Presenting a retrospective analysis in an ecosystem context, Kris Cooreman, ILVO, Belgium and member of the Working Group on Biological Effects of Contaminants (WGBEC) and Working Group on Marine Sediments in Relation to Pollution (WGMS) takes a look at how tributyltin (TBT) has impacted Crangon crangon's life history.
The application of the extremely toxic broad-spectrum organotin biocide tributyltin (TBT) as antifouling agent since the mid-1900s was an economic success story until, at the end of the 1970s, fertility and calcification impairment in oysters and induction of imposex and intersex in female gastropod snails linked TBT to the decline and local extinction of mollusk species, providing the strongest evidence on the occurrence of endocrine disruption in the marine environment.
The economic damage to shellfish industries compelled countries to implement measures to assure the sustainable management of these resources. France was the first country to respond, issuing a ban in 1982 on the application of organotin-based anti-fouling paints on the waterline of ships less than 25 metres in length and fish farms. Between 1987 and 1991, other countries throughout the North Sea followed suit with similar bans. In 1989, the European Union (EU) imposed measures on all Member States and the IMO acted accordingly on a global scale in 1990. In 2001, the IMO adopted the International Convention on the Control of Harmful Antifouling Systems, formalizing a global ban on the application of organotin antifouling agents on marine vessels after the deadline of 17 September 2008. The EU transposed the 2001 Antifouling Systems Convention into a Regulation which banned the application of TBT on EU-flagged vessels from 1 January 2003. That regulation further obliged all ships visiting EU ports from 1 January 2008 onwards to be free of TBT or at least to bear a barrier coating.
Beyond mollusks
In contrast to the vast scientific datasets and assessments on endocrine TBT disruption in mollusks at population-relevant endpoints up to extinction, metabolic to apical effects on other taxa were seldom identified until the last two decades. In 2004, the outcome of the EU project Sources, Consumer Exposure and Risks to Organotins accumulated in seafood (OT-SAFE) reported alarming TBT body burdens in the economically and ecologically important Crangon crangon (brown shrimp). The project raised serious concerns about the environmental risks related to the transfer of accumulated TBT in C. crangon to the human food chain, the health of the large southern North Sea population (its main habitat), as well as the entire marine environment.
The latter concern resulted in a research initiative on the impact of TBT on the welfare of C. crangon in the marine environment. The initial research focused on agonistic interference of TBT at the metabolic pathways regulating growth and reproduction of C. crangon. Metabolic, topical and population-relevant biological endpoints in C. crangon and other crustaceans have been evaluated in relation to the temporal and spatial trends on TBT's occurrence and distribution in the field during and after the introduction of the TBT restrictions and endocrine-related incidents. Arguments have been forwarded to relate the German Bight incident on growth and reproduction failure in the C. crangon population to the pernicious impact of TBT in 1990/91 and previous years, despite the lack of direct evidence at that time. The extreme occurrence of TBT in C. crangon from other parts of the southern North Sea in the following years and evidence on the high body burdens as dose metrics of exposure also feeds the suspicion on detrimental TBT impacts in those areas. This research further demonstrated the complexity of the individual roles of unrelated stressors on a population in an integrated evaluation of the health of the marine environment at the ecosystem level.
TBT levels in the marine environment
It is undeniable that several routes of bioaccumulation generated high TBT body burdens in species from different taxa in TBT-charged marine areas in the past. However, the understanding of the severity of the status of the health of the marine environment has been crossed by knowledge on species-specific TBT catabolism. The high distribution ratios of the degradation products dibutyltin and monobutyltin to TBT in several species pointed to enhanced TBT-metabolizing efficiency thereby complicating the interpretation of body burden data in those metabolizing species while this was not the case in other species. In C. crangon, the unquantifiable dibutyltin and monobutyltin levels were a sign of minimal or no catabolism making the measured high TBT levels the actual body burdens.
In contrast to the common scientific opinion that TBT would have a long-lasting presence in the marine environment, following the local TBT restrictions and the global ban, sufficiently high elimination rates have drastically reduced the TBT levels in matrices such as biota and (aerated) sediment in a time span of a few years and have led to a large-scale progressive recovery of the marine ecosystems, albeit at differing timescales. The differing recovery time is not surprising as the restrictions on small ships in 1989 had a rapid positive local impact in predominantly recreational, production, and small ships areas.
The biogeochemistry, distribution, and bioavailability of TBT explained
Distribution, bioavailability, and uptake of this ionizable chemical is a complex process steered by several surrounding and confounding parameters in different matrices. That makes TBT in biogeochemical and biotical conditions very different from non-ionic hydrophobic compounds.
The scientific debate on the biogeochemical behavior of TBT in the marine environment did not result in a common understanding of the biogeochemical occurrence, distribution, or uptake process in biota. The predominant occurrence in coastal conditions at pH 8 of the so-called neutral ( ̴hydrophobic) TBT forms, in marine systems was an important indication to link the partition behaviour of TBT to that observed for neutral hydrophobic substances and biogeochemical predictions based on the octanol-water equilibrium partitioning coefficient Kow.
However, partitioning equilibrium values such as Kow are not very suitable to predict bioaccumulation of TBT since its derivatives are strongly pH-regulated. The sorption of the majority of the neutral TBT forms to the organic carbon in sediments from coastal waters at average pH 8 might seem the principal process. However, a second seemingly less significant and underestimated process is the moored stable metal-type fixation of the remaining approximately 3% TBT cations to electronegative ligands in the sediment, thereby creating a quasi-continuous disequilibrium in the lipophilic partitioning of the neutral forms forcing into the direction of metal-type fixation on available ligands and steady state conditions. It is just this metal-type fixation that explains the accumulation in biota. But there is more. In biota, the metal-type fixation, accumulation, distribution and bioavailability to cellular processes of bio-accumulating TBT may substantially be accelerated by the acidic intracellular pH conditions (pHi). The existence of intracellular compartments with acidic pH values, far below the pH 8 in the surroundings in coastal waters, was already demonstrated in the 1970s. The compartmentalized nature of the pHi and the predominant acidic conditions in the cells provide new insights in the process of distribution and behavior of ionizable toxins. In the case of TBT, acidic pHi values close to the acid-dissociation constant pKa value of TBT maximize the ratio of the cation, compared to the 'neutral' forms and increase the proportional interactions of the metal-type behavior of TBT by complex formation with ligands in phospholipids, proteins and more. In other words, the neutral forms appear to partition only to the lipid fraction of the biomembrane making this form of intracellular uptake minor to the process of the metal-type uptake of TBT anions in cells. The pH and electrochemical equilibrium of the extracellular fluid between cells and the cytoplasmic conditions is an additional argument on the tandem of enhanced metal-type uptake. Conclusively, it is argued that the metal-type fixation in the acidic conditions in biota occurs at a higher ionic activity compared to the metal-type process in sediments at higher pH.
Mode of action of TBT in crustaceans
Our current understanding of the principal endocrine mechanism of action of TBT in crustaceans is very similar to that in mollusks and refers to the interactions at the physical, functional, and gene-expression levels of the nuclear receptors regulating growth and reproduction. This is not exceptional since these nuclear receptors are highly conserved throughout the animal kingdom and mankind. It is very likely that TBT's mechanism of action is very similar in all taxa.
Hormones trigger the specific regulation of several of the growth and reproduction genes in different tissues and developmental stages of crustaceans. TBT interferes agonistically with the nuclear receptors in competition with the natural hormone triggers of these receptors. Similarly, TBT-mediated endocrine disruption of analogous receptors in sensitive gastropods has led to the decline and local extinction of mollusk species. It is the ligand-dependent nature that makes many receptors susceptible for exogenous chemicals at extremely low exposure doses. TBT causes a strong in vivo down-regulation of growth and reproduction genes, especially in the ovaries of C. crangon. The ovary-specific inhibition is an initial indication of evolving reproductive impairment, in a way similar to the impact on mollusks.
TBT toxicity and topical and population-relevant endpoints
In the 1980s and 1990s, data on the status of TBT in the marine environment were commonly addressed via dose–response relationships in toxicity tests in TBT-exposed water pathways. These assays brought imposex and intersex prevalence in mollusks to the forefront as key indicators in monitoring programs in times of challenging developments of specific and sensitive analytical techniques to measure TBT as a residue. In laboratory tests on crustaceans, TBT-induced macroscopic changes on the growth and reproduction morphology have been reported for population-relevant endpoints, such as intersex, fecundity, percentage of ovigerous females, reproduction, and larval development.
The development of Species Sensitive Distribution models (SSD models), based on the lowest No Observed Effect Concentration and Lowest Observed Effect Concentration (NOEC and LOEC data) from water-exposure pathways, have shown that many aquatic species from different taxa, including crustaceans, are in the same range of sensitivity as mollusks with some fish species showing higher sensitivity. The accumulation of TBT in organisms in most TBT-charged areas in the past were therefore undeniably transcending the vulnerability of crustaceans and species from other taxa for population relevant endpoints.
Knowledge gaps
A critical gap in toxicological research on TBT exposure is the need for information on the impact of the TBT body burdens on biological processes in affected organisms with the aim to more realistically assess the overall TBT impact in the field. It was postulated that the tissue residues reflect the bioavailability and effective target doses more accurately than the toxicity based on the water-exposure pathway. Several studies have found that when TBT toxicity is expressed as a tissue residue, the variability between species, time periods, and exposure conditions is greatly reduced. Unequal tissue distribution and TBT behavior may additionally influence the effective target dose to responsive tissue-specific biological endpoints.
The German Bight incident
The adverse impacts on mollusks–female sterility associated with
imposex and intersex–were attributed to TBT, and recovered following the TBT restrictions. In contrast, the effect of TBT-mediated deregulation of the
C. crangon population remained unclear and hypothetical since this species was commonly regarded as non-endangered for more than half a century.
The most striking potential link between TBT and population-relevant endpoints in C. crangon may have happened before the TBT bans were introduced. Research on the seasonal spawning cycle and reproductive success in areas of the German Bight in the North Sea in the period 1958–2005 linked reproductive impairment to low percentages of ovigerous females on the basis of morphological information. A decrease in the proportion of ovigerous females began in the western part of the German Bight in the second half of the 1970s, where it dropped below 50% of the previous status. The absolute minimum was observed in the late 1980s with ovigerous female proportions below 10%.
The subsequent low reproduction and recruitment caused dramatic drops in landings of shrimp for consumption since the catches in the German Bight accounted for approximately 90% of the total European catch. Correlation analyses with common parameters, such as water temperature, river runoff, North Atlantic Oscillation climate index, and the more obvious indicators–predators and fishing mortality–did not show any plausible proximate cause for this large-scale population impact.
It was our former prominent ICES colleague Volkert Dethlefsen, Germany, and co-authors who reported on the German Bight incident already in 1983. In their research, the authors postulated pollution-induced "dissolutions" of the shell and subsequent secondary infections. Unfortunately, a lack on data from other areas prevented larger geographical-scale comparisons and TBT-induced biological endpoints on growth and reproduction were at that time only extensively described in mollusks from TBT-charged areas. The nature of the observed disorders, low percentage ovigerous females, bad recruitment, and shell disorders in the German Bight surveys were later diagnosed in conventional full life-cycle exposure tests on several crustacean species. Now we know that the described dissolutions of the shell may in turn refer to ecdysis and vitellogenin disruption in the process of the endocrine toxicity on expression of cuticular proteins and eventually vitellogenin.
Another indication of the putative deterioration by TBT is the stock rebound in the German Bight over the course of the 1990s, shortly after the European 1989 TBT ban was introduced. A rapid recovery was remarkable in yachting areas while industrialized maritime areas showed delays until the implementation of the global TBT ban.
TBT: a putative bottom-up regulator of C. crangon's population?
An increased abundance of predators is, in the event of the German Bight incident and the disastrous 1990/91 breakdown of the C. crangon catches, not a justifiable argument for the population-level occurrence of overall growth, reproduction, and recruitment failure. The German Bight incident is, in our opinion, fully attributable to endocrine disruption by TBT, with predation mortality at that time perhaps playing a secondary impact on the stock.
A different scenario has manifested post-2000 until the present day, complicating the distinction between the proportional influences of the different established population drivers. The gradual increase in landings of shrimp for consumption peaked at 37 000t in 2014. The clear relationship between the stepwise temporal and spatial rebound of the stock, the implementation of the 1989 partial ban on TBT, the global ban in 2008, and the fading out of TBT prevalence to threshold values may have restored sustainable shrimp fisheries, making other drivers–predation, fishing effort, and climate change– more visibly accountable.
So far, the fluctuating influences of predation mortality on C. crangon's population have interfered with a putative impact of TBT preventing a clear signal on a population relevant endpoint. The elimination of TBT from the marine environment has ensured that it's putative role on the health of the population has, in the meantime, been minimized. Recently, another stressor, fishing effort, previously not regarded as a potential threat, has reached the highest efforts since 2013. An uncontrolled increase in effort resulted in overfishing and the need for a management plan to restore sustainable shrimp fisheries. 2015 was the turning point, with much lower commercial landings again in 2016–the lowest level since 1995 (ICES advice in 2016 and 2017).
The suspected reason for overfishing can be found in the high number of undersized shrimp in current catches. These undersized shrimp are discarded less nowadays causing recruitment overfishing despite a survival range of discards between 75 and 80%. The impact of this change in fisheries attitude is huge.
Final words
For a long time, (sub-)chronic TBT toxicity in crustaceans was not an issue of particular interest because of the initial toxicity to mollusks, as well as C. crangon's non-endangered status. Despite the lack of complete life-cycle toxicity information on topical endpoints in C. crangon, the available information on biological endpoints in other crustacean species, delivers sufficient arguments to strengthen the hypothesis on population-relevant endocrine disruption to clarify the German bight incident before and during the 1990/91 debacle.
The high TBT accumulation in TBT-charged areas, especially in organisms lacking TBT catabolism, suggests that biomagnification played a more distinct role than could be expected from bioconcentration. The extremely high body residues of TBT in the past and our model on enhanced intracellular partitioning allow to suggest that the TBT toxicity in the marine environment has been underestimated, compared to the water exposure pathway toxicity theory.
The question of whether TBT prevalence was an important indicator of C. crangon population health became old news as a result of the TBT bans. However, it remains a scientific necessity and societal obligation to communicate about the former threats to the population and ecosystem level through the use of this chemical from an environmental and economic perspective.
Read details and references in the open access paper Tributyltin: A Bottom–Up Regulator of the Crangon crangon Population? online.
