Regulation of the Endosymbiotic Algae in Hydra by Digestive Cells and Tissue Growth

Endosymbiotic algae in hydra release photosynthetically fixedcarbon to the host which augments the animals nutrition duringstarvation. The algae may acquire metabolites from hydra asa source of nutrition when the animals are maintained in thedark. The kind of algae acquired by aposymbiotic hydra is determinedby a recognition phenomenon involving the hosts digestive cellsand potential endosymbionts. The size of the algal populationis related to the necessity of light for maximum algal multiplication,the rate of cell division in hydra, and the potential for heterotrophyby the algae.     

The Establishment of the Hydra-Chlorella Symbiosis: Recognition of Potential Algal Symbionts

The epithelial cells lining the gastric cavity of the freshwater hydra, Hydra viridis, harbor unicellular algal symbionts of the genus Cblorella. It has long been known that these hydra cells can readily phagocytose algal cells and will sequester those algae that have the potential to form a symbiotic association. In this paper the evidence is discussed for when and how recognition of potential symbionts by hydra cells occurs, i.e. during phagocytosis or during the subsequent intracellular events leading to sequestration of algal symbionts.     

Brain size and ecology in small mammals

Relative brain size (measured as gross brain size after body size effects are removed) differs systematically between families of rodents, insectivores and lagomorphs. The Sciuridae have the largest relative brain size, the Soricidae and Bathyergidae the smallest.
These results are discussed and compared with previous analyses of relative brain sizes among primates and bats. These differences complicate comparisons between relative brain size across phylogenetically diverse species and attempts to relate differences in relative brain size to ecological variables. To overcome these problems, best fit relationships were estimated for each family , and values for each genus were expressed as deviations from the lines of best fit. We refer to these values as Comparative Brain Size (CBS).
Differences in CBS are related to differences in habitat type (forest-dwelling genera have larger CBS' than grassland forms), in diet (folivores have smaller CBS' than generalists or insectivores, frugivores and granivores), in zonation (arboreal genera have larger CBS' than terrestrial ones) and in activity timing (nocturnal genera have larger CBS' than dirurnal ones). However, these ecological categories are interrelated and, when the effects of other ecological differences are taken into account using analyses of variance, only the differences associated with diet, and possibly habitat remain.     

Interpatch movement of the red milkweed beetle,Tetraopes tetraophthalmus: individual responses to patch size and isolation

Individual movement patterns and the effects of host plant patch size and isolation on patch occupancy were examined for red milkweed beetles, Tetraopes tetraophthalmus, residing in a heterogeneous landscape. Male beetles were found to move both more often and farther between host plant patches than female beetles, and this difference affected the patterns of patch occupancy observed. Overall, unoccupied milkweed patches were smaller and more isolated than patches occupied by beetles. Patches uninhabited by females tended to be more isolated, but not necessarily smaller, than patches with female beetles, indicating that females may be affected more by patch isolation than patch size. Presence of male beetles on patches showed a stronger response to patch size than to patch isolation. Differences in movement between males and females illustrate the need for demographically based dispersal data. Comparisons of Tetraopes interpatch movement patterns between landscapes composed of patches of different size revealed that landscapes with overall smaller patches may have greater rates of interpatch movement.     

Coralline algal nodules off Fraser Island, eastern Australia

Summary Calcareous red algal nodules growing on mobile substrates have been sampled from 28 to 117m off Fraser Island in southern Queensland, eastern Australia. This is a subtropical, transitional area between the tropical Great Barrier Reef to the north and temperate, cooler waters to the south. Red algal nodules are the most common components in bioclastic gravels that extend from about 50 to 110 m and locally cover 40–50% of the seafloor. Variations in the overall character and floristic composition of the nodules with depth can be observed. Algal nodules comprise algal covered pebbles/cobbles and rhodoliths in depths shallower than 60 m whereas only rhodoliths occur in deeper settings. No changes in nodule shape occur but shallower algal nodules have larger mean size with higher standard deviation than the deeper ones (39.2 vs. 30.5 mm and 20.5 vs. 6.3 mm s.d.). Living and subrecent red algae in nodules shallower than 60 m are mainly Melobesioideae and peyssonneliaceans with minor Lithophylloideae and Mastophoroideae. Most plants belong to a few species of the generaPhymatolithon andLithothamnion. Below 68 m, rhodoliths are dominated by the family Sporolithaceae, melobesioids and peyssonneliaceans.Sporolithon is the main component below 80 m. Algal growth forms are mostly smooth encrusting to warty with no depth variation. Maximum plant thickness, however, decreases with increasing depth. Thallus thickness in the deeper water samples is more than three times smaller than in those from shallower waters. These data are important for understanding the paleoenvironmental context of deposition of the abundant coralline algal limestones with similar algal nodules found in the geological record.     

Phytoplankton cell size and the development of microenvironments

Abstract The effect of cell size on the development of extracellular microenvironments of pH produced by individual photosynthesizing phytoplankton cells was investigated. The presence of pH microenvironments was determined by detection of a chemical reaction known to occur in microenvironments of high pH produced by algal photosynthesis, specifically the extracellular oxidation of Mn(II) to MnO_x. The dye leukoberbelin blue was used to detect the reaction. It was experimentally determined that individual algal cells larger than 20 μm (length and/or width) produced microenvironments, while smaller cells did not unless present as cell aggregates larger than 20 μm. A mathematical model is presented and discussed.     

Ultraviolet irradiation initiates ectopic foot formation in regenerating hydra and promotes budding

We have studied the effects of ultraviolet-C (UVC) and Ultraviolet-B (UVB) on growth and pattern formation inPelmatohydra oligactis. UVC brings about a significant increase in budding in intact hydra while UVB does not exhibit such an effect. Excessive budding could be a response for survival at wavelengths that damage biological tissues. If the head or base piece of a bisected hydra is irradiated and recombined with the unirradiated missing part, regeneration proceeds normally indicating that exposure of a body part with either an intact head or foot to UVC does not influence pattern formation. Most significantly, in the middle piece, but not in the head or the base piece of a trisected hydra, UVC leads to initiation of ectopic feet formation in almost one third of the cases. Thus, UV irradiation interferes with pattern formation in regenerating hydra, possibly by changing positional values, and promotes budding in intact hydra. This is the first report on induction of ectopic feet formation by UV in regenerating hydra and opens up the possibility of using UV irradiation as a tool to understand pattern formation in the enigmatic hydra     

Effective size and the persistence of ecosystems

Summary Ecosystems distributed in space have an effective size, reflecting both their absolute size (extent) and their fine-scale physical structure (viscosity). In this paper, a general mathematical model of a predator-prey interaction is presented via the phase-plane graphs of Rosenzweig and MacArthur (1963) to show one reason why ecosystems of larger effective size should persist longer than smaller ones: oscillations of population densities tend to be displaced farther from extinction thresholds—even in spatially homogeneous systems. Experimental results obtained by Gause and Luckinbill with protozoa and Huffaker with mites are interpreted in this context.     

Glucose uptake by symbiotic Chlorella in the green-hydra symbiosis

There is a correlation between the ability of symbiotic Chlorella algae to take up glucose and their survival in green hydra grown in continuous darkness. Although normal symbionts of European green hydra, which persist at a stable level in dark-grown animals, possessed no detectable constitutive ability to take up glucose when grown in light, uptake was induced after incubation in a medium containing glucose. Further, symbionts isolated from hydra grown in darkness for two weeks had acquired a constitutive uptake ability. Neither NC64A nor 3N813A strains of algae, in artificial symbiosis with hydra, persisted in dark-grown animals, and they showed little or no uptake ability, although in culture NC64A possessed both constitutive and inducible glucose-uptake mechanisms. In contrast, mitotic indices in all three types of algae in symbiosis with hydra increased after host feeding, indicating that the factor which stimulates algal cell division is not identical to the substrate utilised during heterotrophic growth.Abbreviations E/E normal Hydra-Chlorella symbiosis - E/NC, E/3N artificial symbioses between Hydra and Chlorella strains NC64A and 3N813A, respectively - 3-OMG 3-O-methyl-D-glucose - SDS sodium dodecyl sulfate     

Manipulation of parasitoid size using the temperature-size rule: fitness consequences

The phenotypic effects of rearing temperature on several fitness components of the koinobiont parasitoid, Aphidius colemani, were examined. Temperatures experienced during development induced a plastic linear response in the dry and fat masses of the immature stage and a non-linear response in the growth rate as well as in the size of adults. We investigated if the phenotypic morphometrical plasticity exhibited by parasitoids reared at different temperatures can induce variations in fitness-related traits in females. We did not find any difference in immature (pupal) mortality in accordance to rearing temperature. However, when examining adult longevity, we found an inverse linear relation with developmental temperature, confirming the usual rule that larger and fatter wasps live longer than smaller ones. The pattern of female fecundity was non-linear; wasps that developed at high and low temperatures were less productive. We suggest that when development is short, the accumulated reserves are not adequate to support both fecundity and survival. By manipulating adult size through changes in the rearing temperature, we showed that the usual shape of the size/fitness function is not always linear as expected. Developmental temperature induced a plasticity in energy reserves which affected the functional constraints between survival and reproduction.     

Nitrogen limitation and amino-acid metabolism of Chlorella symbiotic with green hydra

Chlorella algae symbiotic in the digestive cells of Hydra viridissima Pallas (green hydra) were found to contain less amino-N and smaller pools of free amino acids than their cultured counterparts, indicating that growth in symbiosis was nitrogen-limiting. This difference was reflected in uptake of amino acids and subsequent incorporation into protein; symbiotic algae incorporated a greater proportion of sequestered radioactivity, supplied as ¹⁴C-labelled alanine, glycine or arginine, than algae from nitrogen-sufficient culture, presumably because smaller internal pools diluted sequestered amino acids to a lesser extent. Further experiments with symbiotic algae showed that metabolism of the neutral amino acid alanine differed from that of the basic amino acid arginine. Alanine but not arginine continued to be incorporated into protein after uptake ceased, and while internal pools of alanine were exchangeable with alanine in the medium, those of arginine were not exchangeable with external arginine. Thin-layer chromatography of ethanol-soluble extracts of algae incubated with [¹⁴C]alanine or [¹⁴C]arginine showed that both were precursors of other amino acids. The significance of nitrogen-limiting growth of symbiotic algae is discussed in terms of host-cell regulation of algal cell growth and division.     

Species diversity preserved in different numbers of nature reserves of the same total area

The expected number of species occurring in different numbers of reserves of the same total area is examined on different assumptions of the spatial distribution and the probability of extinction. The advantage of one large reserve or several smaller ones of equal total area depends on the spatial distributions of species and the stage after the establishement of reserves. In general, several smaller reserves maintain more species immediately after the establishments unless the spatial distribution are uniform or random, whereas one large reserve excels several smaller ones after some rare species have gone extinct unless the spatial distributions are strongly contagious. Since the extinction of rare species must be facilitated as the size of each reserve reduces, the area of a reserve should be larger than the critical area that ensures the persistence of the species. Hence it is concluded that one or a few large reserves are a better strategy in order to maintain the species diversity.     

Patterns of natural selection on size at metamorphosis in water frogs

Strategies for optimal metamorphosis are key adaptations in organisms with complex life cycles, and the components of the larval growth environment causing variation in this trait are well studied empirically and theoretically. However, when relating these findings to a broader evolutionary or ecological context, usually the following assumptions are made: (1) size at metamorphosis positively relates to future fitness, and (2) the larval growth environment affects fitness mainly through its effect on timing of and size at metamorphosis. These assumptions remain poorly tested, because data on postmetamorphic fitness components are still rare. We created variation in timing of and size at metamorphosis by manipulating larval competition, nonlethal presence of predators, pond drying, and onset of larval development, and measured the consequences for subsequent terrestrial survival and growth in 1564 individually marked water frogs (Rana lessonae and R. esculenta), raised in enclosures in their natural environment. Individuals metamorphosing at a large size had an increased chance of survival during the following terrestrial stage (mean linear selection gradient: 0.09), grew faster and were larger at maturity than individuals metamorphosing at smaller sizes. Late metamorphosing individuals had a lower survival rate (mean linear selection gradient: -0.03) and grew more slowly than early metamorphosing ones. We found these patterns to be consistent over the three years of the study and the two species, and the results did not depend on the nature of the larval growth manipulation. Furthermore, individuals did not compensate for a small size at metamorphosis by enhancing their postmetamorphic growth. Thus, we found simple relationships between larval growth and postmetamorphic fitness components, and support for this frequently made assumption. Our results suggest postmetamorphic selection for fast larval growth and provide a quantitative estimate for the water frog example.     

Size,shape and orientation of cells in budding hydra and regulation of regeneration in cell aggregates

Summary Changes in the number, shape, volume, orientation and vacuolization of cells involved in the budding of hydra were measured in histological sections. Before evagination, a group of about 800 epithelial cells are visibly recruited for the bud to be produced and this number increases to about 5,000 within a day. Thereafter, bud size increases mainly by proliferation of the cells within the bud. Upon recruitment for budding, the epithelial cells assume a columnar shape, with a smaller contact area facing the mesoglea, accompanied by a decrease in volume which is mostly accounted for by devacuolization. In later stages, cells progressively resume the form typical for non-budding areas of hydra. Evagination proceeds without reorientation of epithelio-muscular fibers, whereas elongation of the bud is accompanied by fiber reorientation.The process of sorting out and regeneration in aggregates of previously dissociated hydra cells was followed using various ratios of endodermal to ectodermal epithelial cells. From different initial compositions, the ratio in the regenerate rapidly approaches 11, the ratio found in normal hydra tissue.The experimental findings are discussed in the context of theoretical notions on pattern formation, evagination, elongation and stability of layered structures.     

Predator detection and avoidance by lotic mayfly nymphs of different size

We studied antipredatory responses of lotic mayfly (Baetis) nymphs in a factorial experiment with four levels of fish presence: (1) a freely foraging fish (the European minnow,Phoxinus phoxinus), (2) a constrained fish, (3) water from a fish stream, (4) water from a fishless stream. LargeBaetis nymphs drifted mainly during night-time in treatments involving either the chemical or actual presence of fish, whereas no diel periodicity was observed when the water was not conditioned with fish odour. The response was strongest when the fish was uncaged, which suggests that visual or hydrodynamic cues are needed in addition to chemical ones for an accurate assessment of predation risk. Fish presence had no effect on the drift rates of small nymphs. Instead, they increased their refuge use in the presence of a live fish. Chemical cues alone did not have any effect on the refuge use of any of theBaetis size classes. Our results indicate active drift entry by mayfly nymphs. Because predation pressure is spatially and temporally variable, nymphs must sample the environment in order to locate predator-free areas or areas with low predation risk. Drifting should be the most energy-saving way to do this. To avoid the risk from visually feeding fish, large individuals can sample safely (i.e. enter drift) only at night-time, while the small ones can also do this safely during the day. We suggest that, contrary to some earlier assumptions, mayfly drift is not a fixed prey response. Instead,Baetis nymphs are able to assess the prevailing predation pressure, and they adjust their foraging behaviour accordingly.     

Parasite prevalence and the size of host populations: an experimental test

Although important in epidemiological theory, the relationship between the size of host populations and the prevalence of parasites has not been investigated empirically. Commonly used models suggest no relationship, but this prediction is sensitive to assumptions about parasite transmission. In laboratory populations, I manipulated the size of Tribolium castaneum flour beetle populations and measured the prevalence and distribution of a parasitic mite, Acarophenax tribolii. I found that parasite prevalence did not vary for a wide range of host population sizes. However, prevalence was lower in populations with less than 40 hosts. This effect cannot be attributed to changes in host population density because host density was held constant among treatments. The reduction in prevalence of small populations below a threshold that I observed is predicted by the extinction debt model, but it is not expected from models of host-parasite interactions that assume density-dependent transmission. The distribution of parasites, measured using Lloyd's patchiness index, was not affected by host population size. The mean crowding of parasites, however, was negatively related with host density. Finally, the prevalence of parasites in large populations did not differ from that found in sets of smaller patches as long as the smaller populations in aggregate were equivalent in size to the large population.     

Regulation of Numbers of Algae in the Hydra-Chlorella Symbiosis

Algae injected into green hydra are taken up rapidly and in relatively large numbers, resulting in an increased symbiont population in digestive cells. The increase is temporary and the normal stock population size is restored within 24 hours after injection. Restoration of the normal population size is taken as evidence for regulation, and the technique offers a means for eliciting and studying at least one type of regulatory mechanism. Similarly, a method is described for rapid assay and visualization of digestive cell mitosis. This method may facilitate the investigation of other potential regulatory mechanisms which may involve interaction between host and algal cell division. Finally, symbiont overgrowth of host cells resulting from nutrient enrichment is described. The observations lend support to the hypothesis that nutrient limitation may regulate population size of algal symbionts.     

The Hydra viridis/Chlorella symbiosis. Growth and sexual differentiation in polyps without symbionts

To investigate interactions between the basal metazoan Hydra viridis and its symbiotic Chlorella algae, we generated aposymbiotic hydra lacking algae and compared them to symbiotic ones with regard to growth and sexual differentiation. Under standard feeding conditions aposymbiotic polyps proliferated similarly to symbiotic polyps. Under moderate and low feeding conditions asexual growth was reduced in polyps lacking algae, indicating that the symbionts supply nutrients to their hosts. In addition, the Chlorella symbionts had a strong influence on the sexual reproduction of Hydra viridis: in most cases female gonads were produced only when symbiotic algae were present. Spermatogenesis proceeded similarly in symbiotic and aposymbiotic polyps. Since during oogenesis symbionts are actively transferred from endodermal epithelial cells to the ectodermal oocytes, this oogenesis promoting role could indicate that the symbionts are critically involved in the control of sexual differentiation in green hydra.     

The effect of grazer size manipulation on periphyton communities

Summary We examined the effect of grazer size on periphyton biomass, size structure, and species compostion by removing the largest invertebrate grazers on artificial macrophytes planted in the littoral of Lake Memphremagog (Que-Vt). A series of exclosures with increasingly fine mesh prevented colonization by large invertebrates but allowed in smaller grazers. Oligochaetes, chironomids, and cladocerans effectively replaced snails so that total grazer biomass in the various treatments was not significantly different from the controls. With one exception, algal biomass, measured as chlorophyll a, did not differ significantly among the various treatments. However algal size and taxonomy were affected because the dominance of large blue-green colonies was apperantly related to the presence of large grazers. The results of the size manipulations were qualitatively similar to those induced in phytoplankton communities by size selective zooplankton grazing and are consistent with models based on general allometric equations.Contribution 181 of the Lake Memphremagog Project, Limnology Research Centre     

When does greater mortality increase population size? The long history and diverse mechanisms underlying the hydra effect

The phenomenon of a population increasing in response to an increase in its per-capita mortality rate has recently been termed the 'hydra effect'. This article reviews and unifies previous work on this phenomenon. Some discrete models of density-dependent growth were shown to exhibit hydra effects in 1954, but the topic was then ignored for decades. Here the history of research on the hydra effect is reviewed, and the key factors producing it are explored. Mortality that precedes overcompensatory density dependence always has the potential to produce hydra effects. Even when mortality follows density dependence, hydra effects may occur in unstable systems due to changes in the amplitude and/or form of population cycles. An increase in resource productivity due to lower consumption rates following increased consumer mortality can also produce a hydra effect. Lower consumption can come about as the result of increased satiation of the consumers or changes in behaviour of either consumer or resource species that reduce the mean attack rate. Changes in species composition of a resource community may also decrease the average attack rate. Population structure can promote hydra effects by allowing separation of the timing of density dependence and mortality, although stage-specific density dependence usually decreases hydra effects.