Organometallic Compounds - Organic Pollutants: An Ecotoxicological Perspective - page 176

Fent et al. 1998). Thus, inhibition of P450s other than aromatase might cause ele-
vation of testosterone levels.
Apart from gastropods, harmful effects of TBT have also been demonstrated in
oysters (Environmental Health Criteria 116, Thain and Waldock 1986). Early work
established that adult Pacific oysters ( Crassostrea gigas ) showed shell thickening
caused by the development of gel centers when exposed to 0.2 μg/L of TBT fluo-
ride (Alzieu et al. 1982). Subsequent work established the no observable effect level
(NOEL) for shell thickening in this, the most sensitive of the tested species, at about
20 ng/L. It has been suggested that shell thickening is a consequence of the effect of
TBT on mitochondrial oxidative phosphorylation (Alzieu et al. 1982). Reduced ATP
production may retard the function of Ca ++ ATPase, which is responsible for the
Ca ++ transport that leads to CaCO 3 deposition during the course of shell formation.
Abnormal calcification causes distortion of the shell layers.
Other disturbances have been shown to be caused by TBT in oysters. These include
effects on gonad development and gender in adult oysters, and on settlement, growth and
mortality of larval forms. In one experiment, European flat oysters ( Ostrea edulis ) were
exposed to 0.24 or 2.6 μg/L TBT over a period of 75 days (Thain and Waldock 1986).
There was a large production of larvae in a related control group, but none in either of
the treated groups. On subsequent examination of the treated oysters, no females were
found in either of the two treated groups (20% of the control group were females). In
the group receiving the highest exposure to TBT, 72% were undifferentiated.
TBTO has appreciable toxicity to fish, LC 50 values ranging from 1-30 μg/L for
most of the species that have been tested. Acute oral LD 50 values for the rat and the
mouse are in the range 85-240 mg/kg. Recently, evidence has come forward that
TBT can cause masculinization of fish (Shimasaki et al. 2003). Japanese flounder
( Paralichthys olivaceus ) fed a diet containing 0.1 μg/g TBTO showed a 26% increase
in the ratio of sex-reversed males. Interestingly, several authors have reported that
TBT can inhibit cytochrome P450 forms in fish (Fent and Bucheli 1994, Morcillo
et al. 2004), so once again, elevation of testosterone levels following inhibition of
aromatase is a possible explanation.
8.3.5 e c o l o g i c a l e f f e c T s of f TbT
A striking example of the harmful effects of low levels of TBT was the decline of
dog whelk populations around the shores of Southern England (completely from
certain shallow waters such as marinas, estuaries, and harbors) where there were
large numbers of small boats. It was found that females showed the first signs of
imposex with levels of TBT as low as 1 ng/L in water, and that development of male
characteristics increased progressively with dose. TBT concentrations exceeding 5
ng/L caused blockage of the oviduct because of the proliferation of the vas deferens
and led to breeding failure. Before 1987, levels high enough to have this effect were
common in shallow waters with large numbers of small craft. In 1987, a ban on the
use of TBT as an antifoulant on small boats was introduced in Britain, after which
recovery of the population began.
An interesting local phenomenon connected with TBT pollution in Southern
England is the so-called Dumpton Syndrome . In the eponymous coastal area in Kent,