Limited genetic variation and structure in softshell clams (Mya arenaria) across their native and introduced range

To offset declines in commercial landings of the softshell clam, Mya arenaria, resource managers are engaged in extensive stocking of seed clams throughout its range in the northwest Atlantic. Because a mixture of native and introduced stocks can disrupt locally adapted genotypes, we investigated genetic structure in M. arenaria populations across its current distribution to test for patterns of regional differentiation. We sequenced mitochondrial cytochrome oxidase I for a total of 212 individuals from 12 sites in the northwest Atlantic (NW Atlantic), as well as two introduced sites, the northeast Pacific (NE Pacific), and the North Sea Europe (NS Europe). Populations exhibited extremely low genetic variation, with one haplotype dominating (65–100%) at all sites sampled. Despite being introduced in the last 150–400 years, both NE Pacific and NS Europe populations had higher diversity measures than those in the NW Atlantic and both contained private haplotypes at frequencies of 10–27% consistent with their geographic isolation. While significant genetic structure (F _ST = 0.159, P < 0.001) was observed between NW Atlantic and NS Europe, there was no evidence for genetic structure across the pronounced environmental clines of the NW Atlantic. Reduced genetic diversity in mtDNA combined with previous studies reporting reduced genetic diversity in nuclear markers strongly suggests a recent population expansion in the NW Atlantic, a pattern that may result from the retreat of ice sheets during Pleistocene glacial periods. Lack of genetic diversity and regional genetic differentiation suggests that present management strategies for the commercially important softshell clam are unlikely to have a significant impact on the regional distribution of genetic variation, although the possibility of disrupting locally adapted stocks cannot be excluded.     

Patterns of p53, p73 and mortalin gene expression associated with haemocyte polyploidy in the soft-shell clam, Mya arenaria

The molecular mechanisms by which haemocytes of clams are transformed in the course of haemic neoplasia remain by far unknown. The aim of this study was to quantify the expression of p53/p73 and mortalin genes, in relation with the ploidy status of clam haemocytes and to correlate the p53 expression with mortalin expression. For this purpose, soft-shell clams, Mya arenaria, were collected from an endemic zone for neoplasia. The ploidy of haemocytes was assessed for each individual clam by flow cytometry using a propidium iodide protocol, while p53/p73 and mortalin gene expressions were quantified by real-time RT-PCR. Results show that haemocytes of some clams with a moderate percentage (15-50%) of tetraploid cells have a significantly high level of p53 and p73 in comparison with clams belonging to categories with low (<15%) or high levels (>50%) of tetraploid cells, where low levels of expression of these genes were observed. Furthermore, mortalin gene expression is strongly correlated (r² = 0.68, p < 0.01) with p53 gene expression level. This reinforces the hypothesis of a cytoplasmic p53 sequestration mechanism in clam haemic neoplasia. Further studies are needed to confirm these preliminary results and further unravel the molecular pathways involved in this process. Our results are believed to provide phenotypic foundation for such studies to be undertaken.     

Assessment of haemic neoplasia in different soft shell clam Mya arenaria populations from eastern Canada by flow cytometry

Diagnosis of haemic neoplasia (HN) in the soft shell clam, Mya arenaria, is often achieved by hematocytology and histology. Since neoplastic cells display tetraploid DNA contents, haemocyte cell cycle analysis was developed for use as a diagnosis tool. The aim of this study was to assess the application of a flow cytometry procedure of cell cycle analysis established for the common cockle, to clams and to evaluate different thresholds of value for the percentage of tetraploid cells for establishing HN disease status of individual clams and clam populations. HN status of six clam populations from eastern Canada was determined. Results of the present study demonstrate a flow cytometry procedure to be useful for HN diagnosis in clams. Individual clams were considered to be affected by HN when presenting at least 20% of haemocytes in S-4N phase; and negative when presenting less that 5% of haemocytes in S-4N phase. As discussed in this paper, intermediate cases represent uncertain diagnoses including either false-negative or false-positive clams, which are difficult to discriminate. At a population level, an additional threshold of 15% for the mean intensity of the disease is proposed, which means having in the population several individual clams presenting more than 20% of their haemocytes in S-4N phase. Based on these thresholds of value, only one population was considered as free of HN disease, and one population was unequivocally affected by HN. For the four other clam populations, further investigations are needed toward development and use of specific and objective biomarkers of HN.     

Sex alteration in soft-shell clams (Mya arenaria) in an intertidal zone of the Saint Lawrence river (Quebec,Canada)

The purpose of this study was to verify whether any changes in sex ratio might occur in soft-shell clams (Mya arenaria) located in an intertidal harbor zone located at the mouth of the Saguenay Fjord in the Saint Lawrence estuary (Baie Sainte-Catherine (BSC), Québec, Canada) likely to be contaminated by organotin compounds. Bivalves were harvested at the BSC harbor site and from two reference sites. Condition index (weight to length ratio), gonado-somatic index, sex ratio, vitellin-like proteins, organotin concentrations in gonad tissue, maturation stages of the gonads, the number of estradiol-17beta binding sites and the capacity of female gonad extracts to produce estradiol-17beta were determined in collected animals. Results showed that sex ratio in clams was significantly skewed toward males. Moreover, the condition and gonad-somatic indices, vitellin-like proteins in female gonads and the capacity of female gonads to produce estradiol-17beta were significantly reduced at the harbor site with respect to the reference sites. Maturation status of male gonads was clearly delayed at the harbor site. Additionally, gonad tissue contained tributyltin (TBT) at an average level of 109+/-18 ngSn/gdry wt. at the harbor site while organotins were not detected from the reference sites. Finally, female gonads had a higher number of unoccupied estradiol binding sites at the harbor site indicating low levels of this steroid in this tissue. Overall, this paper is first to report that clams collected in the vicinity of a TBT contaminated harbor are subject to masculinizing effects which seems to be consistent with biological effects that organotins are known to exert toward some other marine invertebrates.     

Larval settlement of the bivalvesMya arenaria andM. uzenensis on scallop collectors in Vostok Bay, Sea of Japan

This paper discusses data on the horizontal and vertical distribution of the spat ofMya arenaria andM. uzenensis on collectors for the cultivation of the scallopMizuhopecten yessoensis in Vostok Bay. The maximum settlement of the mollusks on scallop collectors occurred in the innermost part of the bay. The spat density ofM. arenaria on collectors was 2.6 times that ofM. uzenensis. M. arenaria larvae preferred settling on the upper portions of collectors, while the distribution ofM. uzenensis was random. The time of collector placement in the sea influenced spat numbers.     

Predation by a water mite (Piona exigua) on enclosed populations of zooplankton

Short term, replicated experimental alteration of densities of a predatory water mite Piona exigua Viets, inside 0.45–0.475 m³ litre enclosures, revealed little evidence of the effects of predation on the number and relative abundance of the enclosed zooplankton species. Predation rates more closely approximated those estimated from single prey functional response experiments in the second experimental period (December) than in the first (March). In December Daphnia was the only susceptible taxon present in large numbers, whereas in March, Ceriodaphnia and Chydorus were also present. This result is consistent with laboratory findings that predation rates are lowered in the presence of more than one prey type.The difficulty of obtaining evidence for significant effects of these planktonic predators is in part due to changes in the preferred prey species in the diet of Piona depending on stage and sex of the mite and to aspects of experimental design. The wide variability between replicate enclosures at each predator density reduced the power of the statistical analyses used to test the null hypothesis. Enclosures with no predators are necessary to investigate the effects of enclosure on the zooplankton prey, since these effects may outweigh those due to predator consumption.     

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.     

Density Dynamics and Size Composition of Bivalve Mollusk Larvae of the Genus Mya in Peter the Great Bay (Sea of Japan)

The seasonal population size of planktonic larvae of mollusks of the soft-shell clam genus Mya were studied in Vostok Bay. The larvae occurred in plankton from the first third of June to early September. The highest density of larvae (279.7 ± 98.7 specimens/m³) was found in late June. In early July, larval numbers decreased sharply, and at the end of the first third of July, only single Mya larvae were found in the plankton. Another increase in larval population was observed in early and late August. The larvae varied in length from 175 to 325 m in June–July and from 150 to 250 m in August. It is likely that the larvae of two species of Mya, M. arenaria and M. uzenensis, are present in the plankton of Vostok Bay.     

Predation on mammals by the chimpanzee (Pan troglodytes)

With respect to prey selectivity and predation frequency, chimpanzees (Pan troglodytes) show local differences as well as diachronic variability within the same population. When data on predation from three long-term studies at Mahale, Gombe, and Tai are compared, some differences and similarities emerge; Mahale is more like Gombe than Tai in regard of prey selection but features of hunting at Tai with respect to predation frequency are not conspicuous. The most responsible factor for diversity in prey selectivity is a distinct “prey image” maintained by chimpanzees of different populations, although it is necessary to clarify in future studies why and how such tradition develops. Relative body size of chimpanzees to prey species and/or the degree of cooperation among members of a hunting party may explain the variability in prey size selected at each site, the latter influencing the frequency of successful hunts at the same time. Although various degrees of habituation and different sampling methods including artificial feeding might have obscured the real differences, recent data from the three populations do not seem to be biased greatly by such factors. Nevertheless, it is still difficult to make strict comparisons due to the lack of sufficient standardized data across the three populations on the frequency of hunting and predation. It is suggested that the size or demographic trend of a chimpanzee unit-group, especially the number of adult males included, necessarily influences its hunting frequency as well as its prey profile. It is also suggested that factors which bring these males together into a party (e.g. fruit abundance, swollen females, conflict between unit-groups etc.) strongly affect theactual hunting and kill rates. Other possible factors responsible for the local differences are forest structure (e.g. tree height), skilful “hero” chimpanzees, and competition with sympatric carnivorous animals. A total of at least 32 species have been recorded as prey mammals of chimpanzees from 12 study sites and the most common prey mammals are primates (18 species), of which 13 species are forest monkeys. Forest monkeys, colobine species in particular, are often the most common victims of the predation by chimpanzees at each site. We may point out a tendency toward selective hunting for the forest monkeys in terms of the selectivity of prey fauna among all three subspecies of chimpanzees, including populations living in drier environment. The mode of chimpanzee hunting seems to correspond to the highest available biomass of gregarious, arboreal monkeys in the forest, colobine species in particular. In contrast, bonobos (P. paniscus) are less carnivorous than chimpanzees, only rarely preying on a few species of small mammals. The sharp contrast of the two allied species in their predatory tendencies appears to have something to do with the differences in the structure of primary production between their habitats.     

The feeding behaviour of Leptodora kindti and its impact on the zooplankton community of Neusiedler See (Austria)

Leptodora kindti is a very efficient invertebrate predator. Its searching mode of preying is tactile. The setae of the first thoracic limb act as mechanoreceptors, the other thoracic limbs, thorax and head together form the shape of an open basket in which after encounter the prey is pushed in by the aid of the first thoracic limbs and the furca. In Neusiedler See, small individuals of Diaphanosoma brachyurum (0.6–0.9 mm) are the preferred prey, rarely copepods are taken. The predation rate is influenced by temperature, prey density and predator size and varies between less than one and 12 prey items per predator per day. At high predator densities, Leptodora will have a substantial effect on the Diaphanosoma population of Neusiedler See.     

Delayed gametogenesis and progesterone levels in soft-shell clams (Mya arenaria) in relation to in situ contamination to organotins and heavy metals in the St. Lawrence River (Canada)

Wild carp, Cyprinus carpio, were sampled in January and March 2000 in a section of the Anoia River (NE Spain) known to be polluted by estrogenic compounds. At each sampling time, three groups were distinguished: (1) apparently normal males; (2) apparently normal females; and (3) affected fish. The latter were characterized by the simultaneous development of male and female tissue in their gonads at a macroscopical level (six out of 31 fish sampled at this particular point), or testicular atrophy (three out of 31). Plasmatic and hepatic vitellogenin (VTG) levels and plasma testosterone (T) and estradiol (E2) were measured to observe the particular estrogenic response of the affected fish. Moreover, the response in the xenobiotic metabolizing capacity in liver was tested. This involved the analysis of mixed function oxygenase (MFO) system such as: total cytochrome P450 content, NAD(P)H cytochrome c reductases and the associated CYP1A1, EROD activity. Also, glutathione S-transferase (GST) and UDP-glucuronosyltransferase (UDPGT) as detoxifying enzymes were measured. Our results showed: (1) a highly variable VTG content in all fish groups; (2) an increase in sex hormones content in March for the female group; and (3) an enhanced xenobiotics metabolism in the affected fish group, measured as total cytochrome P450, EROD activity in the January survey and cytosolic GST in March. The observed increase in VTG, sex hormones and in most of the enzymatic activities from January to March that could also be attributed to higher water temperature.     

Predation of Arternia cysts by water-tiger larvae of the genus Anacaena (Coleoptera,Hydrophilidae)

Artemia cysts are preyed upon by the larvae of Anacaena (Coleoptera, Hydrophilidae) in the wet supralittoral of the hypersaline Solar Lake near Elat. The larvae are specialized feeders of the cysts, as shown by laboratory experiments.     

Orchard trials in Canada on control ofCydia pomonella (Lep: Tortricidae) by granulosis virus

Cydia pomonella (L.), a major pest of apples in Canadian orchards, is susceptible to a granulosis virus (CpGV). Orchard trials with different formulations of CpGV in Ontario, whereC. pomonella has two generations per year, indicated that as many as seven applications of virus would be needed to maintain the percentage of fruit with deep larval entries below 4%. In Nova Scotia, where there is only one generation ofC. pomonella per year, two applications of CpGV usually were needed although, in some cases, one sufficed. The protection of fruit conferred by CpGV equalled that by the organophosphate insecticides, azinphosmethyl or phosmet, in some tests. The advantages of a selective treatment, such as CpGV, over use of broad-spectrum insecticides are discussed.

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Résumé Cydia pomonella (L.), un ravageur important du pommier dans les vergers canadiens, est sensible au virus de la granulose. Des essais en vergers utilisant deux préparations de ce virus en Ontario, oùC. pomonella présente deux generations par année, indiquent qu'il faudrait jusqu'à sept pulvérisations pour maintenir à moins de 40% la proportion de fruits portant des infestations larvaires. En Nouvelle Ecosse, oùC. pomonella ne compte qu'une seule génération par an, deux pulvérisations furent nécessaires, quoique dans certains cas une seule a suffi. Dans certains essais, le niveau de protection des fruits égalait celui fourni par les esters phosphoriques, l'azinphos-méthyle ou le phosmet. On discute des avantages des traitements sélectifs tels que le virus de la granulose en comparaison avec l'emploi des insecticides de grande polyvalence.

     

Predation on earwigs: A novel diet specialization within the genusLeptogenys (Formicidae: Ponerinae)

Summary Leptogenys sp. 13 (nearkraepelini) exclusively hunts on earwigs (Gonolabis electa andParalabis sp.). This unusual kind of prey specialization is shown by field data, prey midden investigations, and prey selection experiments. The adaptive value of the species' prey specialization is discussed.     

Predation by juvenile Platichthys flesus (L.) on shelled prey species in a bare sand and a drift algae habitat

Due to increasing eutrophication of the coastal Baltic waters, drifting algae are a common phenomenon. Drifting algal mats accumulate on shallow sandy bottoms in late summer and autumn, and affect the ambient fauna. Juvenile flounder, Platichthys flesus, utilize these habitats during their first few years. They feed on benthic meio- and macrofauna; part of their diet consists of shelled species, such as Ostracods, and juvenile Hydrobia spp. and Macoma balthica. Earlier studies have shown that up to 75% of ostracods and 92% of hydrobiids survive the gut passage of juvenile flounder, while all M. balthica are digested by the fish. We conducted laboratory experiments to study how the shelled prey responded to a drift algal mat, and the predation efficiency of juvenile P. flesus on these prey species on bare sand and with drifting algae (50% coverage). Hydrobia spp. utilized the drift algae as a habitat and, after 1 h, 50% had moved into the algae; ostracods and M. balthica were more stationary and, after 96 h, only 23 and 12%, respectively, were found in the algae. For the predation efficiency of P. flesus, a two-way ANOVA with habitat (algae, bare sand) and predation (fish, no fish) as factors revealed that both algae and predation affected negatively the survival of all three prey species. The algae, thus, affected the predation efficiency of juvenile P. flesus and the consumption of prey was much reduced in the algal treatments compared to the bare sand. This was due probably to increased habitat complexity and the ability of prey, especially hydrobiids, to use the algal mat as a refuge. Altered habitat structure due to drift algae, together with the resultant changes in habitat (refuge) value for different prey species, may profoundly change the structure of benthic communities.     

Effects of current speed, shell length and type of sediment on the erosion and transport of juvenile softshell clams (Mya arenaria)

The erosion and transport of juvenile softshell clams (Mya arenaria) was studied in a laboratory flume in relation to free-stream velocity (0, 7, 16, 29 and 35 cm s^− 1), shell length (0-5, 5-10, 10-15, 15-20 mm) and type of sediment (mud, sandy-mud, sand and gravel). Our results showed that these factors interact together on the erosion of clams from the sediment. Juveniles were eroded in great numbers in sand while mud retained them more easily. Bedload transport was initiated at speeds of 16 cm s^− 1. Most of the clams were eroded in sandy sediments at speeds of 29 and 35 cm s^− 1. The smallest individuals were highly vulnerable to erosion compared to the other size classes studied. A results-based model using the logistic regression statistics was proposed. This allowed the estimation of erosion probabilities for a given hydrosedimentary environment. A field validation of the model was then carried out. Field results confirmed the importance of free-stream velocity, shell length and type of sediment on the erosion rate of clams. The differences observed between predicted and field results suggest that the model underestimated the erosion rate in the field. Results are discussed in the context of hydrosedimentary environments found off the eastern coast of Canada.     

Predation by Podisus maculiventris (Hemiptera: Pentatomidae) on Plutella xylostella (Lepidoptera: Plutellidae) larvae parasitized by Cotesia plutellae (Hymenoptera: Braconidae) and its impact on cabbage

Biological control offers potentially effective suppression of the diamondback moth (DBM), Plutella xylostella, a serious pest of Brassica crops. Little is known of whether multiple natural enemies have additive, antagonistic, or synergistic effects on DBM populations. No-choice and choice tests were conducted to assess predation by Podisus maculiventris on DBM larvae parasitized by Cotesia plutellae and unparasitized larvae. In no-choice tests, P. maculiventris preyed on greater numbers of parasitized than unparasitized larvae and greater numbers of young larvae than old larvae. In choice tests with early third instar DBM, there was no difference in predation between parasitized or unparasitized larvae. However, in choice tests with older prey, P. maculiventris preyed on more parasitized than unparasitized larvae. Two field studies were conducted to test if this predator and parasitoid have additive, antagonistic or synergistic effects on DBM populations and plant damage in cabbage (Brassica oleracea var. capitata). In 2002, DBM populations were significantly lower in the presence of C. plutellae but not in the presence of P. maculiventris. There was not a significant interaction between the natural enemies. Plant damage was reduced only with C. plutellae. In 2003, DBM populations were significantly lower in the presence of C. plutellae and P. maculiventris, although the combination of natural enemies did not lead to a non-additive interaction. Plant damage was unaffected by the presence of either natural enemy. Because of its greater predation on parasitized larvae, P. maculiventris could be an intraguild predator of C. plutellae. Yet, their overall combined effect in the field was additive rather than antagonistic.     

Predation potential of three flatworm species (Platyhelminthes: Turbellaria) on mosquitoes (Diptera: Culicidae)

We conducted a field survey for flatworms to select species as potential biological control agents against Aedes aegypti and Culex pipiens (Diptera, Culicidae) breeding in artificial containers. Laboratory experiments were performed to determine the daily predation rate, differential predation on each mosquito larval instar, selective predation on either A. aegypti or C. pipiens, and predator tolerance to water from artificial containers. Girardia anceps (Tricladida, Paludicola, Dugesiidae), Mesostoma ehrenbergii and Bothromesostoma cf. evelinae (Rhabdocoela, Typhloplanoida, Typhloplanidae) were found in temporary puddles and permanent pools. In the laboratory, they killed between 52% and 100% of immature mosquitoes coexisting in the same habitat. No preference of flatworms for mosquito preys was detected. Predation rate was related to predator size and instar of preys. Girardia anceps and B. evelinae survived after a dry period and when re-flooding occurred, they laid eggs. Tolerance to water from artificial containers was highest in G. anceps and this species could be a suitable predator to reduce mosquito populations from artificial containers using an inoculative approach.