Environmental and behavioural factors affecting activity in the intertidal gastropod Hydrobia ulvae

Laboratory microcosms were used to investigate the mud snail Hydrobia ulvae (Pennant) bioturbation activities and behavioural changes in response to snail density, algal food, sediment moisture content, light regime and water cover conditions. Density-dependent kinetics of bioturbated muddy areas were described by von Bertalanffy equations, which provided reliable estimates of mud surface covering rates by snail tracks (m² h⁻¹ snail⁻¹). Snails need a wet habitat to be active either covered by seawater or by moving in fluid layers for low-tide conditions. Light and microphytobenthic biomass, which are less potent to affect snail activity, are positively interrelated to increase covering rates in the tested chl a concentrations within the range of 1-15 μg g⁻¹. Experimental results suggested us the relevance of microphytobenthos migration processes in affecting crawling activities of H. ulvae that appeared to adjust their foraging efforts in response to benthic algal biomass. Behavioural processes of H. ulvae, in terms of floating, crawling, burrowing and inactive snails, were described using a Markov model. Finally, an empirical model based on von Bertalanffy equations was proposed to describe kinetics of sediment covering by snail tracks under the influences of snail density, sediment moisture content, chl a concentrations and the four combinations of presence/absence of light and seawater. This model should provide a base for further development of a hydrosedimentary model to simulate the effects of H. ulvae bioturbation activities on the resuspension of the intertidal cohesive sediment-water interface for various in situ conditions.     

Ingestion rate of the deposit-feeder Hydrobia ulvae (Gastropoda) on epipelic diatoms: effect of cell size and algal biomass

The effects of cell size of epipelic diatoms and sediment Chl a content (as an index of algal biomass) on the ingestion rate of Hydrobia ulvae adults and juveniles were investigated in experimental microcosms. Results showed that both adults and juveniles ingested small and large diatoms without exhibiting cell size selection behaviour. The functional response of H. ulvae, juveniles (<4 mm) and adults (>4 mm), over a wide range of sediment Chl a content, was characterized by an increase of the ingestion rate according to a power law. Ingestion rate varied from 0.75 to 10 ng Chl a ind⁻¹ h⁻¹ for juveniles and from 1 to 52 ng Chl a ind⁻¹ h⁻¹ for adults, in the range ca. 10-100 μg Chl a (g dry weight sediment)⁻¹. The ingestion rate was about three times higher for adults than for juveniles. Based on these experimental results, we further proposed a mechanistic approach, using an individual based-model, to identify simple feeding mechanisms that might be involved in H. ulvae functional response.     

Long-term changes in amphipod population dynamics in a temperate estuary following ecosystem restoration

The Mondego estuary (Portugal) has suffered severe ecological stress over the last two decades, as manifested in the replacement of seagrasses by opportunistic macroalgae, degradation of water quality and increased turbidity. A restoration plan was implemented in 1998, which aimed to reverse the eutrophication effects, and especially to restore the original natural seagrass (Zostera noltii) community. This article explores the long-term changes in Ampithoe valida and Melita palmata (Amphipoda) populations in response to eutrophication (with consequent seagrass loss and macroalgal proliferation) and to the subsequent restoration plan (with progressive seagrass recovery and macroalgal biomass decline). Until the early 1990s, high densities of A. valida and M. palmata were recorded in the Mondego estuary, especially during the occurrence of the macroalgal bloom and during all the periods in which green macroalgae were available. After the implementation of the restoration plan, species abundance, biomass and production levels decreased considerably due to the progressive decline of green macroalgae. This implied the virtual disappearance of the amphipod population, mainly A. valida. Distinct behaviours displayed by the two species could be related to different food strategies and habitat preferences. Ampithoe valida showed feeding preferences for ephemeral softer, filamentous or bladed algae (e.g. Ulva sp.) due to its high caloric content, using the Z. noltii bed only as a habitat for protection against predators or shelter from wave action. On the other hand, M. palmata did not suffer a strong decline in its population density, biomass and production, which may indicate that this species is probably not a primary consumer of green macroalgae and may readily shift to alternative ecological niches. Handling editor: P. Viaroli     

Community regulation by herbivore parasitism and density: Trait-mediated indirect interactions in the intertidal

The abundant herbivorous mud-snail Hydrobia ulvae is an ecosystem engineer in soft-bottom intertidal habitats due to its grazing and bioturbation activity. However, mud snails are commonly infected by trematodes that reduce their overall activity, which in turn may affect their impact on the surrounding benthic community. To test this hypothesis, we performed field experiments manipulating both the abundance of uninfected snails (0, 7500 and 15.000 ind. m^- 2) and the level of snail parasitism (0, 33 and 100% trematode prevalence) on a Danish mud-flat. The results showed that increasing snail abundance and parasitism generally had opposite effects on the community of microphytobenthos and zoobenthos. Increasing snail density increased the chlorophyll-a concentration in the substrate (enhancement), whereas increasing parasitism decreased it. In accordance, the benthic primary producers were generally less nutrient limited at high snail density and mostly so at high levels of snail parasitism. Moreover, epipsammic diatoms were favoured over epipelic diatoms at increasing snail density, whereas the opposite was evident at increasing snail parasitism. At the community level, increasing snail density increased evenness among epipelic diatoms, whereas increasing snail parasitism decreased evenness and species diversity. Probably through the action of trophic cascades and varying levels of disturbance, the zoobenthic community was influenced by experimental treatments as well. The indirect effects of snail parasitism influenced significantly the abundance of more faunal species (seven) than did snail density (two). At the community level, increasing snail density decreased evenness and lowest species richness coincided with intermediate snail density. In contrast, increasing snail parasitism resulted in increasing evenness and peaking species richness at intermediate level of parasitism. Together, the results show that parasites solely through their impact on the behaviour of a single community member can be significant indirect determinants of community organisation and function.     

Comparing shoot population dynamics methods on Posidonia oceanica meadows

Prediction capacity of three main shoot population dynamics methods (age structure, net shoot recruitment per plagiotropic rhizome and shoot census) have been tested for a period of four years (2002-2006) on a Posidonia oceanica meadow. Accuracy of each method was checked by comparing measured and predicted densities at the end of the study period. Predicted densities came from the evolution of initial densities (measured in 2002) by a basic exponential model of population growth. The exponential model used the different net shoot recruitment rate estimates by each population dynamics method on three depths (upper, medium and lower limit) and three localities at each depth. Predictions performed by shoot census and net shoot recruitment per plagiotropic rhizome methods matched with measured densities at the end of the study period. Conversely, age structure method underestimated shoot densities at each depth, indicating an unreal decrease of shoot population in the meadow.     

Impact of hypoxia on the survival, egg production and population dynamics of Acartia tonsa Dana

Concern for the increasing occurrence of coastal zone hypoxia has generally focused on the direct, short-term impact of reduced dissolved oxygen (DO) concentrations on the survival of commercially important species such as fish and crabs. Copepods, especially the naupliar stages, are important pelagic food web components, yet only a few studies have considered the effect of reduced DO concentrations on their survival and population dynamics. This study considers the impact of both lethal and sublethal DO concentrations on copepods. Acartia tonsa were reared at 25 °C at saturating DO (normoxic control) and reduced (hypoxic) DO concentrations of 1.5 or 0.7 ml l⁻¹. Oxygen concentrations were maintained in replicate flasks, by bubbling seawater with air (control), or mixtures of nitrogen and oxygen. Egg production, but not survival, was significantly higher in the controls compared to the 1.5 ml l⁻¹ DO treatment. Survival and egg production were significantly lower at 0.7 ml l⁻¹ DO compared to the control. The results suggest that the sublethal as well as the lethal effects of hypoxia may have important repercussions on population and community dynamics in coastal systems.     

Influence of salinity, nutrients and light on the germination and growth of Enteromorpha sp. spores

In many shallow coastal areas worldwide, several species of opportunistic macroalgae (mainly Chlorophyta) have an excessive growth, as a consequence of eutrophication processes. Therefore, bloom-forming macroalgae become the dominant primary producers within these coastal systems. However, frequently the ecology and the ecophysiology of adult macroalgae have been insufficient to explain their seasonal abundances. Thus, it is essential to understand the factors that regulate the germination and growth of spores of opportunistic green macroalgae. In the present work, we assessed the effects of nutrients (N and P), salinity and light on the germination and growth of Enteromorpha spores. Overall, the results highlight the fact that, such as for adult macroalgae, spore germination and growth are adversely affected by low salinities. Growth of the spores is significantly decreased at 5 psu, while salinities of 20 and, especially of 35 psu, clearly promote the spore growth. Additionally, Enteromorpha spores seem to be particularly sensitive to PO₄-P limitation and to NH₄-N toxicity, which suggests a higher sensitivity to the variation of external nutrient concentrations than adult macroalgae. The present results contribute to increase the understanding about the factors that control macroalgal growth at its early phases of development. In particular, the results suggest that the growth of spores from opportunistic green macroalgae is strongly salinity-dependent. Consequently, in highly hydrodynamic systems such as most shallow estuaries, salinity variations may play a determinant role in the yearly abundances of green macroalgae, since it controls macroalgal growth from the spores to the adults.     

The identification of a new Giardia duodenalis assemblage in marine vertebrates and a preliminary analysis of G. duodenalis population biology in marine systems

Giardia duodenalis is an intestinal parasite of many vertebrates. The presence of G. duodenalis in the marine environment due to anthropogenic and wildlife activity is well documented, including the contributions from untreated sewage and storm water, agricultural run-off and droppings from terrestrial animals. Recently, studies have detected this protistan parasite in the faeces of marine vertebrates such as whales, dolphins, seals and shore birds. To explore the population biology of G. duodenalis in marine life, we determined the prevalence of G. duodenalis in two species of seal (Halichoerus grypus, Phoca vitulina vitulina and Phoca vitulina richardsi) from the east and west coasts of the USA, sequenced two loci from G. duodenalis-positive samples to assess molecular diversity and examined G. duodenalis distribution amongst these seals and other marine vertebrates along the east coast. We found a significant difference in the presence of G. duodenalis between east and west coast seal species. Only the zoonotic lineages of G. duodenalis, Assemblages A and B and a novel lineage, which we designated as Assemblage H, were identified in marine vertebrates. Assemblages A and B are broadly distributed geographically and show a lack of host specificity. Only grey seal (Halichoerus grypus) samples and one gull sample (Larus argentatus) from a northern location of Cape Cod, Massachusetts, USA, showed the presence of Assemblage H haplotypes; only one other study of harbour seals from the Puget Sound region of Washington, USA previously recorded the presence of an Assemblage H haplotype. Assemblage H sequences form a monophyletic clade that appears as divergent from the other seven Assemblages of G. duodenalis as these assemblages are from each other. The discovery of a previously uncharacterised lineage of G. duodenalis suggests that this parasite has more genetic diversity and perhaps a larger host range than previously believed.     

Effect of retentive markers on the dynamics of settlement: The case of arthropod silk

During a settlement decision, the presence of conspecifics is crucial to species subject to Allee effects, for which the number of founders affects the subsequent growth of the colony. Marking the area (physically or chemically) conveys information about the number of conspecifics present in a new patch. Here, we study how an individual affinity for the marker affects the dynamics of a foundation process. A generic population model is presented, in which marking and affinity for the marker are at stake. Our results show that population size thresholds can appear, below which settlement is not possible. This model is then used to study the dynamics of migration and aggregation in a set of interconnected populations. We show that affinity for the marker can induce asymmetries in the population distribution. Anelosimus eximius is a social spider subject to Allee effects, for which silk potentially acts as a marker. We test our predictions with field experiments involving two populations of A. eximius in a Y-shaped setup. The agreement between our experimental and theoretical results strongly supports the validity of the model. This allows us to use the model to estimate a realistic set of parameters of biological significance to this social spider.     

新疆北部棉区作物景观多样性对棉铃虫种群的影响

如何从景观尺度上实现对害虫的科学管理已经成为昆虫生态学的研究热点.利用频振式杀虫灯诱集技术,从2007-2009年在新疆北部棉区16-17个农场近240km2作物范围内,监测和评估棉田周边作物景观对棉铃虫种群的影响.结果表明:农业景观多样化显著地影响棉铃虫种群数量,复杂作物系统中(棉花比例＜50％作物面积)棉铃虫成虫数量明显大于简单作物系统(棉花比例≥50％作物面积;棉铃虫种群数量与景观多样性指数(Simpson's Reciprocal Index)呈正相关;同时棉铃虫成虫与加工番茄、玉米和小麦的比例成正相关,但与棉花比例呈负相关.研究结果为转基因棉花抗性管理提供科学依据,同时农田景观多样性指标可作为修正棉区棉铃虫预测模型的重要指标.     

Effect of the snail Ilyanassa obsoleta (Say) on dynamics of the amphipod Corophium volutator (Pallas) on an intertidal mudflat

We investigated interactions between two dominant invertebrate species of intertidal soft-sediment environments of the northwest Atlantic, the mud snail Ilyanassa obsoleta and the burrowing amphipod Corophium volutator, on a mudflat of the upper Bay of Fundy, Canada. Distribution of I. obsoleta on the mudflat was highly patchy and negatively correlated with density of C. volutator. Manipulation of snail density in cages showed that I. obsoleta influences C. volutator; specifically, increasing density of snails reduced density, increased patchiness in distribution, decreased recapture rates and decreased immigration of C. volutator. Ilyanassa obsoleta seems to be affecting C. volutator through an influence on survival rate and emigration rate, although temporal variation in these effects was observed. Given that both I. obsoleta and C. volutator show a preference for tide pools, an important microhabitat on mudflats, snails might have a profound impact on C. volutator population dynamics.     

Trypanosoma vivax displays a clonal population structure

African animal trypanosomiasis, or Nagana, is a debilitating and economically costly disease with a major impact on animal health in sub-Saharan Africa. Trypanosoma vivax, one of the principal trypanosome species responsible for the disease, infects a wide host range including cattle, goats, horses and donkeys and is transmitted both cyclically by tsetse flies and mechanically by other biting flies, resulting in a distribution covering large swathes of South America and much of sub-Saharan Africa. While there is evidence for mating in some of the related trypanosome species, Trypanosoma brucei, Trypanosoma congolense and Trypanosoma cruzi, very little work has been carried out to examine this question in T. vivax. Understanding whether mating occurs in T. vivax will provide insight into the dynamics of trait inheritance, for example the spread of drug resistance, as well as examining the origins of meiosis in the order Kinetoplastida. With this in mind we have identified orthologues of eight core meiotic genes within the genome, the presence of which imply that the potential for mating exists in this species. In order to address whether mating occurs, we have investigated a sympatric field population of T. vivax collected from livestock in The Gambia, using microsatellite markers developed for this species. Our analysis has identified a clonal population structure showing significant linkage disequilibrium, homozygote deficits and disagreement with Hardy-Weinberg predictions at six microsatellite loci, indicative of a lack of mating in this population of T. vivax.     

Macroinvertebrate response to different species of macroalgal mats and the role of disturbance history

Over the last 20 years, the Mondego estuary, Portugal has experienced excessive growth of macroalgae especially in the inner parts of the system, with several algal species implicated. In this study, we compare the effects of morphologically different species, the red alga Gracilaria verrucosa and the green macroalga Enteromorpha intestinalis on macrobenthic assemblages, by a field experiment whereby the biomass of algae was manipulated and the resultant changes in macrofauna abundance evaluated. The experiments were carried out in different areas (a relatively undisturbed sea grass bed and an upstream eutrophic area) experiencing different degrees of overall enrichment.Measurements of sediment redox potential revealed a rapid anoxia with a significant increase in algal biomass after 4 weeks. The effects of macroalgae were different at the two sites, being more marked in the eutrophic area. In addition, the effects of Gracilaria and Enteromorpha were significantly different, with Enteromorpha having a greater detrimental effect for most of the macrofauna, in particular Cyathura carinata, Scrobicularia plana, Cerastoderma edule and Alkmaria romijni. However, three of the most abundant invertebrates (Hydrobia ulvae, Hediste diversicolor and Capitella capitata) showed significant increases in abundance in weed affected compared to weed-free plots. Gracilaria had less of an impact on macrobenthic assemblages leading to a more enriched community. Between-site differences in overall impact were related to their previous disturbance history.     

石榴园西花蓟马种群动态及其与气象因素的关系

2007-2008年,对云南省建水石榴园西花蓟马种群动态进行了系统调查,并采用回归分析(逐步回归分析、通径分析)、主成分分析及灰色系统分析就气象因子对该虫种群动态的影响进行了系统分析。结果表明,西花蓟马在建水石榴园常年发生,冬季较低,夏季最高,成虫全年种群消长呈单峰型,高峰期为5月份。相关性分析结果表明,西花蓟马种群数量与月相对湿度间呈极显著正相关性(P<0.01),与月均气温和月最低气温间呈显著正相关性(P<0.05),与月最高气温、月均降雨量和月均蒸发量间无相关性(P>0.05)。回归分析结果表明,石榴园西花蓟马种群动态的决定因子中影响最大的气象因素是月最低气温,而月均气温和月相对湿度是影响种群数量变动的主要因素。主成分分析表明,月最低气温是主要成分,其累积方差贡献率达73.03%。灰色系统分析结果表明,影响石榴园6种蓟马种群动态最关键的因子是月相对湿度;年度间影响最大的是年总降雨量;石榴花期各蓟马的种群数量与气象因素间关联度最大的是月最低温;果期各种蓟马的种群数量与气象因素间关联度最大的是月均降雨量。     

Further study on Ascogregarina culicis in temperate Argentina: Prevalence and intensity in Aedes aegypti larvae and pupae

The bionomics of South American strains of Ascogregarina spp. are poorly known and the first studies were performed a few years ago. Our main objective was to characterize Ascogregarina culicis population in Aedes aegypti immatures in temperate Argentina. A total of 1800 water-filled containers were inspected within a cemetery of Buenos Aires City through a reproductive period of the host (October 2006-June 2007). The parasite was detected in 16.7% (329/1974) of the immatures and 8.5% (15/177) of the breeding sites. The prevalence decreased from 19.9% in larvae to 6.5% in pupae. In those infected breeding sites, about 85% of the immature mosquitoes harbor the parasite with a median intensity of nine trophozoites per larva and six gametocysts per pupa. The prevalence in shaded containers was higher than in sun exposed ones but the intensity of the infection was quite similar between both lighting conditions. Sun-exposed containers recorded water temperatures significantly higher than those under shade throughout the study period. Parasite trophozoites were only found from January to May with a clear seasonal pattern of prevalence. Monthly values of parasite prevalence and mosquito host (percentage of breeding sites and number of immatures) were significantly correlated at p < 0.05 when a temporal delay of two months was considered. Our results suggest that parasite prevalence is spatially and temporally heterogeneous in temperate urban Argentina, and these variations are associated with the host abundance.     

The deubiquitinase emperor's thumb is a regulator of apoptosis in Drosophila

We have characterized the gene emperor's thumb (et) and showed that it is required for the regulation of apoptosis in Drosophila. Loss-of-function mutations in et result in apoptosis associated with a decrease in the concentration of DIAP1. Overexpression of one form of et inhibits apoptosis, consistent with et having an anti-apoptotic function; however, overexpression of a second form of et induces apoptosis, indicating that the two forms of et may have competing functions. et encodes a protein deubiquitinase, suggesting it regulates apoptosis by controlling the stability of apoptotic regulatory proteins.