Effects of green algal mats on bivalves in a New England mud flat

Concurrent with the spread of green algal mats on tidal flats, reports of macrofauna dieoffs under dense algal mats have increased in numbers. Bivalves seem to be particularly affected by persistent dense algal mats. Bivalve species with a long extendible siphon seem to be less affected underneath algal mats, but no distinction has been made in the past between species with short and those with long siphons,Mya arenaria andMacoma balthica, on an intertidal mudflat in New England. Abundances ofM. arenaria declined substantially during the study period when a thick green algal mat covered the mudflat for several months. Numbers of the small bivalveGemma gemma also decreased substantially, whereas abundances ofM. balthica showed minimal variation during the time of algal coverage. In algae removal/addition experiments numbers ofM. arenaria decreased, but effects were only significant in an algal addition to previously algal-free mudflat areas. Abundance ofM. balthica did not change significantly in the algal removal/additition experiments. Over the time period of the experiment (9 weeks),M. arenaria showed measurable size increase in uncovered mudflat areas, but not underneath algal mats. Similarly,M. balthica only increased in size in the uncovered mudflat area. From these results it is concluded thatM. balthica can survive time periods of dense algal coverage because it is able to penetrate through the algal mat with its long extendible siphon, and thus can reach well-oxygenated water layers above the mat.M. arenaria with its thick, less extendible, siphon cannot push through dense algal mats and therefore is more likely to die underneath persistent algal mats.     

The functional group approach to bioturbation: II. The effects of the Macoma balthica community on fluxes of nutrients and dissolved organic carbon across the sediment-water interface

We have examined the effects of individual members of the two dominant functional groups of the Macoma balthica community on fluxes of nutrients and dissolved organic carbon between sediment and overlying water. The biodiffusers M. balthica and Mya arenaria and the gallery-diffuser Nereis virens were added to microcosms containing sieved tidal flat sediment, using identical biovolume for each treatment to facilitate comparison. Both functional groups enhanced fluxes over a control without macrofauna. The gallery-diffuser had the greatest effect. The two biodiffusers had opposite effects on the flux of nitrate. M. balthica, which lives near the sediment surface, caused nitrate release whereas M. arenaria caused nitrate uptake. Both organisms increased the release of ammonia, but M. arenaria had the greater effect. We attribute this intra-functional difference not so much to different functionality as to interactions between organisms and the depth distribution of pore water constituents. Because the burrow of M. balthica is located within the nitrification zone of the sediment, the depth averaged concentration of nitrate in its burrow is higher than in the overlying water, and burrow flushing leads to net release of nitrate. In contrast, the burrow of M. arenaria includes both nitrification and denitrification zones, which lower the depth averaged concentration of nitrate below the concentration in the overlying water. The functional group approach needs to take the processes that control sediment chemistry into account in order to predict fluxes sucessfully.     

Physiological responses of marine Dendryphiella species from different geographical locations

The saprobic, cosmopolitan, marine fungi Dendryphiella arenaria and Dendryphiella salina, isolated from various plant and algal substrates from different geographical locations and climatic zones, were studied for their adaptations to the abiotic and biotic parameters commonly found in their natural marine habitats. All the tested strains of D. arenaria and D. salina grew optimally on culture media with added marine salts, at pH values between 6.5 and 8.0 and at an incubation temperature of 25°C. The D. arenaria strains had faster mean colony extension rates under all conditions of culture. All strains exhibited an increased salt optimum with increasing incubation temperature. The TLC profiles of strains of the two species were similar. The culture extracts were antimicrobial, though production of the biologically active metabolites was strain-specific. There were no significant correlations between source of origin and responses to the investigated parameters. These results demonstrate phenotypic plasticity and the ability of each isolate to adapt to diverse biotopes.     

The clam sipho as indicator for growth indices in the soft-shell clam <Emphasis Type="Italic">Mya arenaria</Emphasis>

Due to the species’ deep burrowing behaviour, growth parameters of the soft-shell clam Mya arenaria L. 1758 are difficult to estimate, especially in deeper habitats which are not directly accessible. In this study, we analysed 192 specimens of M. arenaria and found, contrary to results of most other studies that the sigmoid Gompertz growth model (GGM) is appropriate to describe the growth. Predictions from this model confirm the finding of a life span up to 8 years and as a consequence of this, a maximum shell-length of 60 mm. Individual growth rates calculated from the first deviation indicate a nearly exponential growth in young individuals. The relationship between the clam sipho-width and age, shell-length and biomass conform to the GGM. This is a new approach to assess growth of M. arenaria and to solve practical problems that arise in field studies.     

Predation on soft-shell clams (Mya arenaria) by the nemertean Cerebratulus lacteus in Atlantic Canada: implications for control measures

The nemertean, Cerebratulus lacteus Verrill (Nemertinea: Heteronemertini), has been identified as an important threat to soft-shell clam (Mya arenariaL.) populations in Atlantic Canada. The biology of this species, however, is still largely unknown. Field and laboratory studies were undertaken in 1998 and 1999 in Prince Edward Island, Canada, to test certain control measures to reduce predation on soft-shell clam populations and to better describe the relationship between C. lacteus and M. arenaria. Field abundance of C. lacteus, M. arenaria and Nereis virens Sars were evaluated in relation to particular habitat modifications that were used as control measures. Sediment manipulations tested were: (1) addition of shells and (2) use of a hydraulic rake. No difference was observed on the abundance of C. lacteus, M. arenaria and N. virens after treatments were applied. In the laboratory, C. lacteus was shown to be an efficient predator of M. arenaria. Clam mortalities reached 100% in the presence of C. lacteus while 0% mortality was observed in its absence. A complementary set of experiments was carried out to see if the sympatric polychaetes N. virens and Glycera dibranchiata Ehlers had any impact on the relationship between C. lacteus and M. arenaria. N. virens showed no impact on C. lacteus predation on clams. The presence of G. dibranchiata significantly reduced the nemertean predation rate. Analysis of clam size selection revealed no significant preference by C. lacteus. Other experimental studies revealed that high predator densities did not impede predation on clams and that C. lacteus preferred soft-shell clams among other commercial bivalve species presented (Mercenaria mercenariaL., Crassostrea virginica Gmelin and Mytilus edulisL.). This study should provide a better understanding of the relationship between C. lacteus and M. arenaria and lead to the development of improved nemertean control measures.     

Meiofauna affect the macrobenthic biodiversity–ecosystem functioning relationship

The biodiversity–ecosystem functioning (B–EF) relationship has become of main interest in the last few decades, mostly because of the worldwide increase in species extinctions. However, most of these studies only consider species within single size‐class or trophic level, thereby most likely underestimating ecosystem complexity. To reach more realistic scenarios, we assessed the role of meiofauna (lower size‐class level) on the relationship between macrofaunal biodiversity and multiple benthic ecosystem properties. Experiments took place under controlled conditions using three macrofaunal species (Alitta (formerly Nereis) virens, Macoma balthica and Mya arenaria). A total of eight combinations of zero to three functionally different macrofaunal species were maintained in microcosms for 34 days in either the presence or absence of a different size‐class grouping (a meiofaunal mixture). The organic matter content and bacterial abundance in sediments and the oxygen and nutrient (NH₄⁺, NO_x^?, PO₄^3?) fluxes across the sediment‐water interface were measured and used as proxies of ecosystem properties. Overall, macrofaunal species richness did not modify any of the measured properties; however, we observed changes associated to the presence of A. virens and M. balthica. This study also revealed a strong impact of the presence of meiofauna on ecosystem properties. They changed interactions between macrofaunal species, which led to modifications in the ecosystem properties. Thus, even if this size‐class group has been poorly considered in previous studies, this experiment suggests that future studies should consider the meiofauna with greater attention, particularly in the context of B–EF.     

Influence of water availability on competitive interactions between plant species on sandy soils

The gravimetric soil water content of four different successional stages of inland dune vegetation revealed pronounced seasonal changes and particularly a major drought period in the late summer. In order to analyze the effect of these seasonal water changes on the competitive interactions of two dominant species (Carex arenaria andDeschampsia flexuosa) in one of these stages root development was compared under experimentally manipulated water conditions in a minirhizotron experiment. Under well-watered conditions, the root growth ofC. arenaria andD. flexuosa was not influenced by competition, but was significantly reduced forC. arenaria at low water availability. Accordingly, the relative yield based on total biomass was significantly lower under water shortage than under well-watered conditions forC. arenaria, indicating higher interspecific competition compared to intraspecific competitive interactions. These results indicate (i) that soil water status is an important factor for belowground competitive interactions and (ii) that seasonal changes in soil water content may have a pronounced effect on the outcome of competition and, in consequence, on the structure of the respective plant communities.     

Habitat matters – Strong genetic and epigenetic differentiation in Linum catharticum from dry and wet grasslands

Plant species differ in their ecological amplitude, with some species occurring in very different habitats under strongly differentiated environmental conditions. We were interested in to what extent the occurrence of Linum catharticum in dry calcareous grasslands (Bromion) and wet litter meadows (Molinion), two habitats on opposing ends concerning, for example, moisture level, is reflected on the genetic and epigenetic level. Using AFLP (amplified fragment length polymorphisms) and MSAP (methylation sensitive amplification polymorphisms) analyses, we studied the genetic and epigenetic variation of L. catharticum from calcareous grasslands and litter meadows. From each habitat, we included five study sites with 16 individuals per sampling location. We observed lower genetic than epigenetic diversity, but considerable differentiation among habitats, which was stronger on the genetic than the epigenetic level. Additionally, we observed a strong correlation of genetic and epigenetic distance, irrespective of geographic distance. The dataset included a large portion of fragments exclusively found in individuals from one or the other habitat. Some epigenetic fragments even occurred in different methylation states depending on the habitat. We conclude that environmental effects act on both the genetic and epigenetic level, producing the clear differentiation among plant individuals from calcareous grasslands and litter meadows. These results may also point into the direction of ecotype formation in this species.     

The functional group approach to bioturbation: The effects of biodiffusers and gallery-diffusers of the Macoma balthica community on sediment oxygen uptake

The Macoma balthica community, which is widely distributed in intertidal soft sediments bordering the north Atlantic, is dominated by two functional groups with different sediment mixing modes: the biodiffusers M. balthica and Mya arenaria and the gallery-diffuser Nereis virens. To compare the effects of these two groups on sediment oxygen uptake rates, we used experimental microcosms with identical biovolumes to measure the influence of each species on oxygen uptake. The two biodiffusers had similar effects on oxygen uptake in spite of different space occupation and different feeding, ventilation and burrowing modes. Biodiffusers and gallery-diffusers had different effects on oxygen uptake. Periodic ventilation by the gallery-diffusers stimulated the oxygen uptake by the sediment more than the steady activities of the biodiffusers. Temporal variation in oxygen fluxes in bioturbated microcosms was linked to construction and maintenance of biogenic structures. The results confirm that the functional group approach to bioturbation is a useful tool for quantifying the effects of intertidal benthic communities on benthic fluxes.     

Meloidogyne javanica Parasitic on Peanut

Peanut fields in four governorates of Egypt were surveyed to identify species of Meloidogyne present. Fourteen populations obtained from peanut roots were all identified as M. javanica based on perineal patterns, stylet and body lengths of second-stage juveniles, esterase phenotypes, and restriction fragment length polymorphisms of mtDNA. Three of 14 populations, all from contiguous fields in the Behara governorate, had individuals with a unique two-isozyme esterase phenotype. All populations of M. javanica tested on peanut had levels of reproduction on the M. arenaria-susceptible peanut cultivar Florunner that were not different from M. arenaria (P = 0.05), and had lower levels of reproduction on the M. arenaria-resistant genotype TxAG-7 than on Florunner (P = 0.05). Reproduction of the five Egyptian populations of M. javanica tested was lower on root-knot nematode resistant tomato cultivars Better Boy and Celebrity than on the root-knot nematode susceptible cultivar Rutgers (P = 0.05). These data are evidence that some populations of M. javanica are parasitic on peanut and that the peanut and tomato genotypes resistant to M. arenaria are also resistant to these populations of M. javanica.     

Within and between species variation in response to environmental gradients in Polygonum pensylvanicum and Polygonum virginianum

Summary The responses of Polygonum pensylvanicum L., an early successional annual, and of Polygonum virginianum L., a late successional perennial, were examined along discontinuous gradients of soil moisture, light and nutrient availability. In the case of P. virginianum both individuals grown from seed and individuals grown from rhizomes were examined. The results show that variation in the response of individuals of a species of different age to environmental variation is as great as that found between the two congeneric species of different successional habitats. In general, individuals of the two species were more similar to one another in response to the resource gradients when both were started from seed, than were individuals of P. virginianum grown from seed and from rhizomes. Potential differences in stored reserves (starting capital) between rhizomes and seeds appeared to have little effect upon responses to resource availability. A number of plant characters were found to vary along the gradients in ways that were unique to the character, the gradient, and the age of the individual. These characters included aspects of leaf size, shape, and orientation, as well as whole plant architecture. Many if not all of these characters are likely to have significant effects upon the functioning of plants. The origin of this difference in response to the gradients of individuals of P. virginianum of differing age may be ontogenetic or may reflect differences in genetic composition created by recombination.     

Effects of the deposit-feeding benthic bivalve Macoma balthica on meiobenthos

To evaluate the effects of a deposit-feeding bivalve on meiobenthic assemblage structure in muddy habitats, a laboratory experiment was performed at the Askö Laboratory in the northwestern Baltic proper. Microcosms, surface area 104 cm², containing a c. 7-cm thick layer of sieved (0.5 mm) sublittoral mud were established in June 1990. Two months later the tellinid bivalve Macoma balthica was added in quantities varying from 0 to 40 individuals per microcosm. After 5 months the effects of the bivalves on the meiofauna were surprisingly small. The density of harpacticoid copepods was lowest (P <0.05) in microcosms containing a high density of M. balthica. It is suggested that competition for food resources was responsible for this pattern. For all other meiofaunal groups, including nematodes which were the most abundant taxon (99%), no significant differences (P>0.05) were observed among treatments. The assemblage structure of the nematodes was similar between treatments. The vertical distribution of both major taxa and nematode species appeared to be unaffected by the presence of the bivalve.     

Mahonia invasions in different habitats: local adaptation or general-purpose genotypes?

Rapid evolutionary adaptations and phenotypic plasticity have been suggested to be two important, but not mutually exclusive, mechanisms contributing to the spread of invasive species. Adaptive evolution in invasive plants has been shown to occur at large spatial scales to different climatic regions, but local adaptation at a smaller scale, e.g. to different habitats within a region, has rarely been studied. Therefore, we performed a case study on invasive Mahonia populations to investigate whether local adaptation may have contributed to their spread. We hypothesized that the invasion success of these populations is promoted by adaptive differentiation in response to local environmental conditions, in particular to the different soils in these habitats. To test this hypothesis, we carried out a reciprocal transplantation experiment in the field using seedlings from five Mahonia populations in Germany that are representative for the range of habitats invaded, and a greenhouse experiment that specifically compared the responses to the different soils of these habitats. We found no evidence for local adaptation of invasive Mahonia populations because seedlings from all populations responded similarly to different habitats and soils. In a second greenhouse experiment we examined genetic variation within populations, but seedlings from different maternal families did not vary in their responses to soil conditions. We therefore suggest that local adaptation of seedlings does not play a major role for the invasion success of Mahonia populations and that phenotypic plasticity, instead, could be an important trait in this stage of the life cycle.     

Resistance of maize to Meloidogyne arenaria and Meloidogyne javanica

Summary A diallel cross of eight maize, Zea mays L., inbred lines was analyzed for reaction to two species of root-knot nematodes, Meloidogyne arenaria (Neal) Chitwood and M. javanica (Treub) Chitwood. Egg production following inoculation of F₁ hybrid seedlings with nematode eggs was determined in a greenhouse experiment. Data were analyzed using Griffing's Method 4, Model I. General combining ability was a significant source of variation in egg production of both M. arenaria and M. javanica; specific combining ability was not a significant source of variation for either. The correlation between egg production of the two nematode species on the 28 F₁ hybrids was highly significant. Hybrids with Mp313 or SC213 as one parent were the most resistant to both species. This indicates that germ plasm is available for developing inbred lines and hybrids with resistance to both M. arenaria race 2 and M. javanica.This article is a contribution of the Crop Science Research Laboratory, U.S. Department of Agriculture, Agricultural Research Service, in cooperation with the Mississippi Agricultural and Forestry Experiment Station, Journal No. J-7481.     

Chemically induced predator avoidance behaviour in the burrowing bivalve Macoma balthica

The responses of the burrowing bivalves Macoma balthica and Cerastoderma edule to chemical cues emitted by feeding shore crabs Carcinus maenas were investigated. M. balthica held in the laboratory and exposed to chemical signals in effluent water discharging from tanks containing C. maenas fed 20 M. balthica day^− 1 reacted by increasing their burial depths from approximately 30 mm to depths of > 60 mm, over a period of several days. When the signal was removed the bivalves gradually returned to their original depth over 5 days. C. edule similarly exposed to effluent from crabs feeding on conspecifics showed no response. In an attempt to identify the signal inducing this burrowing response, M. balthica were exposed to a variety of chemical signals. Crabs fed M. balthica elicited the strongest response, followed by crabs fed C. edule. There were also small responses to effluent from crabs fed on fish, crabs previously fed on M. balthica and to crab faeces, but no responses to starved crabs, crushed M. balthica, or controls. We conclude that increased burrowing depth of M. balthica is induced by some as yet unidentified chemical cue produced by feeding crabs and is strongest when the crabs were fed on M. balthica. Unexpectedly, neither the presence of crabs themselves, nor of damaged conspecifics, was effective in eliciting a burrowing response. The mortality rates of M. balthica and C. edule selected by crabs when burrowed at normal depths and after exposure to effluent from feeding crabs were different. Crabs selected 1.5 times more C. edule than M. balthica when both species were burrowed at their normal depths, but 15 times more after the tanks had been exposed to effluent from feeding crabs for 5 days. The burrowing response of M. balthica thus appears to reduce mortality significantly by displacing predation pressure on to the more accessible C. edule.     

Can postlarval bivalves select sediment type during settlement? A field test with Macoma balthica (L.) and Cerastoderma edule (L.)

Many soft-bottom benthic invertebrates display sediment-associated patterns of dispersion. Habitat selection experiments have shown that the larvae of some species can choose to settle in favourable habitats, and this process could establish patterns observed in the field. However, many soft-bottom infauna, including the bivalves Cerastoderma edule and Macoma balthica, show postlarval relocation. Such movements could obviate patterns established at initial settlement, and active habitat selection in these postlarval stages may be responsible for adult distributions. We investigated sediment selection in postlarval stages of these bivalves on an intertidal mudflat using a field survey and experiment. Our 1225 m² grid survey showed a strong spatial correlation between the smallest individuals of both species, which showed no correlation with sediment characteristics. Larger size groups showed increasingly strong associations with sediment type: C. edule with sandy sediments and M. balthica with muddy. In our sediment choice experiment, the numbers of bivalves recruited into muddy, sandy and muddy depression (to detect small-scale hydrodynamic effects) treatments after 2 days in the field were recorded. There were no significant treatment effects in either C. edule or M. balthica. All bivalves were too large to be initial settlers; they were thus relocating individuals. Neither the survey nor the experiment provided any evidence that relocating individuals of these two species could choose the sediment in which they alight. The sediment-related patterns observed in adults of these species must therefore be due to other mechanisms, possibly postsettlement predation by Carcinus maenas and/or an ability to leave less-preferred sediments in a series of migrations until the preferred habitat is found.     

Rivers shape population genetic structure in Mauritia flexuosa (Arecaceae)

The Mauritia flexuosa L.f. palm is known as the “tree of life” given its importance as fundamental food and construction resources for humans. The species is broadly distributed in wet habitats of Amazonia and dry habitats of the Amazon and Orinoco river basins and in the Cerrado savanna. We collected 179 individuals from eight different localities throughout these habitats and used microsatellites to characterize their population structure and patterns of gene flow. Overall, we found high genetic variation, except in one savanna locality. Gene flow between populations is largely congruent with river basins and the direction of water flow within and among them, suggesting their importance for seed dispersal. Further, rivers have had a higher frequency of human settlements than forested sites, contributing to population diversity and structure through increased human use and consumption of M. flexuosa along rivers. Gene flow patterns revealed that migrants are sourced primarily from within the same river basin, such as those from Madeira and Tapajós basins. Our work suggests that rivers and their inhabitants are a critical element of the landscape in Amazonia and have impacted the dispersal and subsequent distribution of tropical palm species, as shown by the patterns of genetic variation in M. flexuosa.     

Genetic,morphological, and dietary changes associated with novel habitat colonisation in the Canary Island endemic grasshopper Acrostira bellamyi

1. The large flightless grasshopper Acrostira bellamyi Uvarov, endemic to the island of La Gomera (Canary Islands), inhabits two different environments: the xeric euphorb shrubland, as is typical for congeneric Canarian species, and the humid laurel forest, a novel habitat for the genus. 2. We investigate genetic, morphological, and ecological variation among individuals of A. bellamyi from the two habitats. DNA sequence data were used to evaluate whether grasshoppers from the two environments represent distinct lineages. Morphological and trophic analyses were performed to assess phenotypic differentiation between the two different habitats. 3. Population genetic analyses support the hypothesis that the euphorb shrubland is the ancestral habitat for this species. Female laurel forest specimens are larger than those inhabiting the euphorb shrubland, and some external body parts exhibit significant morphometric differences between the two populations. Diet of shrubland individuals is completely different from that of laurel forest individuals. Although in each habitat they consume the most abundant plants, individuals are able to select food plants, which appear to be explained by their nutrient content. 4. Our results suggest that A. bellamyi has colonised laurel forest from shrubland, and that this habitat shift has resulted in genetic, morphological, and ecological changes, perhaps as an adaptation to this new habitat.     

A Field Test of the Intermediate Disturbance Hypothesis in the Soft‐Bottom Intertidal

The intermediate disturbance hypothesis has had success in explaining changes in local diversity in many habitats. Recent laboratory work has shown that disturbance may act to increase diversity in soft‐bottom marine communities as predicted by the hypothesis. In this paper, we present the results of a field experiment which tested the impacts of physical disturbance on soft‐bottom, intertidal macrobenthic communities. Five disturbance treatments were used, differing in the frequency of applied disturbance events; the highest frequency treatments were dug every week, lowest every eight weeks. The experiment was run for a total of 25 weeks over the winter of 1997/98. Our experiment controlled for differences in recovery time after disturbances. Abundances of Pygospio elegans, Macoma balthica, Hydrobia ulvae and Streblospio benedicti were all significantly reduced in high disturbance treatments, as was the total number of species. No species showed significant increases in abundance in disturbed treatments, and there was no evidence of an increase in diversity in any treatments. These results are discussed in the context of the intermediate disturbance hypothesis. Our results suggest that the interspecific competitive effects postulated by the hypothesis are not important in structuring this low diversity, intertidal community. However, unequivocally rejecting the hypothesis is difficult because it contains many ambiguities, and acts more as a conceptual model than as a falsifiable hypothesis.