Genetic assessment of species boundaries in the North American Daphnia longispina complex (Crustacea: Daphniidae)

Species boundaries in the North American Daphnia longispina group have proved difficult to establish on the basis of morphology alone. This confusion may be due to hydridization, phenotypic plasticity or the existence of sibling species. We therefore used genetic analysis to delineate species boundaries by examining 27 North American populations belonging to the longispina complex for variation at 15–26 allozyme loci. The populations consisted of Daphnia thorata from two western sites and two eastern sites, Daphnia galeata mendotae from its type location and seven sites across its range, and Daphnia rosea from eight temperate and seven arctic sites. Two populations from the Eurasian longispina complex were also included for reference. Populations assigned to D. galeata mendotae formed a genetically cohesive group, whereas a genetic dichotomy was found between temperate and arctic D. rosea , suggesting that this taxon includes two species. Genetic analysis also confirmed the distinctness of western D. thorata from other members of the longispina group. Unexpectedly, eastern populations resembling D. thorata were genetically more similar to temperate D. rosea than to any helmeted species ( D. galeata, Daphnia hyalina or D. thorata ). Our results suggest that the helmet character is a poor indicator of phylogenetic relationships, as the genetic ability to produce this feature has been lost or acquired several times in the evolution of the longispina group.     

Feeding ecology of larval and juvenile American shad (Alosa sapidissima) in a small pond

Release ponds are used as part of a multifacet effort to restore American shad ( Alosa sapidissima ) in the upper Susquehanna River basin. Little information exists, however, on the feeding ecology of young shad in small ponds. Consequently, we examined feeding ecology and prey selection of 299 larval and 299 juvenile American shad in a small pond during spring and summer. Larval shad mainly consumed copepods (37.7%) and cladocerans (37.4%) whereas juvenile shad ate chironomids (43.1%) and ostracods (28.4%). Larval and juvenile shad exhibited diel variation in diet composition and feeding periodicity. Food consumption by shad was minimal at night; feeding activity was highest during the day, peaking at 2000 h for both larvae and juveniles. Electivity values of shad larvae for prey taxa were highest for cladocerans (+0.27) and lowest for ostracods (−0.07). Electivity values of juvenile shad were highest for chironomids (+ 0.21) and ostracods (+ 0.09), and lowest for copepods (− 0.08) and baetids (− 0.14). Our data indicate differences in diet composition, prey preference and, to a lesser extent, feeding patterns between larval and juvenile American shad in small pond environments.     

Population dynamics ofDaphnia rosea in a small eutrophic pond

The population dynamics ofDaphnia rosea were studied in a small, eutrophic fish-free pond during November 1986–November 1987.D. rosea showed two density peaks in 1987, a high peak in spring and a slightly lower one in autumn, but they disappeared in mid- and late summer when a smaller cladoceran,Ceriodaphnia reticulata, dominated. The rapid decline in density ofD. rosea in mid-May and the low density in June/July were preceded by or coincided with a period of increased male density, the appearance of ephippial females, smaller body length of adult females, decreased brood size, and increased frequency of empty brood chambers, all of which indicate a worsening food situation. The rapid density decline ofD. rosea in mid-May was considered attributable neither to increasing water temperature, nor predation by the dipteranChaoborus flavicans, but to insufficient food conditions for the maintenance of stable reproduction. Although edible phytoplankton was abundant in June/July, the general food situation was not suitable forD. rosea due to blooms of the large-celled dinoflagellate,Ceratium hirundinella, which probably inhibited the feeding activity ofD. rosea. TheC. hirundinella blooms might also be important for the shift of the dominant herbivore from the largerD. rosea to the smallerCeriodaphnia reticulata in August/September.     

Microspatial heterogeneity in the distribution of ciliates in a small pond

Five transects of contiguous samples from the surface of a small pond and one transect from its bottom were collected in order to quantify microspatial heterogeneity in the distribution of ciliated protozoa. Examination of the frequency-abundance relations for these transects suggests that they can be approximated by negative binomial distributions with a commonk of 1.87. Contagiousness or crowding increases with population density.Mean patch size and mean interpatch distance were measured for 4 transects as 1.5 to 2 cm and 3 to 4 cm, respectively. This heterogeneity is suggested to arise from behavioral aggregation about discrete food sources and be very ephemeral.Blocking of adjacent contiguous samples was used to investigate the effect of sample size on the apparent correlation between the numbers of pairs of taxa. In all cases examined, taxa were relatively independent in their distribution at small sample sizes and became more negatively or positively associated as samples were combined. This may reflect that the small scale patches are essentially monospecific.     

Planktivory by bluegill (Lepomis macrochirus) on Leptodora kindti in a small North American lake

Leptodora kindti (Crustacea: Cladocera) is a large species of zooplankton (2–18 mm length) that is exceptionally transparent. This transparency is believed to be a means by which it successfully coexists in lakes with planktivorous fishes. We investigated the gut remains of bluegill (Lepomis macrochirus) that had been feeding on L. kindti and Daphnia (D. galeata and D. retrocurva) in the wild (Lake Zurich, Illinois) and found that bluegill readily preyed on L. kindti as small as 3–5 mm length, and strongly selected L. kindti over Daphnia galeata and Daphnia retrocurva. The large compound eye of L. kindti is one half to one complete order of magnitude larger than Daphnia's eye, consistent with the hypothesis that eye area is an important visual cue for fishes. Moreover, the slope of the relationship between eye area and body length is an order of magnitude shallower in L. kindti than Daphnia, suggesting that eye area has been under stronger negative selection in L. kindti. Results suggest that L. kindti's large and dark eye compromises the transparent nature of its body.     

Seasonal changes in the abundance of Microcrustacea in a small pond

Sampling methods, and methods used in extracting, identifying and counting the Microcrustacea in samples are outlined. Numbers of parthenogenetic females and young stages of Daphnia obtusa were highest from March/June to Sept./Nov., with a trough in July. Ephippial females were recorded during some or all of the months from March to Nov. with two numerical maxima, one in early summer and one in autumn. Males were present during the same period as ephippial females. Naupliar, copepodid and adult stages of Cyclops strenuus strenuus were recorded in every month; numbers of all three stages tended to be high from March to Nov. and low from Dec. to Feb. Females were present in smaller sumber than males. Females with eggs sacs occurred every month with two peaks of abundance in April–July and Oct. Adult Canthocamptus staphylinus were absent during the summer/early autumn from June/July/Aug. to Sept./Oct. Numbers were highest from Nov. to May/June with a peak in spring. Females carrying an egg sac were recorded each month with the highest proportion occurring sometime in the period Feb. to May. Possibly two peaks, one in early summer and one in autumn, of total numbers of Cyclocypris ovum were recorded. Comparisons with other available information on the life-cycle of the species are made. The effects on the species of a six-week period of drought in the autumn of 1969 are considered.     

Density-dependent predation ofChaoborus flavicans onDaphnia longispina in a small lake: the effect of prey size

Functional response curves of fourth instar larvae ofChaoborus flavicans preying on two size classes ofDaphnia longispina were examined throughout three summer seasons in a small forest lake. Data for each size class were fitted to Holling's disc equation. The parametersa (attack rate) andTh (handling time) were calculated for each prey size from these curves. Attack rate was greater and handling time was shorter for small (0.77 mm) than for large (1.82 mm)Daphnia. In 1:1 mixture of these prey size classes the predation rates ofChaoborus on smallDaphnia at prey densities above 20 l^–1 were greater than predicted from the single size-class experiments. The observed predation rates on largeDaphnia were lower than predicted at all prey densities. Since both single size-class and two size-class experiments were run during the same period of time the difference in observed and predicted predation rates could not be attributed to seasonal changes in prey preference ofChaoborus larvae. In experiments with a concentrated mixture of lake zooplankton (dominated byD. longispina)Chaoborus preference forDaphnia decreased as prey body size increased. There was no obvious correlation between selectivity coefficients and size-frequency distributions ofDaphnia. When medium-sizedDaphnia were omitted from calculations the preference of small over large prey did not differ significantly from the predictions of the single size-class model.     

Cyclomorphosis in Daphnia lumholtzi induced by temperature

• 1 Cyclomorphosis is a well known phenomenon in Daphnia that involves a regular, seasonal, or induced change in body allometry. Long helmets and tail spines were induced in laboratory cultures of Daphnia lumholtzi with temperature of 31 °C as the proximal cue (temperature of locally occurring peak abundance in Kentucky Lake). The effect was greater in embryos than juveniles or adults exposed to the temperature cue.
• 2 The temperature cue appears to have a threshold value (animals cultured at 25 or 28 °C did not develop elongated helmets or spines). The helmet and spine length receded both with D. lumholtzi kept at a constant 31 °C temperature and when water temperature was decreased.
• 3 The induced helmet in this experiment (0.66 mm, 1.0 mm animal) was significantly longer than values reported in the literature for induction by planktivorous fish kairomones (0.25 mm, 1.2 mm animal). The strong response to a proximal cue of temperature may require the second weaker chemical cue for maintenance. It is suggested that a synergistic explanation with two cues may be more appropriate for cyclomorphosis induction and maintenance in Daphnia lumholtzi that could be tested with further studies.

     

Light and assimilation number in a small desert,recharged-groundwater pond

Summary Measurements of irradiance, chlorophyll a concentration, respiration, and net and gross photosynthesis were carried out in a small desert pond containing recharged-groundwater from the Flushing Meadows water reclamation project at Phoenix, Arizona. Over the 6-month period May–October, chl. a concentration averaged 119 mg m^-3; and gross primary productivity in terms of the mean daily carbon fixation rate was 8.29 g C m^-3, which is roughly equal to productive fish and farm ponds but less than many polluted eutrophic lakes and streams. Both net and gross assimilation numbers were found to be satisfactorily represented by pure sine waves that implied total inhibition of those two processes near an underwater irradiance value of 0.32 cal cm^-2 min^-1.The maximum value of the gross assimilation number at 0.16 cal cm^-2 min^-1 of 30 mg C hr^-1 per mg chl. a ranked with some of the highest values that have been found world-wide.Joint contribution of the Department of Zoology, Arizona State University, and the Agricultural Research Service, U.S. Department of Agriculture. Portions of this work were supported by the Grace Ellis Ford Fellowship, American Association of University Women, awarded to Joyce M. Foster on July 1, 1971.     

Abundancy estimations of aDaphnia magna population in a small artificial pond

Practical difficulties in the application of correct statistical methods in field ecology are illustrated by a plankton abundancy estimation in a small pond. Random sampling was performed over a limited number of previously chosen sample places and times. A cylindric sampler was used, which caught the plankton within a whole water column (1 m). Relatively high numbers of samples were needed to achieve reliable estimates of population parameters, which was demonstrated by simultaneous sampling. The probability distribution of the number of daphnids per sample was shown to be approximately lognormal. However, the variances of the log-transformed data were not uniformly homogeneous, which reduces the applicability of familiar tests for parameter comparison. Arguments are presented to make comparisons of population abundancies through transformed data only.     

Bilateral distribution of implantation sites in small mammals of 22 North American species

The distribution of activity between the left and right sides of the reproductive tract, as measured by numbers of CL, embryos and placental scars, was studied in small mammals of 22 species. Shrews ovulate from the two ovaries in a distribution that does not differ from the binomial. Implantation of blastocysts in the two uterine horns is more nearly even ('balanced') than would be predicted from the binomial distribution. Balance in this group apparently is achieved by transuterine migration of blastocysts, perhaps in conjunction with some spacing mechanism within the uterus. Some cricetid rodents show little or no balance, but in others the distribution of activity sites (embryos, CL and placental scars) departs significantly from the binomial distribution. Reproductive activity sites of heteromyid and geomyid rodents (Geomyoidea) are highly balanced; uterine balance apparently is achieved by means of ovarian rather than uterine control. We know of no previous reports of ovarian balance and suggest that physiological mechanisms controlling numbers of ovulations in the species exhibiting this characteristic may differ from those in species exhibiting a random distribution of ovulation sites. Hypotheses regarding evolutionary aspects of balance are considered in phylogenetic and ecological terms, generating several testable research questions for physiologists, anatomists, and evolutionary ecologists.     

Patterns in the structure of Asian and North American desert small mammal communities

Aim We compared assemblages of small mammal communities from three major desert regions on two continents in the northern hemisphere. Our objective was to compare these with respect to three characteristics: (1) species richness and representation of trophic groups; (2) the degree to which these assemblages exhibit nested community structure; and (3) the extent to which competitive interactions appear to influence local community assembly. Location We studied small mammal communities from the deserts of North America (N=201 sites) and two regions in Central Asia (the Gobi Desert (N=97 sites) and the Turan Desert Region (N=36 sites), including the Kara-Kum, Kyzyl-Kum, NE Daghestan, and extreme western Kazakhstan Deserts). Method To provide baseline data we characterized each desert region in terms of alpha, beta, and gamma diversity, and in terms of the distribution of taxa across trophic and locomotory groups. We evaluated nestedness of these communities using the Nestedness Temperature Calculator developed by Atmar & Patterson (1993, 1995) , and we evaluated the role of competitive interactions in community assembly and applied a null model of local assembly under varying degrees of competitive interaction ( Kelt et al., 1995, 1996 ). Results All three desert regions have low alpha diversity and high beta diversity. The total number of species in each region varied, being highest in North America, and lowest in the Turan Desert Region. The deserts studied all present evidence of significant nestedness, but the mechanism underlying this structure appears different in North American and Asia. In North America, simulations strongly implicate interspecific competition as a dominant mechanism influencing community and assemblage structure. In contrast, data from Asian desert rodent communities suggest that these are not strongly influenced by competition; in fact, they have greater numbers of ecologically and morphologically similar species than expected. These results appear to reflect strong habitat selection, with positive associations among species that share similar habitat requirements in these communities. Our analyses support earlier reports suggesting that predation and abiotic forces may have greater influences on the assembly and organization of Asian desert rodent communities, whereas interspecific competition dominates assembly processes in North America. Additionally, we suggest that structuring mechanisms may be very different among the two Asian deserts studied. Gobi assemblages appear structured by trophic and locomotory strategies. In contrast, Turan Desert Region assemblages appear to be randomly structured with respect to locomotory strategies. When trophic and locomotory categories are combined, however, Turan species are positively and nonrandomly associated. Main conclusions Very different ecological dynamics evidently exist not only between these continents, but within them as well. These small mammal faunas differ greatly in terms of community structure, but also appear to differ in the underlying mechanisms by which communities are assembled. The underlying role of history and geography are strongly implicated as central features in understanding the evolution of mammalian faunas in different deserts of the world.     

Cyclomorphosis as a factor explaining success of a Daphnia hybrid in Tjeukemeer

Recent studies have shown that Daphnia hybrids are intermediate to their parental species for several life history traits, and that they combine advantageous traits of the parental species, thereby decreasing their risk of being preyed upon by planktivorous fish. In this study D. galeata, D. cucullata and their interspecific hybrid are compared with respect to cyclomorphosis and juvenile growth as a potential mechanism to avoid invertebrate predation. Helmets and tail spines are known to reduce predation risk, whereas fast juvenile growth reduces the period that Daphnia are vulnerable to predation. In laboratory cultures, I tested the hypothesis that, with regard to cyclomorphosis, juvenile hybrids are more comparable to the juveniles of D. cucullata, and with respect to growth rate, they are more comparable to D. galeata. The hybrid did not differ significantly from D. galeata in the time needed to reach 0.9 mm body length. In contrast, the relative spine and helmet length of juvenile hybrids were more comparable with those of D. cucullata, whereas in adult instars these characters were more similar to D. galeata. Thus during the first juvenile instars, the hybrids combine the relative long helmet and spine of D. cucullata, with the faster growth of D. galeata. This combination of characteristics might reduce hybrid vulnerability to invertebrate predation in field situations.     

Carbon and nitrogen dynamics in a small pond in the maritime Antarctic

Small ponds and puddles are extremely common throughout the ice-free areas of the maritime Antarctic. The carbon and nitrogen dynamics in a typical pond on Livingston Island, South Shetland Islands were investigated during summer 1991. The pond vegetation consisted of a benthic mat of cyanobacteria, diatoms and chlorophytes. The mat was not limited by nutrient availability, both phosphorus and nitrogen being available in the overlying water and N:P ratios in both the water and the mat indicating a roughly balanced supply. Maximal rates of carbon fixation of 0.1–0.2 mgC g^–1 dry weight h^–1 were similar to those of other perennial Antarctic mat communities. Productivity appeared to be limited by physical factors, but the effects of irradiance and temperature could not be separated. Although carbon fixation rates were low, carbon loss processes were minimal leading to an accumulation of material in the mat approximating to one doubling per year. Atmospheric nitrogen fixation was not a significant component of the nitrogen budget of the pond, accounting for only 0.1 % of the nitrogen accumulation by the mat. Nitrogen uptake was largely from dissolved nitrogen sources, in particular as dissolved organic nitrogen. It is concluded that ephemeral water bodies may play a significant role in the nutrient dynamics of maritime Antarctic ecosystems.     

Seasonal succession of a microphagotroph community in a small pond during litter decomposition

Temporal dynamics of a lentic microphagotroph community were studied during leaf litter decomposition from December to May. Small plastic vessels containing leaf litter were placed on a pond bottom. They were sampled periodically to collect microphagotrophs. Three abiotic factors and abundance of two food items were also measured to analyze the autogenic and allogenic phenomena during a microphagotroph succession. Three behavior types were recognized in dominant taxa: a free-swimming type, a vagile (creeps on substratum, sometimes swims) type, and a voluntarily fixed type. Dominant taxa changed from the free-swimming to the vagile type up to mid-March, and the reverse change occurred from mid-April. Principal component analysis (PCA) indicated four factors affecting the dynamics of the community: water temperature as a seasonal factor, detritus volume on the litter surface as a habitat factor, and densities of bacteria and small flagellates as food factors. Taxa replacement appeared to occur through two mechanisms. (1) Dominance of small holotrichs, a free-swimming type, was brought about by a high bacterial density caused by seasonal events, i.e., leaf fall in December and detritus formation by litter feeders in mid-April. This is an allogenic aspect of community dynamics. (2) The free-swimming type was replaced by the vagile one during the period with high taxa diversity. This replacement occurred through intertaxa competition for scarce food and/or selective predation by larger microphagotrophs. It is an autogenic process within the community.     

Cyclomorphosis in natural populations of Daphnia cephalata King

SUMMARY. Daphnia cephalata King is a major component of the zooplankton in freshwater ponds of eastern Australia. Phenotypic variation in head shape both within and between populations is striking. Some of this variation is ontogenetic, but among individuals of similar size head length still varies by a factor of two. The causes of this variation have been investigated by analysing head size in thirty-four populations of D. cephalata over a 17-month period. An annual cycle in crest size, apparently related to seasonal changes in mean temperature, was evident when the data were pooled. Large differences in head size were commonly observed, however, among samples collected simultaneously. Some of this variation was correlated with local differences in resource availability, which were due, in part, to local variation in Daphnia density. Other consistent interpopulation differences in head size were related to stable environmental differences among sites and probably also to genetic differences among populations.     

Landing distance in a synchronic North American firefly

Abstract.  The flash communication system of Photinus carolinus in Great Smoky Mountain National Park (Tennessee) is characterized by male firefly synchrony. Photinus carolinus males signal a conspecific female with synchronic trains of flashes. A solitary responding female attracts several males, which is not common in North American rover fireflies. The female and the group of males that she attracts are called a cluster. It is hypothesized that the first male attracted to the female would land closer to the female than would additional males because there would be less tendency for visual confusion. This hypothesis is explored under controlled conditions by replacing the responsive female with an appropriately flashing light-emitting diode (LED) located in the center of a flat target area. When infra-red videography is used to measure the first male and the additional males' landing distances from the counterfeit female (LED), most fireflies land within 15 cm of the target LED, and the first male does not land closer than the additional males. It is suggested that cluster formation is a by-product of male synchrony and is facilitated by the tendency of males to land near, but not on, females.     

Plant-herbivore interactions in a North American mixed-grass prairie

Summary Studies were conducted during the 1979 growing season to examine how North American bison (Bison bison) use prairie dog (Cynomys ludovicianus) colonies in Wind Cave National Park, South Dakota. Objectives included (1) determining whether bison selected for prairie dog towns parkwide; (2) characterizing in greater detail bison use patterns of a 36-ha colony in Pringle Valley as a function of time since prairie dog colonization; and (3) relating these bison use patterns to measured changes in structure and nutritional value of vegetation on and off the dog town.During midsummer, prairie dog towns were one of the most frequently used habitats by bison parkwide. Day-long observations at Pringle Valley revealed that bison exerted strong selection (nearly 90% of all habitat use and feeding time) for the dog town, which occupied only 39% of the valley. While there, they partitioned their use of the colony by grazing in moderately affected areas (occupied <8 years by prairie dogs) and by resting in the oldest area (>26 years occupation).Prairie dogs facilitate bison habitat selection for a shortgrass successional stage in this mixed-grass community by causing a broad array of compositional, structural, and nutritional changes in the vegetation.