season but rose to a value near 50% in each of four populations. It is hypothesized that after they have an initial series of female broods, individual females begin to alternate the sexes of their broods in response to an environmental cue. Such an alternation of brood sexes would explain the population sex-ratio pattern observed and would satisfy theoretical requirements for population and individual secondary sex ratios of 1:1. In studies on cladocerans, environmental cues inducing the sex-ratio response must be distinguished from cues with a sex-determining effect.

Daphniopsis ephemeralis sp. n. (Cladocera, Daphniidae): a new genus for North America

The range of the cladoceran genus Daphniopsis is extended to North America with the description of a new species, Daphniopsis ephemeralis. Its morphological intermediacy suggests that Daphniopsis is ancestral to the genera Daphnia and Simocephalus. The genus appears to have a relict distribution and is restricted to marginal habitats.

Ecology of the dominant copepod species at a Low Arctic site

Four species of calanoid copepod regularly co-occur in tundra ponds near Churchill, Manitoba. Despite the physical uniformity of the habitats they occupy, twofold variation in body size and fivefold variation in brood size existed among conspecific populations of each species. Size shifts were concordant in the sexes, but the extent of sexual dimorphism was size dependent in each species. Size differences among populations persisted through two summers. The three herbivorous species showed parallel variation in body sizes and fecundities, while the predatory species did not. The size covariation in the herbivorous taxa may reflect significant overlap in resource use despite their size divergence.

The induction of sexual reproduction in Daphnia magna: genetic differences between arctic and temperate populations

Even when provided with abundant food and grown in isolation, Daphnia magna from arctic Canada responds to declining photoperiods by producing male and then ephippial offspring. In contrast, clones of English origin do not alter their reproductive behaviour when exposed to photoperiod shifts. Hybrid clones show an intermediate response in terms of ephippial egg production, but a variable response in male production. The results indicate that variation exists at gene loci controlling sexual behaviour in local populations as well as between distant populations. The significance of this variation is discussed in relation to the environmental heterogeneity encountered by D. magna over its extensive range.

A laboratory study of the feeding behavior of the rhabdocoel Mesostoma ehrenbergii on pond Cladocera

The predation rates of the rhabdocoel Mesostoma ehrenbergii were determined on a number of cladoceran species. Two means of capturing prey are employed by the flatworm but only active predation was explored in these experiments. It was found that this flatworm is a size-dependent predator with a preference for prey of about 1.2 mm. It was also observed that prey behavior was not an important factor in determining susceptibility to predation. M. ehrenbergii is a voracious predator; individuals consume more than 10 prey/day.

Selective predation and the species composition of arctic ponds

The zooplankton species composition of 135 ponds in the Churchill, Manitoba, area was determined. The predaceous copepod Heterocope septentrionalis was present in 99 of these habitats. Several species of calanoid copepod, but only a single morph of Daphnia middendorffiana, coexisted with this predator. Two additional species of Daphnia and a second morph of D. middendorffiana were found in ponds lacking the predator. Experiments showed that these latter species were much more vulnerable to predation than the Daphnia morph that coexisted with Heterocope.

A revision of the taxonomy of the gennus Daphnia (Crustacea : Daphnidae) in south-eastern Australia

The taxonomy of Daphnia in south-eastern Australia has been revised on the basis of conventional morphological studies supplemented by electrophoretic analysis. Ten species are recognized; of these five have not previously been described and three others have been treated as forms of Daphnia carinata. Detailed descriptions, with figures of the females of all species and of the males of eight species, are included as well as a key to the adult females. Comment is made on the ecology and distribution of the species and on the zoogeographical relationship of Australian Daphnia.