What in the 1950s seemed like a solution in the naval industry to prevent biofouling or the accumulation of microorganisms, algae or bivalves in structures and materials that were continuously submerged in the sea, would be confirmed years later as a serious problem for many marine organisms to the point that international authorities banned its use. Tributyltin, TBT, is a biocidal compound that began to be used as an active ingredient in anti-fouling paints in order to prevent the growth of organisms, however, over the years and its worldwide diffusion, it was confirmed that it was not discriminatory and was highly toxic to some organisms. The abnormal thickening detected in oyster shells in Arcachon (France) was added in the 1970s in England and the United States by the observation of the presence of a penis in females of the gastropods Nucella lapillus and Nassarius obsoletus. This phenomenon, called imposex, consists of the imposition of male sexual characteristics, such as the penis or the vas deferens in females of some species of gastropods. Imposex was not linked to TBT pollution until 1981, when it was observed that levels of this abnormality were higher near ports. Successive European directives and the International Convention for the Control of Harmful Antifouling Paints, held in 2008, led to a worldwide ban on the use of TBT on all ships.
“In the most severe states of imposex, females can be sterile, which leads to a decrease or extinction of the population. Currently, there are records of this effect in more than 260 gastropods,” explains Filipe Laranjeiro, a researcher at the University of Aveiro and the Ecotox Group at the University of Vigo. The researcher points out that the fact that imposex is a highly specific response to TBT makes it one of the best biomarkers for environmental monitoring and an effective and low-cost tool for detecting contamination by this compound in the aquatic environment. Given the great utility of this biomarker, organizations such as Ospar or Helcom consider imposex a mandatory element in their environmental monitoring programs.
Imposex in Galicia and a case study in the port of Vigo
Studies carried out in the 1990s in the Galician estuaries confirmed that those of Ferrol, Coruña, Arousa and Vigo were the areas most contaminated by TBT, due to the fishing ports and the merchandise they house. Later work revealed that the application of the European directives on the prohibition of the use of tributyltin were effective, registering a decrease in the level of this compound in the environment, something that has also been observed in other European countries, although in specific cases populations with high levels of imposex continue to be recorded.
The Ecotox group of the University of Vigo, under the auspices of the Sediport project, funded by the Ministry of Science and Innovation, carried out several studies in the port area of Vigo. In a recently published article they demonstrate the advantages of using the gastropod species Nassarius reticulatus in a bioassay carried out for the detection of TBT contamination in sediments. To carry out the bioassay, individuals of this species were collected at a control station previously defined for their low imposex levels, and subsequently exposed to the sediments to be tested for 28 days. “The results showed that in three of the six sediments collected in 2010 in different areas of the port of Vigo there was a statistically significant increase in imposex levels. This was indicative of the presence of bioavailable TBT in them, thus posing a potential risk to both gastropods and other organisms due to its high toxicity,” explains the Ecotox researcher.
The value of bioassay with sediments
On the other hand, monitoring of N. reticulatus populations in the areas where the sediments were collected showed that between 2008 and 2013 imposex levels had decreased only in the areas containing the sediments that had not caused any effect in the 2010 bioassay. It was also confirmed that imposex levels had remained considerably high during that period, in one of the areas whose sediment had induced an increase in imposex levels in 2010. “All this shows that the sediment bioassay is a low-cost tool that can detect TBT contamination in areas where there are no native gastropod populations and without the need for expensive chemical analyses. This bioassay can also identify contaminated areas early and anticipate future imposex levels or, conversely, the recovery of imposex levels in populations,” says the Portuguese researcher. The results of this work allowed the design of a protocol that was included in the Manual of Basic Methods in Aquatic Pollution, published by Ecotox.
Filipe Laranjeiro warns that, although in Galicia the natural trend is the decrease in imposex levels in areas that include shipyards, fishing ports and merchandise, TBT pollution can continue to be detected because its degradation rate in sediments is low.
Next conference at ECIMAT
Next Wednesday the 15th, Filipe Laranjeiro will be the guest of the Ecimat’s “Coffee with salt” scientific presentation series, in which he will present the paper entitled “Sex changes on the beach: endocrine disruption in gastropods”.
For more information visit our blog
