Females develope into males!

In the North Sea during 1991 imposex was discovered for the time, females developing male characteristics such as a penis and sperm canals. This phenomenon is often found in areas with intensive shipping and its intensity and occurence appears to increase with the intensity of the shipping. The reason for this is the very poisonous anti-fouling tributyltin (TBT) that leaks from the hulls of the passing boats. When this TBT-based paint was introduced for about 25 years ago, no symptoms of imposex had been observed. TBT is added to the anti-fouling to counteract Marine growth on boat hulls, but unfortunately is also very poisonous for many other organisms. Because of this, restrictions in the use of TBT have been introduced and in many countries TBT-based anti-fouling can only be used on boats under 25 metres. These restrictions have resulted in a decrease in the amount of TBT found in the water, on the sea bottom and in whelks, but TBT is still supplied to the water when old anti-fouling is removed and with seepage from bottom sediments. To further decrease the presence of TBT in the Marine environment, steps need to taken to stop the use of TBT-based anti-fouling on all ships. Another source of TBT contamination is the dumping of TBT polluted dredged masses. Areas that are close to dumping sites and shipping lanes can recieve increased doses of TBT from both sources.
   TBT causes the collection of male hormones which results in females becaming sterile and possibly death. It is expected that populations with a high degree of imposex are dominated by older males. Injury caused by the presence of TBT has been proven in over 100 specie of snails in different parts of the world. It is also probable that other groups of organisms are effected and result in TBT being carried further up the food chain.

TBT - not the only cause?

To study the association between TBT and imposex, whelks have under Laboratory conditions been subjected to varying quantities of TBT. Young snails developed imposex that was dose related, that is, low concentrations of TBT - few snails showed signs of imposex, while with high concentrations - many snails showed signs of imposex. Older snails, although showing a high TBT content in their body had no signs of imposex, therefore we can deduce that older snails are not as sensitive to TBT. Imposex has even been observed in areas where TBT has not been noted in the water. Penis development is probably in this case a result of TBT being stored in the sediment or their food source, but no sedimentary analysis has been conducted. The occurence of TBT is not necessarily the only reason why whelks develope imposex, other reasons can be compounds of tin, etanol, coppar or environmentally related stress.

Only mild symptoms in Sweden

In Sweden, imposex has been found in 23-50 % of the females depending on where the surveys were conducted, only lesser developed symptoms of imposex have been observed, that is a hint of a penis. Imposex has not advanced so far so the whelks have became sterile and reproduction has been effected.
   At the Tjärnö Marine Biological Laboratory, intensive studies are in progress to find an alternative and environmentally sound anti-fouling paint for ships hulls. If this research is successful, then we really have performed a good deed for both the Marine environment and its organisms.

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