Strain Concentrations Surrounding an Ellipsoid Model of Lacunae and Osteocytes

Direct cell sensing of tissue matrix strains is one possible signaling mechanism for mechanically mediated bone adaptation. We utilized homogenization theory to estimate bone tissue matrix strains surrounding osteocytes using two sets of models. The first set of models estimated the strain levels surrounding the lacunae and canaliculi, taking into account variations in lamellar properties. The second set estimated strain levels in the osteocyte and the surrounding matrix for different cellular mechanical properties. The results showed that the strain levels found in and surrounding osteocytes, 1700 to 2700 microstrain (denoted as μe; 1 μe =.0001% strain), were significantly greater than the trabecular tissue level strains of {1325 μe, 287 μe, 87 μe} used for model input. Variation in lamellar properties did not affect strain levels, except at lamellar boundaries. Strain in and surrounding the osteocyte was not significantly affected by cellular stiffness ranging between 28 and 28, 000 Pascals (Pa). Strain levels surrounding lacunae and canaliculi were approximately equivalent.     

A stream by any other name: does β-diversity differ between Nearctic and Neotropical streams?

Hydrobiologia - Beta diversity typically increases with decreasing latitude, with explanations of this pattern often based on tropical origins of most clades. Black flies (Diptera: Simuliidae) are...     

The effect of temperature and host species on the development of the trichomycete Smittium culisetae (Zygomycota)

We examined the growth and development of the trichomycete Smittium culisetae (Harpellales: Legeriomycetaceae) in the larval hosts Simulium vittatum Zetterstedt (Diptera: Simuliidae) and Aedes aegypti (L.) (Diptera: Culicidae) at three temperatures, 17, 22 and 30 C. Trichospore maturation of Sm. culicetae external to the host as well as the ability of these trichospores to colonize new hosts also was investigated. Although the development of Sm. culisetae varied with both temperature and host there was a pattern of maximum trichospore production at 48-72 h postinoculation. In addition thalli under laboratory conditions are capable of spore production after extraction from a host and these trichospores can colonize new hosts. Furthermore this was noted to occur in both host species. These results suggest that synchrony between host and symbiont development is not as tightly coupled as previously assumed.     

Laboratory investigations of trichomycete prevalence, abundance and fecundity in a Smittium-simuliid model

Smittium, the most speciose genus of the "gut fungi" (Zygomycota: Trichomycetes), is found attached to the hindgut cuticle of larval aquatic Diptera. Smittium spp. colonize several host families (e.g., Smittium culisetae in Chironomidae, Culicidae and Simuliidae), but some species appear to be specific to a single host family (e.g., Smittium morbosum Sweeney in Culicidae). The specificity of Smittium spp. within a host family has been difficult to resolve. This research presents evidence that certain Smittium spp. differentially colonize particular species of black fly (Diptera: Simuliidae) hosts as measured by differences in prevalence, abundance and fecundity. Reasons for this differential occurrence and fecundity in hosts are unclear but might include fungal responses to variations in host morphology, physiology, distribution or behavior. Variable fitness of Smittium spp., within a suite of available hosts, could be a factor in the diversity of this fungal group.     

The roles of abiotic factors, dispersal, and species interactions in structuring stream assemblages of black flies (Diptera: Simuliidae)

ABSTRACT: BACKGROUND: The patterns and drivers of species assemblages represent the core of community ecology.We focus on the assemblages of a single family of ubiquitous lotic insects, the Simuliidae(black flies), of which the larvae play a critical role in resource turnover in steams. We useMantel tests and null models to tease out the potential influence of abiotic stream conditions,species interactions, and dispersal on the assemblage patterns of larval black flies over twospatial scales (within and across ecoregions) and two seasons (spring and summer). RESULTS: When stream sites were considered across ecoregions in the spring, stream conditions anddispersal were correlated significantly with species similarity; however, within ecoregions inthe spring, dispersal was important only in the Piedmont and Sandhills and abiotic factorsonly in the Mountains. In contrast, results of the summer analyses within and acrossecoregions were congruent; assemblage similarity was significantly correlated with streamconditions both across and within ecoregions. Null models suggested that patterns of speciessegregation in the spring were consistent with a community structured by competition,whereas patterns in the summer were consistent with species assemblages influenced byabiotic factors. CONCLUSIONS: Species composition of black flies at streams sites is correlated with dispersal factors andstream conditions, but results vary over spatial and temporal scales. Communities of blackflies can be viewed within a metacommunity context; local assemblages are consistent withspecies sorting and mass effects. Given that black flies have a terrestrial stage, with femalesdeciding where to place the eggs, a full understanding of the processes that determine local aquatic assemblages will require integration of the dynamics of the aquatic immature stagesand the terrestrial adults.     

Scale, time, space, and predictability: species distributions of preimaginal black flies (Diptera: Simuliidae)

This study examines species-specific distributions of preimaginal black flies (Diptera: Simuliidae) over two large spatial scales (within and across ecoregions) and two seasons (spring and summer). The study area in the western two-thirds of South Carolina, United States, was divided into three ecoregions (Mountains, Piedmont, Sandhills). The mean correct classification of species distributions among stream sites was 81.3%. Predictability of species distributions varied with spatial scale, location, and time. Species occurrence was not independent of ecoregion; distributions of 12 of the 13 species examined at this scale were predictable on the basis of changes in water column (temperature, percentage dissolved oxygen) and channel characters (velocity, streambed-particle size). However, with the exception of the Mountains during the summer, predicting species distributions within ecoregions was far less successful than predicting distributions across these regions; predictability was particularly poor in the Sandhills. Seasonal effects on predictability were most pronounced in the Mountains. As stream sites became more homogeneous, predictive capability declined. Species-level identifications, aided by cytogenetic techniques, were of paramount importance in recognizing the association between species distributions and stream conditions, emphasizing that considerable ecological information is lost when species are not considered. This study emphasizes the critical role of taxonomic resolution, linked with spatial and temporal scale of observation, in elucidating patterns of species assemblages. It also demonstrates that predictability of species distributions at a fixed scale can vary with geographic location and time. Received: 2 May 1997 / Accepted: 22 September 1997     

Alpha richness and niche breadth of a widespread group of aquatic insects in Nearctic and Neotropical streams

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Species composition of local riffle beetle (Coleoptera: Elmidae) assemblages in small coastal streams of the Gulf of Mexico: the influences of local and regional factors

Using Partial Mantel tests to isolate distance and environmental effects from each other, we examined species composition of riffle beetle assemblages (Coleoptera: Elmidae) of 22 local streams from the Gulf of Mexico, as a function of dispersal and environmental constraints. In addition, we examined how different stream topologies (Euclidean, Network) influenced dispersal constraints on species composition. In general, when the influences of local and regional influences were isolation from each other (Partial Mantel test), community dissimilarity increased between sites as differences in both stream conditions and distance between sites increased. However, these trends varied over the scale of study (within and across water basins) and stream topology. For example, at the scale of the watershed, overland dispersal did not appear to be a limiting factor in the formation of local elmid assemblages. The relationship between local species composition and distance was not evident in a distance–decay regression, nor was an environmental similarity versus community similarity regression significant. At least within the limits of our study, it was clear that if local and regional influences were not isolation from each other (Partial Mantel test), their associations with local assemblages were not evident. We also argue the utility of restricting study to a single family of insects when attempting to clarify the causal mechanisms responsible for local species assemblages.     

Ecoregions as predictors of lotic assemblages of blackflies (Diptera: Simuliidae)

Local ecological attributes of streams are known to have strong influences on community membership for many aquatic insects. Differences in aquatic insect assemblages, therefore, should be clearly detectable across large scale ecological "units", such as ecoregions. Many studies of aquatic invertebrates however, have suffered from a lack of species level identifications. In addition, many previous studies that examined the influence of ecoregion on aquatic assemblages have the implicit assumption that members of different taxa are responding in the same manner. Our study, therefore, was restricted to an ecologically (lotic) and functionally (mostly filter-feeding) homogenous group, the Simuliidae. In the current study, we examine the relationship between species assemblages of preimaginal blackflies and the landscape through which their stream habitats flow. Accordingly, the larval simuliid faunas from South Carolina, USA, are compared among three ecoregions established a priori: Blue Ridge Mountains, Piedmont, and Sandhills. Using discriminant function analysis, we show that each ecoregion produces a distinct stream habitat; factors responsible for regionalization are quantified. We also show that streams can be assigned correctly to ecoregion of origin 85% of the time on the basis of the simuliid assemblage. We suggest that our results can be interpreted most readily by considering the distribution of individual species.     

Prevalence of the trichomycete fungus Harpella melusinae (Harpellales: Harpellaceae) in larval black flies (Diptera: Simuliidae) across a heterogeneous environment

A total of 2063 mid- to late-instar larval black flies were collected from 64 stream sites in South Carolina and screened for the presence of the trichomycete fungus Harpella melusinae. Sixteen of 18 host species were colonized by H. melusinae on at least one occasion. Prevalence of H. melusinae in larvae of Simulium tuberosum cytospecies "A" was highest in acidic streams with low conductivity, whereas H. melusinae colonized larvae of Simulium verecundum most frequently in slower-moving streams. Ecological conditions, therefore, can serve as predictors of the prevalence of H. melusinae. Prevalence in host larvae was significantly lower in the Piedmont ecoregion than in the Mountain ecoregion. Prevalence did not differ in the host species S. verecundum across ecoregions, suggesting that different prevalences among host species might indicate some host preference. The prevalence of H. melusinae differed significantly between two univoltine host species (Simulium venustum and Prosimulium magnum) at the same site but not between two multivoltine host species (S. tuberosum cytospecies "FG" and S. tuberosum cytospecies "CDE"), suggesting that host life history could be important in determining fungal prevalence.