Cellular and physiological effects of TBT antifouling agent on the clam Venus verrucosa

Abstract

Tributyltin (TBT) is used as an antifouling agent on the hulls of aquatic vessels. As a result of this practice, some of this TBT ends up in the environment where it was shown to cause unacceptable harm especially to sensitive stages of aquatic organisms particularly molluscan species. In Malta, elevated levels of TBT were found in Marsamxett and the Grand Harbour in the surface microlayer water column, sediments and also in biota collected from this area. One inhabitant of these Harbours, which is likely to suffer from this pollution. is the clam Venus verrucosa. The present work evaluates the biological impact of pollution by local environmental range levels of TBT. on the clam Venus verrucosa. This was carried out by exposing clams to different TBT concentrations and then analysing for any effects at the cellular and physiological level. Cellular responses were carried out by performing two types of investigations: cytological and cytochemical, at TBT concentration of 10, 100 and 1000 ng/l. Cytological investigations were done by preparing histological sections and examining them microscopically. The results showed that TBT causes cellular aberrations even at a nominal concentration of 10 ng/l. Such aberrations included an increase in the number of mucus producing cells in the blood tubes of the gills, cytoplasmic degeneration and vacuolation of the epithelia of the mantle and siphons, disruption of the ciliated epithelium of the lower part of the foot to form 'blisters', minimal vacuolation and cilia detachment in gastric epithelial cells and a significant decrease in digestive cell height. Cytochemical investigations involved preparing cryostat sections of the digestive gland and analysing for lysosomal membrane stability and changes in lysosomal sizes. TBT was shown not to cause any significant effect on lysosomes, therefore implying that the damage and atrophy observed in the cytological investigations are not a result of autophagy. Physiological responses involved analysing changes in filtration efficiencies and in the action of frontal cilia (involved in feeding) resulting from exposure to 100 and 1000 ng/l TBT. Filtration efficiencies \Vere not found to be significantly affected by TBT (probably TBT reduces filtration but the effect was not detected due to limitations in the experimental protocol and variability in animal behaviour). The activity of the frontal cilia was found to slow down significantly therefore leading to a probable reduction in feeding. Reduced feeding can result in a reduction in the energy available for growth and reproduction (scope for growth-SFG). Combining all the results it can be concluded that clams exposed to TBT concentrations as low as 10 ng/1. suffer from a reduction in the SFG The concentrations which led to significant effects on the organism are within the range reported for the main Maltese Harbours. The reduced SFG can be translated into effects on growth, reproduction and survival at the individual and population levels. The present study therefore shows that this clam and perhaps other benthic species in these Harbours may be at risk.