Chronic toxicity of tributyltin on development and reproduction of the hermaphroditic snail Physa fontinalis: Influence of population density

Effect of tributyltin exposure on the embryonic development and behavior of a molluscan model species, Lymnaea stagnalis

The presence of the organotin compound tributyltin (TBT) in aquatic ecosystems has been a serious environmental problem for decades. Although a number of studies described the negative impact of TBT on mollusks at different levels, investigations connected to its potential effects during embryogenesis have been neglected. For a better understanding of the impact of TBT on mollusks, in the present study, embryos of previously TBT-treated or not treated specimens of the great pond snail (Lymnaea stagnalis) were exposed to 100 ng L-1 TBT from egg-laying (single-cell stage) until hatching. According to our results, TBT significantly delayed hatching and caused shell malformation. TBT transiently decreased the locomotion (gliding) and also reduced the feeding activity, demonstrating for the first time that this compound can alter the behavioral patterns of molluscan embryos. The heart rate was also significantly reduced, providing further support that cardiac activity is an excellent indicator of metal pollution in molluscan species. At the histochemical level, tin was demonstrated for the first time in TBT-treated hatchlings with intensive reaction in the central nervous system, kidney, and hepatopancreas. Overall, the most notable effects were observed in treated embryos derived from TBT treated snails. Our findings indicate that TBT has detrimental effects on the development and physiological functions of Lymnaea embryos even at a sub-lethal concentration, potentially influencing their survival and fitness. Highlighting our observations, we have demonstrated previously unknown physiological changes (altered heart rate, locomotion, and feeding activity) caused by TBT, as well as visualized tin at the histochemical level in a molluscan species for the first time following TBT exposure. Further studies are in progress to reveal the cellular and molecular mechanisms underlying the physiological and behavioral changes described in the present study.

Environmental conditions affect the food quality of plastic associated biofilms for the benthic grazer Physa fontinalis

With an ever-increasing amount of plastic pollution in the various aquatic ecosystems around the world, the effects on organisms are still not fully understood. Most studies focus on direct effects posed by plastic intake or entanglement, but plastic debris can also affect primary production of biofilms and have an indirect impact on its consumers. This study investigates the primary production on three common plastic types in freshwater and its food quality for a benthic grazer. We hypothesized that different polymer types affect biofilm composition as well as the life parameters of its consumers. We incubated polyethylene (PE), polyethylene terephthalate (PET) and polystyrene (PS) as well as glass (control) in a productive freshwater creek for natural biofilm establishment. To account for changes in the environmental conditions, the experiment was conducted twice during winter and late spring, respectively. These biofilms were offered to the freshwater gastropod Physa fontinalis as sole food source. Growth and reproduction of the snails were measured to monitor sublethal effects. Additionally, biofilm composition was observed using confocal laser scanning microscopy (CLSM). In winter, snails feeding off PET and PE showed a significantly lower egg production and lower growth rates were observed on PET. No such effects occurred in spring. CLSM data revealed, that algal growth was significantly lower on PE and PET during the winter treatment compared to PS and glass. Since we could only find these effects during the colder and darker months (January-March), the microbial colonization on PE and PET was inhibited by the substrate under less favorable conditions of temperature and light. Hence, benign conditions may mask the adverse effects of microplastic on food webs. Our findings show that future studies on the plastisphere will need to consider such variations to further understand the influence of plastic pollution on primary production and higher trophic levels.

The importance of environmental factors and matrices in the adsorption, desorption, and toxicity of butyltins: a review

Butyltins (BTs) are considered as a group of the most important organometallic compounds in industry and agriculture. Due to their widespread use, large amounts of BTs including tributyltin (TBT), dibutyltin (DBT), and monobutyltin (MBT) have entered into the environment, and subsequently causing detrimental effects on humans and aquatic organisms. This work provides a critical review of recent studies on the adsorption, desorption, bioaccumulation, and toxicity of BTs that can notably influence the distribution of BTs in the environment. Influence of environmental factors (e.g., pH and salinity) and adsorbents in the matrices (e.g., minerals, organic carbons, and quartz) on the adsorption, desorption, and toxicity of BTs is particularly addressed.

Active and passive spatial avoidance by aquatic organisms from environmental stressors: A complementary perspective and a critical review

Spatial avoidance is a mechanism by which many organisms prevent their exposure to environmental stressors, namely chemical contaminants. Numerous studies on active avoidance and drift by aquatic organisms, as well as the main approaches used to measure both responses, were reviewed. We put forward a particular recommendation regarding methodological approaches: active avoidance should preferably be evaluated under a dilution gradient in a multi-compartmented system instead of in a bi-compartmented system. Available data on spatial avoidance from contamination indicate that emigration can occur at even lower contaminant concentrations than sub-individual noxious effects (assessed with the traditional forced-exposure assays), challenging the widely accepted paradigm in ecotoxicology that contaminant-driven adverse consequences at the population level result from a time delayed cascade of sequentially linked biochemical, cellular, physiological, and finally whole organism deleterious effects. Therefore, contaminants should not be viewed solely as potential toxicants at the individual level, but also as potential disturbers of habitats, by making the latter, at least partially, unsuited to accommodate life. Also, exposure to contamination is needed to trigger avoidance, but uptake is not mandatory, which demands the concept of exposure to be expanded, to include also the mere perception of the stressor. Since emigration eventually leads to local population extinction, and thus to severe implications for ecosystem structure and functioning, we then recommend that avoidance data be incorporated in environmental risk assessment schemes.

Patchy sediment contamination scenario and the habitat selection by an estuarine mudsnail

Since mudsnails are able to avoid contaminated sediment and that the contaminants in sediment are not uniformly distributed, the mudsnail Peringia ulvae was exposed to cadmium (Cd) spiked sediment and assessed for avoidance response in a heterogeneous contamination scenario. Four Cd concentrations were prepared and disposed in patches on dishes, which were divided in 25 fields (six fields for each sediment concentration); 24 organisms were deployed in the central field, with no sediment. Observations were made at 2, 4 and 6 h (corresponding to immediate response), 8, 10 and 12 h (very short term), and 24 h (short term). A trend to avoid contaminated patches was observed in the immediate and very short term. After 24 h exposure, the organisms exposed to the highest level of contamination seemed to have lost the ability to move and avoid contaminated patches. In a contamination scenario in which non- and contaminated sediment patches are heterogeneously distributed, local mudsnail populations can simply rearrange their locality without needing to move to a different habitat. Such less contaminated patches can become donor areas in a future recolonization scenario.

Adult exposure to the synthetic hormone 17α-ethynylestradiol affects offspring of the gastropods Nassarius burchardi and Nassarius jonasii

The aim of this study was to determine whether adult exposure to endocrine disrupting compounds affects offspring using trans-generational testing. Adult estuarine dwelling gastropods Nassarius burchardi and Nassarius jonasii were exposed to the synthetic estrogen 17α-ethynylestradiol (EE2) to determine the effects on the development and survival of their offspring. Adults were maintained in synthetic seawater controls and EE2 treatments (0.005, 0.05, 0.5, 50µg/L) over a sixteen week period. Egg capsules were collected from the adults following four, ten and sixteen weeks of adult exposure and transferred to different EE2 exposure scenarios. Treatment concentrations were selected to represent changes in EE2 exposure that could occur over different periods in an organism's lifecycle. Egg capsules laid by adults were therefore transferred to control or EE2 treatments (0.005, 0.05, 0.5, 5, 50, 500µg/L) to develop until hatching. The percentage of egg capsules with unviable eggs and abnormalities, number of days for hatching to occur and hatching success were measured. The veliger larvae that hatched from egg capsules following two, eight and fourteen weeks of adult exposure to EE2 and controls were used in 96h acute toxicity tests with controls and EE2 treatments at concentrations of 0.5, 5, 50, 500, 1250, 2500, 4000µg/L. Exposure of adult N. burchardi and N. jonasii to EE2 affected the percentage of egg capsules with unviable eggs, the development and hatching success of embryos and survival of veligers. These toxicity tests produced a complex set of results with different responses in developing eggs and veliger larvae to the adult EE2 treatments and length of adult exposure. This study demonstrates the importance of trans-generational testing and adult exposure scenarios in toxicity investigations.

Reproductive impacts of tributyltin (TBT) and triphenyltin (TPT) in the hermaphroditic freshwater gastropod Lymnaea stagnalis

Tributyltin (TBT) and triphenyltin (TPT) are emblematic endocrine disruptors, which have been mostly studied in gonochoric prosobranchs. Although both compounds can simultaneously occur in the environment, they have mainly been tested separately for their effects on snail reproduction. Because large discrepancies in experimental conditions occurred in these tests, the present study aimed to compare the relative toxicity of TBT and TPT under similar laboratory conditions in the range of 0 ng Sn/L to 600 ng Sn/L. Tests were performed on the simultaneous hermaphrodite Lymnaea stagnalis, a freshwater snail in which effects of TPT were unknown. Survival, shell length, and reproduction were monitored in a 21-d semistatic test. Frequency of abnormal eggs was assessed as an additional endpoint. Triphenyltin hampered survival while TBT did not. Major effects on shell solidity and reproduction were observed for both compounds, reproductive outputs being more severely hampered by TBT than by TPT. Considering the frequency of abnormal eggs allowed increasing test sensitivity, because snail responses to TBT could be detected at concentrations as low as 19 ng Sn/L. However, the putative mode of action of the 2 compounds could not be deduced from the structure of the molecules or from the response of apical endpoints. Sensitivity of L. stagnalis to TBT and TPT was compared with the sensitivity of prosobranch mollusks with different habitats and different reproductive strategies.

Development of an embryo toxicity test with the pond snail Lymnaea stagnalis using the model substance tributyltin and common solvents

The development of a chronic mollusc toxicity test is a current work item on the agenda of the OECD. The freshwater pond snail Lymnaea stagnalis is one of the candidate snail species for such a test. This paper presents a 21-day chronic toxicity test with L. stagnalis, focussing on embryonic development. Eggs were collected from freshly laid egg masses and exposed individually until hatching. The endpoints were hatching success and mean hatching time. Tributyltin (TBT), added as TBT-chloride, was chosen as model substance. The selected exposure concentrations ranged from 0.03 to 10 μg TBT/L (all as nominal values) and induced the full range of responses. The embryos were sensitive to TBT (the NOEC for mean hatching time was 0.03 μg TBT/L and the NOEC for hatching success was 0.1 μg TBT/L). In addition, data on maximum limit concentrations of seven common solvents, recommended in OECD aquatic toxicity testing guidelines, are presented. Among the results, further findings as average embryonic growth and mean hatching time of control groups are provided. In conclusion, the test presented here could easily be standardised and is considered useful as a potential trigger to judge if further studies, e.g. a (partial) life-cycle study with molluscs, should be conducted.

Measuring the avoidance behaviour shown by the snail Hydrobia ulvae exposed to sediment with a known contamination gradient

This study tested the hypothesis that the snail Hydrobia ulvae will recognise differences in the contamination levels of sediment and avoid the more contaminated zones. Instead of testing avoidance behaviour in a two-compartment system, a linear contamination gradient has been devised in which several zones containing sediment with different contamination levels could be "chosen". A sediment of known severe contamination was collected and mixed with a non-toxic sediment to comprise a linear gradient with four concentrations of 0, 30, 60 and 100% contamination. Thus, the individuals were free to move between fields of different concentration (unforced conditions). During 24 h of exposure, the distribution of the organisms on fields was recorded and organisms' percentage that avoided each field was calculated for each concentration. At 30 and 60% of contamination, avoidance was around 50% for all periods of exposure. Avoidance at 100% contamination ranged between 11 and 56% (rates lower than expected). Organisms that did not avoid the 100% contamination revealed that they were inactive, indicating either that non-mobility and retraction within their shell may be alternative defence strategies, or else the high contamination may have impaired their ability to escape. The percentage of preference shown to the uncontaminated sediment ranged between 64 and 74%. The initial hypothesis was supported: H. ulvae avoids the more contaminated sediment. The assay proposed has been shown to be simple, rapid and effective; in addition, it is considered ecologically useful since the effects resulting from the avoidance are similar to the extinction of the population.

Effects of the organophosphate insecticide azinphos-methyl on the reproduction and cholinesterase activity of Biomphalaria glabrata

Azinphos-methyl is an organophosphate insecticide used for pest control on a number of food crops in many parts of the world. The snail Biomphalaria glabrata is a freshwater gastropod widely distributed in South America, Central America and Africa. The aim of the present work was to investigate whether azinphos-methyl causes alterations in the reproduction of B. glabrata. To this end, gastropod pigmented specimens were exposed to various concentrations of the insecticide (0.021, 0.5, 2.5, and 5 mg L(-1)) for either 2 or 14 d. Along 14 d, several reproduction parameters and cholinesterase (ChE) activity were evaluated. In each group, the number of egg masses, the number of eggs per mass, the number of hatchings, the time to hatching, and the survival of the offspring after one month of treatment was evaluated. The results showed that, depending on the concentration and time of exposure, azinphos-methyl induced alterations in the reproduction of B. glabrata. These alterations were mainly represented by a decrease in the number of egg masses, and, in some cases, by a lower number or even the total absence of hatchings. Thus, the gastropods exposed to 2.5 and 5 mg L(-1) of azinphos-methyl for 14 d showed ChE inhibitions higher than 35% along time and completely lost their ability to reproduce. On the other hand, exposure to high acute concentrations or exposure to low concentrations for 14 d resulted in ChE inhibition equal to or lower than 35% between 7 and 14 d of treatment and similar alterations in reproduction. These were represented by a decrease in the number of egg masses. At low pestice levels, the number of egg masses and the number of offspring resulted to be more sensitive biomarkers than ChE inhibition. It is concluded that the insecticide azinphos-methyl can cause a decline in the reproductive performance of B. glabrata.

The unexpected sources of organotin contamination in aquatic toxicological laboratory studies

Unaccounted sources of contamination can be problematic in toxicological studies and can range from the presence of impurities, breakdown products or isoforms of the parent compound to the unexpected compounds leaching from dosing apparatus. As these compounds are not being tested, they may not be measured in the dosed aquaria and hence go undetected, potentially contributing as confounding factors in toxicological assessments. In this paper we report the unexpected detection of butyltin compounds (mono, di and tributyltin) in flow-through aquaria waters of an aquatic toxicological set-up. High and variable leaching rates for dibutyltin of 2.0 and 6.6 microg/h were detected during the first week of each of two separate flow-through studies. Following this initial 'surge' of dibutyltin leachate, a decrease in leachate rate was seen with values of 0.9 and 1.2 microg/h by Day 14 (second week of study). The main source of the butyltin leachates was shown, to be the airline tubing used in the assembly of the air-supply into each flow-through tank. A 24h period of incubation of the airline tubing with clean water led to the leaching of concentrations of 63.8 ng/l TBT-Sn, 1638.8 ng/l DBT-Sn and 4054.6 ng/l MBT-Sn. The concentration of tributyltin detected was within its toxicologically effective range and as such could have potentially confounding effects on the toxicological bioassays being used. These accidental findings could be of enormous relevance to aquatic toxicologists and have an important bearing on the choice of materials used to construct experimental exposure aquaria.

Suitability of the marine prosobranch snail Hydrobia ulvae for sediment toxicity assessment: A case study with the anionic surfactant linear alkylbenzene sulphonate (LAS)

Individuals of the mudsnail Hydrobia ulvae (Pennant) (Mollusca: Prosobranchia) were exposed to sediments spiked with increasing concentrations (1.59-123.13mgkg(-1) dry weight) of the anionic surfactant linear alkylbenzene sulphonate (LAS) which is employed in the formulation of laundry powders and liquids, as well as hand dishwashing products. The suitability of the selected organism, H. ulvae for routine sediment toxicity testing was evaluated by measuring acute toxicity recording survival. Sublethal toxicity was evaluated as total number of produced veliger larvae per treatment throughout the test (9d). Mortality has shown to be a reliable and reproducible indicator of acute toxicity. LC(50) values were comprised between 203.4 (48h) and 94.3mgkg(-1) (9d) dry weight. As sublethal endpoint, the total number of produced larvae showed to be a useful indicator of toxicity for this organism. The number of produced larvae increased at lower exposure concentrations, whereas at the highest LAS concentration, the number of produced larvae decreased. This is the first report of acute and sublethal toxicity of sediment associated LAS for this species.

Endocrine disruption in aquatic pulmonate molluscs: few evidences, many challenges

As compared to other groups of aquatic gastropods, documented examples of endocrine disruption in pulmonates are rather limited. This is quite surprising because the endocrine control of physiological functions has been extensively studied in these animals. In the model-species Lymnaea stagnalis, the neurohormonal regulation of reproduction has been thoroughly investigated, and the primary structure of several peptides and receptors involved in endocrine processes has been established. However, the use of this knowledge has been fairly limited in the context of ecotoxicology, to investigate the effects of endocrine-disrupting chemicals. The present review summarizes the main and more recent findings on the neuroendocrine control of reproduction in aquatic pulmonate snails (Basommatophora). It then comprehensively describes selected in vivo laboratory and semi-field studies which provide evidence for possible endocrine disrupting effects of estrogenic and androgenic test compounds [e.g., ethynylestradiol, methyltestosterone (MT)], and of environmental contaminants [e.g., cadmium (Cd), tributyltin (TBT), and nonylphenol (NP), pesticides]. Finally, challenging perspectives for future research are discussed.

Chronic toxicity of tributyltin to development and reproduction of the European freshwater snail Lymnaea stagnalis (L.)

Chronic toxicity, growth and reproduction were measured in the freshwater gastropod Lymnaea stagnalis exposed to waterborne bis(tri-n-butyltin) oxide (TBTO) over a range of four nominal concentrations (0-10microg TBTl(-1)). Egg development was completely inhibited at 10microg TBTl(-1), whilst abnormal embryonic development was observed at 1microg TBTl(-1). For the solvent control and the 0.01microg TBTl(-1)treatment group, normal development of L. stagnalis was observed. Survivorship of hatchlings was significantly reduced by TBT at 1microgl(-1) while inhibition of shell growth of L. stagnalis was also observed at this concentration. The data were used to determine intrinsic growth rates (r) using two theoretical approaches (the Euler-Lotka equation and a Leslie Matrix). Both approaches showed that survival, fecundity and population growth rate were reduced at 1microg TBTl(-1). Interestingly, at 0.01microg TBTl(-1) snails showed a higher fecundity and growth rate than in the solvent control. The TBT concentration at which the r would equal zero (ECr(0)) and the population NOEC (No Observed Effect Concentration) were estimated. The population NOEC was defined as either the lower 95% confidence or lower 95% pointwise percentile limit of the ECr(0). Values obtained using the two different approaches were similar and thus a geometric mean was calculated to obtain a final representative population NOEC value for L. stagnalis of 2745ng TBTl(-1). The present data together with chronic toxicity TBT data for freshwater organisms, obtained from peer-reviewed literature, were used to construct a species sensitivity distribution (SSD). A predicted no effect concentration was then derived from the SSD (hazardous concentration at 5%, i.e., HC5 or 95% protection level). This SSD was compared with the SSD derived from saltwater species datasets. The HC5 value for saltwater species (3.55ng TBTl(-1); lower confidence limit: 1.93ng TBTl(-1)) was significantly lower than that for freshwater species (30.13ng TBTl(-1); lower confidence limit: 9.23ng TBTl(-1)), indicating that saltwater species are probably more susceptible to TBT than their freshwater counterparts.