Life history variation among low-arctic clones of obligately parthenogenetic Daphnia pulex: a diploid-polyploid complex

Summary Laboratory life table experiments were conducted using nine clones of obligately parthenogenetic Daphnia pulex that were collected from a site in the Canadian low-arctic. Two of the nine clones were diploids, while the other seven clones were polyploids. Significant clonal differences in age at first reproduction, size at first reproduction, number of offspring in each of the first three broods, offsrring sizes for the first two broods, and intrinsic rates of natural increase were detected. Differences in life histories were evident between polyploids and diploids. Generally, polyploid clones reached maturity at later ages, matured at larger sizes, produced smaller broods, and larger offspring than the diploid clones. The data are discussed in reference to potential biotic (i.e. invertebrate predation) and abiotic factors (i.e. physicochemical gradients) that may influence life history variation in this clonal assemblage.     

Final thermal preference in parthenogenetic females of Daphnia magna straus (Crustacea: Cladocera) acclimated to various temperatures

The final thermal preference FTP) range in parthenogenetic females of cladoceran Daphnia magna was assessed by “acute” and “chronic” methods. The first method included 4-month acclimation to different temperatures in the range of 14.2 ± 0.7 to 27.1 ± 0.3°C; the “chronic” method was characterized by long-term acclimation to +20°C. Two ranges of FTP were found for D. magna, 13.3–15.4°C and 20.2–26.2°C. The thermal preference of daphnids and the temperature of acclimation were correspondingly linearly. The range of FTP was independent of the season. The searching activity of D. magna rose in April, when the FTP range increased, and the FTP was less pronounced.     

Pokey,a New DNA Transposon in Daphnia (Cladocera: Crustacea)

We report the complete sequence of two representatives of the transposable element, Pokey, isolated from the ribosomal DNA of the cladoceran, Daphnia pulicaria. We describe the general features of this element, which confirms its classification as a DNA transposon. We show that Pokey is similar to piggyBac and, as such, is a member of the TTAA-specific family of elements. Pokey is putatively autonomous, possessing an open reading frame that encodes a putative protein with similarity to piggyBac's transposase, in addition to putative proteins from Drosophila and human LOOPER elements, and several human proteins of unknown function. We show that these proteins all contain amino acid motifs that are perhaps conserved for similar functions. We demonstrate that Pokey is carrying several regions of similarity to Daphnia pulex IGS sequences, likely the result of some sort of recombination event with the host genome. Pokey inserts into a conserved region of the large subunit ribosomal RNA gene known to contain other arthropod elements, suggesting that this location is a hot spot for insertional activity. However, Pokey is unique in that it is the only DNA transposon yet known to insert into this region, as other such insertions are non-LTR retrotransposons. It is now clear that this "ecological niche" has been effectively exploited by both retrotransposons and DNA transposons.     

Genetic variation in the cellular response of Daphnia magna (Crustacea: Cladocera) to its bacterial parasite

Linking measures of immune function with infection, and ultimately, host and parasite fitness is a major goal in the field of ecological immunology. In this study, we tested for the presence and timing of a cellular immune response in the crustacean Daphnia magna following exposure to its sterilizing endoparasite Pasteuria ramosa. We found that D. magna possesses two cell types circulating in the haemolymph: a spherical one, which we call a granulocyte and an irregular-shaped amoeboid cell first described by Metchnikoff over 125 years ago. Daphnia magna mounts a strong cellular response (of the amoeboid cells) just a few hours after parasite exposure. We further tested for, and found, considerable genetic variation for the magnitude of this cellular response. These data fostered a heuristic model of resistance in this naturally coevolving host–parasite interaction. Specifically, the strongest cellular responses were found in the most susceptible hosts, indicating resistance is not always borne from a response that destroys invading parasites, but rather stems from mechanisms that prevent their initial entry. Thus, D. magna may have a two-stage defence—a genetically determined barrier to parasite establishment and a cellular response once establishment has begun.     

Daphnia (Ctenodaphnia) brooksi (Crustacea: Cladocera), a new species from eastern Utah

A single population of an unusual ctenodaphnid was sampled in a shallow, turbid, temporary pond in eastern Utah in 1965. The animals resemble the African Daphnia (Ctenodaphnia) barbata, but show several morphological differences, and are therefore given the new name Daphnia (Ctenodaphnia) brooksi. Repeated efforts to collect this species since 1965 have failed, suggesting either the species is very rare, or the type population represented an instance of long-distance migration.     

Life history and body size evolution in Holopedium gibberum Zaddach (Crustacea, Cladocera)

SUMMARY. 1. Body size, development, sheath size and relative egg size were examined in two populations of Holopedium gibberum Zaddach. The populations were chosen because of differences in juvenile versus adult mortality.
2. The population subject to food limitation and predation from icthyoplankton (high juvenile mortality) exhibited large size at birth and maturity, large sheath size, and eggs which were relatively large compared to the adult. In contrast, individuals in the population exposed to predation by golden shiner (high adult mortality) were smaller at birth and maturity, had a smaller sheath, and produced relatively small eggs.
3. I contrast body size and relative egg sizes of these H. gibberum populations with literature values for all Cladocera, and propose a general hypothesis for the evolution of relative egg size in Cladocera.     

Life histories of some species of Chydoridae (Cladocera: Crustacea)

SUMMARY 1. Life history parameters of two species of Chydoridae ( Leydigia leydigi and Disparalona rostrata ) were studied in the field and in the laboratory.
2. For both species the number of juvenile instars was variable and tentatively related to initial size of neonate. Egg volume increased with increasing parent length and with decreasing temperature; possible advantages gained by the offspring are discussed in relation to invertebrate predation.
3. The life history strategy employed by the littoral Chydoridae is compared to that of large and small planktonic cladocerans. Small planktonic cladocerans and the Chydoridae (except the Eurycercinae and the Saycinae) produce large young relative to their size at maturity and mature early. However, unlike the small planktonic cladocerans, growth in the Chydoridae is curtailed after the onset of reproduction and in this they resemble the large planktonic cladocerans. This strategy may be related to vertebrate predation and the presence of vegetation in the habitat.     

The invasion history of the exotic freshwater zooplankter Daphnia lumholtzi (Cladocera, Crustacea) in North America: a genetic analysis

Daphnia lumholtzi is a planktonic crustacean native to subtropical regions in Africa, Asia and Australia. Since its invasion to the southern USA in ~1990 it has spread across North America as far north as the Laurentian Great Lakes. We assessed invasion history using microsatellite makers and to explore the influence of mean annual temperature on the genetic structure along a latitudinal gradient in North America. Genotypic data were obtained from 9 microsatellite markers for 178 individuals from 13 populations (eight populations introduced to North America and five populations in the native range). Pairwise F_st values as well as Bayesian clustering showed a strong subdivision between native and introduced populations. Bayesian clustering identified multiple genetic clusters in recently invaded locations, suggestive of multiple invasions from various sources, including Asia and Africa. Using variation partitioning, we determined the amount of variation for genetic clusters of populations in the invaded range due to mean annual air temperature and the year of first detection. The results point to a primary introduction into the southern range of North America, with a subsequent northward expansion, and multiple introductions possibly from both the native range and by secondary spread from previously-invaded locations. Separate analysis of genetic clusters within the invaded range suggests additional effects of temperature conditions on geographic genetic structure, possibly as a consequence of D. lumholtzi’s tropical origin.     

Fish kairomone regulation of internal swarm structure in Daphnia pulex (Cladocera: Crustacea)

In this laboratory experiment, swarms of D. pulex were artificially created by placing thirty individuals in small chambers containing 130 ml of water. The swimming behaviour of the animals was measured using a video camera and a programme to digitize observations. The D. pulex swimming in water with fish kairomones had a more uniform swimming speed compared with animals in control water. If aggregated prey individuals benefit from a reduced predation risk, and this risk is further reduced by uniformity of swarm members, the uniformity of swimming speed can be interpreted as an behavioural adjustment to minimize the vulnerability to predation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of previous feeding conditions on survival in Daphnia pulicaria (Crustacea: Cladocera)

Survival of natural Daphnia populations without previous acclimatization in filtered water under laboratory conditions (starvation) was used as a parameter describing the actual competitive ability of populations in periods of food limitation. Two populations (1 and 2) of Daphnia pulicaria Forbes coming from two enclosures with a low (< 1 mgC l^–1) and a high (about 3 mgC l^–1) seston level were examined in their resistance to starvation. Juveniles of both populations survived better than adult females in the conditions without seston as well as in the control experiment, despite their much lower biomass. Animals from population 1 had lower mortality than those from population 2 in all variants of the experiment. The results confirm the hypothesis that the ability of daphnids to starve depends also on adaptation to previous feeding conditions.     

Ultrastructural investigations on the labral glands of Daphnia obtusa (Crustacea,Cladocera)

The labral glands of Daphnia consist of three distinct functional units on each side: (1) several cells at the base of the head, (2) two large cells at the base of the labrum and one large cell (cell A) in the median part of the labrum and (3) one large cell (cell B) in the median part of the labrum. These gland cells do not form a syncytium, contrary to reports by previous investigators. With the exception of cell B, they have a well-developed rough endoplasmic reticulum and many active Golgi complexes. The Golgi activity changes during the molt cycle. The Golgi activity of the cells of the head base is different from that of the large cells of the labrum. Since clear exocytotic phenomena were not observed, the secretion can be assumed to flow into the hemolymph after accumulation in the enlarged intercellular spaces. Cell B has a distinctive cytoplasmic ultrastructure the function of which is not yet understood. The four large cells of the labrum are in contact with a duct cell (or several duct cells) characterized by a deep infolding of the plasma membrane. This delimits a narrow lumen, which contains no secretion. No passage of substance is visible from the gland cells to the duct cell(s).     

Life history variation in Campanula americana (Campanulaceae): population differentiation

Genetic variation in life history traits has important consequences for life-history evolution. Here we report the results of a greenhouse experiment investigating the broad sense genetic basis of variation in life history traits within and among five populations of Campanula americana distributed along a latitudinal gradient. The populations exhibit differentiation for a number of morphological traits (seed weight, number of branches, final plant size, number of capsules) and the phenological traits, days to emergence, days to bolting, the onset of flowering, and the duration of flowering. Families within populations differed only in days to emergence and seed weight. These results suggest that the life history differences among populations are genetically based. In addition, two life history types—winter annuals and biennials—have previously been reported from natural populations of Campanula americana. This experiment identified a third type—summer annuals from the Florida population.     

A test for the production of sexual pheromones by Daphnia magna (Crustacea: Cladocera)

SUMMARY. (1) Swarming in the Cladocera has often been associated with periods of sexual reproduction and this has led to the suggestion that members of opposite sex are attracted to one another. The most plausible mechanism by which this could occur is through the production of pheromones. The present experiments aimed to determine if Daphnia magna Straus produce such chemicals.
(2) No evidence was found for the presence of sexual pheromones. The behaviour of both males and females was unaffected by the presence of sexually receptive animals of the opposite sex.
(3) It is suggested that the cited relationship between sexual reproduction and swarming may be due to the fact that sexual reproduction occurs during periods of high population density. It is during such periods that swarming behaviour would be most apparent.     

Differences in demographic parameters among electrophoretic clones of Daphnia obtusa Kurz (Crustacea: Cladocera)

In an 18-month electrophoretic study, a Daphnia obtusa population that recently colonized Lake Orta (a large, severely polluted lake in Northern Italy) showed no genetic variation.Populations from other localities were sampled. From clones established by single females, genotype differences were determined by two life table experiments (using Lake Orta and Lake Maggiore water as a culture medium).As expected, in the Lake Orta water the colonizing clone (LO) had longer survival, produced more newborns and had a larger rate of increase than the other clones. In the Lake Maggiore water the LO clone still fared better than the other clones.     

Haemoglobin synthesis in Daphnia magna Straus (Crustacea: Cladocera): ecological differentiation between neighbouring populations

SUMMARY. 1. Variations in the haemoglobin index of two neighbouring populations of Daphnia magna were recorded over a range of dissolved oxygen concentrations (0.5–4.0ml O₂ 1⁻¹, 20°C). Reciprocal transfer experiments between habitats compared haemoglobin synthesis in situ.
2. An inverse relationship was found between the oxygen content of pond water and the haemoglobin indices of laboratory and natural populations.
3. Significant genetic differences in the synthesis of haemoglobin were found between the two populations. Animals from the poorly oxygenated habitat (0.8±0.18ml O₂ 1⁻¹) had consistently higher haemoglobin contents (maximum HI, 87.7±4.5) at all experimental and in situ oxygen levels. D. magna from the well oxygenated pond (4.3±0.59 ml O₂1⁻¹) had a lowered physiological ability to synthesize haemoglobin (maximum HI, 48.3±4.2). The process of ecological differentiation in Daphnia populations is discussed.     

The distribution and variation of Daphnia lumholtzi (Crustacea: Cladocera) in relation to fish predation in Lake Albert, East Africa

In Lake Albert Daphnia lumholtzi is found in two forms. One has a pointed anterior prolongation, or helmet, on the head. The other has a shorter rounded head, and was originally described as a separate species, D. monacha . The latter form dominates the zooplankton in the middle of the lake where planktivorous fish are rare or absent. The helmeted form becomes commoner near the margins of the lake and reaches its greatest abundance in Ndaiga Lagoon, where planktivorousfish are common. The possession of a helmet is associated with a reduction in the size of the carapace compared to the round headed form. The carapace with its contained eggs is the most conspicuous part of a cladoceran, so that the helmeted forms are at an advantage in the presence of planktivorous fish which locate their prey by sight. The mid-lake monacha forms are larger than specimens of the same form in Ndaiga Lagoon, where it is shown that Alestes baremose feeds selectively on the larger specimens of the monacha form.
The helmeted form produces more, but smaller eggs than the monacha form. The total brood volume (= mean egg volume x mean number eggs per female) is greatest in the midlake monacha forms. The selective advantages of variations in egg size and the possession of a helmet are discussed. It is concluded that the data from Lake Albert support the hypothesis of Brooks (1965) concerning the adaptive significance of helmet development in Daphnia .     

Benthic predators and zooplanktonic prey: predation by Crangonyx shoemakeri (Crustacea; Amphipoda) on Daphnia obtusa (Crustacea; Cladocera)

Amphipods and cladocerans are common inhabitants of seasonal ponds throughout North America. Although amphipods are usually considered to be benthic detritivores, Crangonyx shoemakeri was found to capture and consume mosquito larvae and Daphnia obtusa. The rate of capture of Daphnia was independent of the amount of decomposing leaf matter available. However, this rate was dependent on prey density and the presence of light, indicating that these amphipods are visual predators.     

Cladocera (Crustacea) from Nicaragua

Thirty-one Cladocera taxa are recorded from lakes, rivers and ponds of Western Nicaragua. They include Alona bromelicola sp. nov. found in water accumulations in epiphytic Bromeliads. The Cladocera recorded are neotropical and circumtropical. Other invertebrates found are also listed.