TBT and Imposex
This article describes the use of Tributyltin (TBT) in aquatic antifouling paints, its behaviour in the marine environment and one of its powerful negative effects in non-target species - the phenomenon of imposex in marine gastropods - which have led to the partial ban of this compound.
- 1 Introduction
- 2 Why the need of antifoulings?
- 3 Effects in non-target species
- 4 Monitoring of TBT contamination
- 5 Restrictions to TBT
- 6 See also
- 7 References
Tributyltin (TBT) is a biocide compound which integrates certain antifouling paints used on the hulls of vessels to prevent biological fouling - a phenomenon which has considerable economic costs and environmental risks. Although very efficient, TBT has been subject to restrictions due to its toxic effects in non-target species, detected at the end of the 1970s. One of this harmful effects is imposex – the masculinisation of females of certain marine snails in response of the exposure to TBT concentration, in the magnitude of ng.l-1. So far this phenomenon has been described for over 150 species. The sensitiveness and high correlation between the intensity of this phenomenon and the environmental concentrations of TBT allow the use of certain gastropod species as indicators of the degree of contamination in coastal zones. Though the use of TBT has been forbidden in many countries for vessels smaller than 25 m, the contamination levels are still a concern, particularly close to areas of intense boating and associated activities, such as fishing and commercial ports, marinas and dry-docks.
Why the need of antifoulings?
The Problem of Fouling in Vessels
Any submersed rigid structure can work as substrate and be colonized by several marine organisms. It is estimated that there are over 4000 marine fouling species . In the case of vessels, the degree of fouling of the hull depends on the time of submersion, the time the vessel is immobilized or its speed, but mainly on the features of the marine environment. Without an antifouling protection, the fouling can reach 150 kg per square meter, in less than 6 months . This phenomenon leads to an increase in the weight of the vessel and the drag resistance of the hull surface, which directly affect the speed, manoverability and the fuel consumption (increasing up to 40%), leading to more frequent maintenance operations, higher costs and higher emissions of polluting gases . Additionally, the hulls can work as vectors of translocation of organisms from one place to another, increasing the risks of introducing non-native, invasive species .
Antifouling methods and TBT
The problem of fouling in vessels was recognised since the beginning of navigation. The ancient Phoenicians and Carthaginians were thought to have used copper sheathing and the Greeks and Romans both used lead sheathing on their ships’ hulls . More recent methods included the use of paints containing organic compounds of lead, arsenic, mercury and halogens (e.g. DDT) and copper oxide . The later is still widely used. The first antifouling paints using organic compounds of tin started appearing in the second half of the 20th century and quickly dominated the markets during the following decades. Even today, TBT is globaly considered as the most effective solution developed so far to prevent fouling.
Sources and behaviour of TBT in aquatic systems
Antifouling systems represent the biggest and direct source of this pollutant. A TBT-based paint can be composed up to 3% of tin and a large commercial vessel can release more than 200g of TBT to the aquatic environment in only 3 days of permanence in a port . Aditionally, dry-docks and boatyards can also be relevant sources of antifouling paints (and other pollutants), where old paint removal and repaint procedures take place. Most of the residues end up in the surrounding environment.
When released into the water TBT can be degradated into less harmful forms by microrganisms and ultra-violet radiation. However, due to its high affinity to particles it will be easily transported to the sediments, where its concentration is typically higher than in the water. Here, organotin compounds are exceptionally stable and the concentration can remain high for a long time even after the sources have ceased . In the water, TBT can remain for a few days or months but in the sediments its half-life can extend for several months, years or even decades .
Effects in non-target species
The case of the Bay of Arcachon (France)
During the period when TBT was being widely used as antifouling, the production of oisters in the Bay of Arcachon (France) almost collapsed. This coastal area is sumultaneasly a place of production of this shellfish and an area of intense recreative boating . Although the knowledge of TBT was very limited at the time, the French Authorities restricted the use of the compound in antifouling paints in the region, in a rare example of precautionary principle . Later on, it became clear that TBT was responsible for the failures in the reproduction and abnormal shell development of the oisters.
Imposex in marine snails
Also in the beginning of the 70’s certain reproductive abnormalities in other molluscs were discovered, which were later proved to result from exposure to TBT. In certain species of gastropods with separate genders, the females presented a penis and/or vas deferens. The term “imposex” was given as “a superimposition of male features in females” and was first described in dog whelk (Nucella lapillus) . Soon it was clear that this was a generalised phenomenon – not only all the populations of dog whelk analysed in southwest England were affected but worldwide the same phenomenon was reported and for different species of snails, particularly in areas of intense maritime traffic. So far, imposex and intersex (a similiar phenomenon) have been described in over 150 species of marine snails . More developed stages of imposex can lead to the sterilization and premature death of the females, affecting the entire population. However, the most dramatic aspect of this endocrine disruptor is the fact that TBT can act at extremely low concentrations: a few nanograms per litre is enough to trigger imposex in marine snails - the equivalent of 1 g of salt dissolved in a square pool of 100 m side and 100m depth! These are so low that they are almost undetectable.
Effects in other species
The knowledge of TBT, its toxicity and risks to non-target organisms, including humans, is still limited. However, studies suggest several harmful effects on the imune and neurological systems and embrios in mammals  and described toxicity to plankton, algaes, fish and marine birds . It is known that top predators from marine ecosystems can accumulate significant amounts of pollutents. TBT is not an exception and has been already detected in cetaceans and seals, sharks and tunas .
Monitoring of TBT contamination
Imposex as an indicator of TBT contamination
Some species of snails have been used as bio-indicators to evaluate and compare the degree of TBT contamination in aquatic environments. They are suitable species since:
- the stage of imposex reflects the amount of TBT present in the tissues of the organism and in the surrounding environment 
- the imposex is triggered by extremelly low concentrations – close to the level of detection of measuring instruments
- marine snails can be very commun in certain habitats and have restricted mobility
Restrictions to TBT
Since 1988 the International Maritime Organization (IMO), through the Marine Environment Protection Committee (MEPC), has recognised the harmful effects of the antifouling systems, particularly TBT. In 1990 the MEPC recommended the IMO Member States to restrict the usage of TBT in boats smaller than 25m (as the recreation boating was considered to be the main direct input) and to establish maximum release rates for the antifouling paints. As the evidences of the negative impacts and toxicity of TBT increased, IMO adopted the International Convention on the Control of Harmful Anti-fouling Systems on Ships with the intention to globally ban TBT, starting in 2008. The ratification of this proposal was slow and though the number of joining countries has increased, the goals haven't been met. France, in 1982, was the first country to forbid the use of TBT in boats smaller that 25m, followed by the UK in 1987. The rest of the EU gradually joined the action. Japan has banned the organic compounds of tin from antifouling paints in 1990 and has called for a global ban. Other countries such as Switzerland, Austria and New Zealand voluntarily followed the IMO recomendation. Most developed countries have adopted legislation restricting the use of TBT and alternative methods are being used and developed.
- TBT and intersex in periwinkles
- Coastal pollution and impacts
- Endocrine disrupting compounds in the coastal environment
- Pollution laws and regulations
- Differentiation of major algal groups by optical absorption signatures
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