Toxicity of heavy metals to Asellus aquaticus (L.) (Crustacea,Isopoda)

Effects of heavy metals on the isopod Asellus aquaticus (L.) are studied by static toxicity tests. Results demonstrate that the species is sensitive to Cd⁺², Cr⁺⁶, Cu⁺², Fe⁺³, Hg⁺², Ni⁺² Pb⁺² and Zn⁺², but the toxicity of each metal is different. Differences are also found between adults and between adults and juveniles. The comparative analysis of all data on the toxicity has been performed on the concentrations of metal ions and not on metal compound concentrations.Criteria for establishing water quality in order to guarantee protection of the environment are discussed.     

The ultrastructure of the heart of Oniscus asellus L. and Asellus aquaticus L. (Crustacea,Isopoda)

Summary The endocardium of Oniscus asellus L. and Asellus aquaticus L. consists of lipid cells. The epicardium consists of a layer of cells with a vesiculated cytoplasm covered by a thick extracellular fibrous sheet. The myocardium is a single layer of cells, the sarcolemma invaginates at Z disc level forming transverse tubules, and longitudinal tubules branch off from these. At the A-I level' longitudinal tubules form transverse systems, which form couplings with the sarcoplasmic reticulum. The sarcoplasmic reticulum appears as perforated sheets enveloping the myofibrils. Two types of nerve terminal are found: one is embedded in a myocardial cell process, the other lies in a myocardial cell depression. They contain clear and dense-cored synaptic vesicles.This work was supported by grants from the Norwegian Research Council for Science and the Humanities     

Epibionts of Asellus aquaticus(L.) (Crustacea, Isopoda): an SEM study

1. This study presents a qualitative and quantitative survey of epibionts infesting two populations of the freshwater isopod Asellus aquaticus (L.). Using scanning electron microscopy, the prevalence, mean intensity, abundance, spatial preferences and distribution of organisms on various external surfaces were ascertained.
2. A diverse community, consisting primarily of protozoa and rotifers, utilised A. aquaticus as a substrate organism. Every individual A. aquaticus supported some form of epizoic life. Peritrich species comprised 89.2% of all epibionts. Carchesium polypinum, Pseudocarchesium aselli, Pseudocarchesium asellicola, Pseudocarchesium simulans and Opercularia hebes dominated the epifauna.
3. Highest epibiont burdens occurred on the mouthparts, the first few ventral segments and the gills and largely comprised the above peritrich species. Most epibionts exhibited some degree of site preference on the host. A distinct gill epifauna existed. Apart from Acineta tuberosa and Vorticella rotunda , which had equal prevalences on dorsal and ventral surfaces in one culture, most species largely avoided the dorsal surface.
4. Epibionts exhibited varying degrees of adaptation to life on a living substrate. They ranged from facultative species such as A. tuberosa , which are capable of colonising abiotic substrata, to Gymnodinoides aselli whose life cycle relies on interaction with A. aquaticus . Highly specialised species tended to exhibit the most pronounced and consistent site restriction.
5. Asellus aquaticus offers a highly tractable system for further study of substrate species–epibiont relationships.     

Light-dark cycle and mitotic index in Asellus aquaticus (L.) (Crustacea,Isopoda)

The circadian variation of the mitotic index during spermiohistogenesis was studied in Asellus aquaticus (L.). The actual number of metaphases and prometaphases was determined at the end of each hour of light or darkness over a 24 h period in animals bred under LD 12:12. The number of the metaphases and prometaphases decreases during the light period and sharply increases in the last 3 hrs of the dark period. This variation in the proliferative activity suggests that photoperiod can play a role in the synchronization of mitosis.     

GATA repeats in the genome of Asellus aquaticus (Crustacea,Isopoda)

A 500 bp fragment of Drosophila genomic DNA containing 37 copies of the tetranucleotide GATA was used to probe, by Southern DNA blotting and in situ hybridization, two natural populations of the isopod crustacean Asellus aquaticus collected from the Sarno and Tiber rivers. This species does not have a recognizable sex chromosome pair. In a number of males from the Sarno population chromomycin A₃ staining reveals a heteromorphic chromosome pair. The heterochromosome has two blocks of heterochromatin. After digestion of genomic DNA with six restriction endonucleases and hybridization with the GATA probe, the two populations exhibit different fragment length patterns. No sex-linked pattern was observed in either population. In situ hybridization to chromosomes of males and females from the Sarno population does not reveal any sex-specific pattern of labelling and indicates a scattered distribution of GATA sequences on most chromosomes with some areas of preferential concentration. The heterochromatic arcas of the male heterochromosome are not labelled.by E.R. Schmidt     

The distribution of Asellus aquaticus and A. meridianus (Crustacea, Isopoda) in Britain

SUMMARY. Asellus aquaticus and A. meridianus are freshwater isopods common more or less throughout Britain. It has been suggested that the former species is replacing the latter. To investigate this, the distributions of the species were determined in the English Lake District, near Merseyside, and elsewhere. Additionally, some field transference experiments were conducted in Anglesey and near Merseyside. Particular attention was given to changes in distribution with time; many of the observations reported relate to a time span of almost 20 years. Taken together, the results provide qualified support for the suggestion that A. aquaticus is replacing A. meridianus .     

Sensitivity of Asellus aquaticus (L.) and Proasellus coxalis Dollf. (Crustacea,Isopoda) to Copper

The effects of Copper on the life-cycle of two crustacean isopods, Asellus aquaticus (L.) and Proasellus coxalis Dollf., are studied. ST₅₀ indicates that females and males are differently sensitive in comparison with juveniles. The two species do not show differences in sensitivity in the range of 0.01–15 mg·1^-1 between adults and juveniles. The effect of 0.005 mg·1^-1 Copper sulphate was strongest on embryonic and larval stages. Ecological implications are discussed.     

Effect of chronic exposure to cadmium and copper on Asellus aquaticus (L.) (Crustacea,Isopoda)

The effects of chronic exposure to 5 g·1^–1 cadmium or copper on the crustacean Isopod Asellus aquaticus (L.) were studied by analyzing survival and body growth in the first stages of the life-cycle and by determining fecundity and survival of embryo-bearing females. Juveniles survival is differently affected by the two metals in that embryonic development is more sensitive to cadmium while juvenile development is more sensitive to copper. Juvenile body growth is stimulated by cadmium and depressed by copper. Embryo-bearing female survival and fecundity are significantly reduced by cadmium but are not affected by copper. The consequence of environmental contamination by a sublethal cadmium or copper concentration is discussed.     

Geography of morphological differentiation in Asellus aquaticus (Crustacea: Isopoda: Asellidae)

We implemented a detailed morphometry and multivariate statistics to establish a general, large-scale racial differentiation in Asellus aquaticus (L.) sensu Racovitza. We ascertained that in surface populations a set of 11 morphometric characters might equivalently be represented by the pleopod respiratory area size alone. The analyses resulted in a distinct distribution pattern, with the large respiratory area populations disposed mainly along the Dinaric karst between southern Slovenia and western Macedonia and surrounded by the medium respiratory area morph, spatially irregularly substituted by the small area morph. This pattern is in contradiction with the distribution pattern of molecularly defined clades (as shown by Verovnik et al. 2005 ). We could find no ecological, hydrographical or paleogeographical explanations for such distribution pattern either. The only hypothetical explanation would be a preservation of the large respiratory area as a plesiomorphic character in the comparatively sheltered karst habitats, while throughout the easier accessible parts of the species range it was replaced by the 'modern' smaller area size. While a diminution of the respiratory area functionally means an increased sclerotization – hardening of pleopod IV–V exopodites, endopodites of pleopods III–V remain less sclerotized, probably respiratory and osmoregulatory functional.     

Silver staining analysis of nucleolar-organizer activity during spermatogenesis of Asellus aquaticus (Crustacea,Isopoda)

The Ag-staining technique was employed to investigate the activity of the nucleolar organizer during spermatogenesis of the isopod crustacean Asellus aquaticus. The most interesting results of this investigation were: (1) The NORs remain continuously Ag-stained for the whole of spermatogenesis until maturation of the sperm, contrary to the situation in the other species so far described; (2) In the gonial mitotic cells the NORs of a single pair, in the meiotic cells of both pairs of chromosomes that have the NORs, are Ag-stained and therefore active; (3) Some of the individuals examined exhibited additional NORs.     

Quantification of sexual dimorphism in Asellus aquaticus (Crustacea: Isopoda) using outline approaches

A marked sexual dimorphism is often observed in arthropods species in which males perform precopulatory mate guarding. It is generally thought to reflect the influence of sexual selection. Until now, sexual dimorphisms associated with mate guarding have mainly been qualitatively described. However, assessing the effects of sexual selection on sexual dimorphims requires a preliminary quantitative assessment of differences in morphology between sexes. Using Fourier analyses, we tested if morphological dimorphisms could be quantitatively assessed in the isopod Asellus aquaticus . In addition, we checked whether sexual dimorphism in shape was exclusively related to mate guarding through considering characters that are not, a priori , implicated in mating behaviour. To assess the potential role of sexual selection in shaping morphology, we then examined how dimorphic characters could influence males' pairing success. Three characters (pleotelson, paraeopods 4 and 5) differed significantly in shape between males and females. In addition, two characters (pleotelson and paraeopods 4) differed in shape between guarding males and non-guarding males, with the latter being closer in shape to females. This suggests that sexual selection may be partly responsible for the observed morphological divergence between sexes in A. aquaticus .  © 2002 The Linnean Society of London, Biological Society of the Linnean Society , 2002, 77 , 523–533.     

Energetics of a population of Asellus aquaticus (Crustacea, Isopoda): respiration and energy budgets

SUMMARY. Respiration of Asellus aquaticus was determined on tiine occasions throughout the year using a Gilson differential respirometer. On each occasion the determination was made at the lake temperature, which ranged from 2 to 18°C, Linear regressions were derived for log oxygen uptake against log dry weight. There was a significant difference between mean rates of oxygen uptake at the various temperatures but the slopes of the regression lines, which varied from 0,62 to O.85 with a mean of 0,76, were not significantly different.
The temperature of the lake was monitored throughout the year. Using the relationship of oxygen uptake against temperature, and estimates of population density obtained previously, the total annual respiration of the population was calculated as 4571 ml O₂ m^-2, equivalent to an energy loss of 92.3 kj m^-2 year^-1 from a mean biomassof 752 mg m^-2.
Consumption of decayed Alnus glutinosa leaves and faecal production rates, and thus assimilation efficiency, were determined by gravimetric methods in the laboratory at 10°C, close to the mean temperature of the lake (10.rC), In the four size classes investigated, larger animals consumed more food per individual, but less on a weight specific basis. However, it was shown that consumption was greater if the leaf material was more highly decomposed, Assimiliation efficiency was calculated as 23%.
Using data for respiration, population density and the assimilation efficiency, the annual population energy budget was estimated as (kJ m^-2year^-1): consumption, 568.9 (100%); production, 38.5 (6.8%); respiration, 92.3 (16.2%); faeces, 438.1 (77%). The significance of these energy values, and the ecological efficiencies calculated from them, are discussed in relation to other published work.     

The Fine Structure of the Compound Eyes of Shallow-Water Asellotes,Jaera albifrons Leach and Asellus aquaticus L. (Crustacea: Isopoda)

Abstract Both species have small sessile compound eyes. The dioptric apparatus of J. albifrons consists of a biconvex lens and a pyriform crystalline cone, the latter formed by two principal and two accessory cone cells. A. aquaticus has a reduced lens and a round cone formed by two to four principal cone cells with two to no accessory cone cells. Distal pigment cells and pigmented retinular cells lie between the ommatidia in J. albifrons. A. aquaticus has only the pigmented retinular cells. Both species have a fused, continuous (unhanded) rhabdom formed by eight retinular cells (R1—8), one of which (R8) is situated distally. The retinular cells R1—7 form, in J. albifrons, a cylinder-shaped middle portion with three microvillar directions (60° apart) and a proximal star-shaped portion. The entire rhabdom of A. aquaticus is star-shaped. Distal pigment-cell processes and basal cells form the fenestrated membrane in J. albifrons and “eye-cup cells” in A. aquaticus.     

Phylogeography of subterranean and surface populations of water lice Asellus aquaticus (Crustacea: Isopoda)

The water louse Asellus aquaticus is a widespread, euryoecious species, mostly uniform throughout its range. However, six subspecies are known from the Dinaric karst in the northwestern Balkans. They include some specialized subterranean populations. The pattern of genetic variation among subterranean and surface populations in this hydrographically highly fragmented karst region was investigated using a 653 bp fragment of the mitochondrial gene (COI). Sequencing of 168 individuals from 25 localities revealed 72 haplotypes. amova and methods of phylogenetic reconstruction all uncovered hydrographic structuring of genetic variation of the populations. Nested clade analysis pointed out several fragmentation events, along with some range expansions within hydrographical systems. By superimposing the subterranean mode of life on the phylogeographical pattern, three independent cave colonizations could be inferred within a distance of < 100 km. Caves were invaded after the ancestral surface populations became isolated through vicariant fragmentation. A possible scenario of hydrographic history of the region was constructed combining the molecular data with palaeogeographical information.     

The life history and production of Asellus aquaticus (Crustacea: Isopoda) in the River Ely, South Wales

SUMMARY. A population of the isopod Asellus aquaticus was studied for sixteen months in the organically polluted River Ely. South Wales. There were two complete generations each year. The overwintering population released young (the spring-brood) over the period April-June. These animals in turn produced their own offspring (summer-brood) in late July-October. Each generation died after breeding and the overwintering population was comprised entirely of animals from the summer-brood.
Seasonal patterns of mortality were related to water flow, temperature and life-cycle stage. Individual growth rates were affected by both temperature and life-cycle stage.
Annual population production (P) was 195.68 g wet wt m^-2, mean annual biomass (B) was 32.98 g wet wt m^-2 and the annual P/B ratio was 5.93.     

Comparison between some epigean and hypogean populations of Asellus aquaticus (Crustacea: Isopoda: Asellidae)

Samples for benthic meiofauna were collected in the vicinity of a salmon aquaculture farm in Bliss Harbour, Bay of Fundy, Canada in early August 1990. Simultaneously, samples for water content, organic carbon, organic nitrogen were collected, and redox potential and benthic oxygen consumption were measured. Meiobenthic size-spectra of biomass and respiration (calculated using allometric equations) were examined at three locations along a gradient of sediment organic enrichment radiating from the farm. Neither biomass nor respiration size-spectra were significantly different between locations despite a decrease in taxon diversity with increasing sediment organic enrichment. Small nematodes were the single largest contributor to respiration and usually to biomass at all locations, particularly at the most organically enriched location directly under the salmon farm. Calculated meiofauna respiration accounted for a greater proportion of total benthic community respiration in organically enriched sediments than in less enriched sediments.     

Random Amplified Polymorphic DNA Diversity among Surface and Subterranean Populations of Asellus aquaticus (Crustacea: Isopoda)

The ecological and evolutionary processes leading to isolation and adaptation of cave animals compared to their surface ancestors are not yet unequivocally understood. In this study the genetic relations of four cave and three surface population of the freshwater crustacean Asellus aquaticus in the Karst region of SW Slovenia and NE Italy were assessed using RAPDs as genetic markers. The results suggest that specialized populations from two caves invaded their subterranean habitat independently, and that their morphological similarity is a result of convergent evolution. Another, less specialized cave population seems to originate from a later colonization of a cave system already inhabited by a more specialized population, but the two populations do not interbreed. This series of temporally and spatially independent invasions has generated a diversity hotspot of non-interbreeding populations of a ubiquitous freshwatercrustacean, which is uniform over most of its range. Genetic variability estimated by the percentage of polymorphic RAPD fragments was similar (86–91%) in most cave and surface populations. Substantially lower values (as low as 49%) were found in two cave populations affected by heavy pollution. Two a priori groupings of populations, traditional subspecies and hydrologically connected groups, were rejected as not significant by means of nested analysis of molecular variance (AMOVA). On the other hand, groupings revealed by UPGMA clustering displayed a significant component of among-group variance. An analysis of gene flow between populations using estimated migration rates confirmed these findings.     

Parallels between two geographically and ecologically disparate cave invasions by the same species,Asellus aquaticus (Isopoda,Crustacea)

Caves are long‐known examples of evolutionary replications where similar morphologies (troglomorphies) evolve independently as the result of strong natural selection of the extreme environment. Recently, this paradigm has been challenged based on observations that troglomorphies are inconsistent across taxa and different subterranean habitats. We investigated the degree of replicated phenotypic change in two independent cave invasions by the freshwater isopod Asellus aquaticus; the first in a sulphidic aquifer in Romania, the second in a sinking river in the Dinaric Karst in Slovenia. Both ancestral surface populations still live alongside the subterranean ones. Phylogenetic analyses show independence of the two colonization events, and microsatellite analysis shows no evidence of ongoing genetic exchange between surface and subterranean ecomorphs. The overall morphology has changed dramatically at both sites (50 of 62 morphometric traits). The amount of phenotypic change did not reflect differences in genetic diversity between the two ancestral populations. Multivariate analyses revealed divergent evolution in caves, not parallel or convergent as predicted by the current paradigm. Still, 18 traits changed in a parallel fashion, including eye and pigment loss and antennal elongation. These changes might be a consequence of darkness as the only common ecological feature, because Romanian caves are chemoautotrophic and rich in food, whereas Slovenian caves are not. Overall, these results show that morphologically alike surface populations can diverge after invading different subterranean habitats, and that only about one‐third of all changing traits behave as troglomorphies in the traditional sense.