Impairments of Endocrine Functions: Case Studies - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 223

routes, or the absence of replicate experiments throughout the physiological (reproductive)
seasonal states of the animals. In the hypothesis, TBT is fixed onto the retinoid X receptor
in the cerebral ganglia. This leads to signaling to the pedal ganglia, which in turn start
secreting “morphogenetic penis factor” (MPF). This molecule activates the establishment
of male sexual organs. Subsequently, these organs will secrete testosterone and via posi-
tive retro-control, MPF will continue stimulating their own growth. Moreover, this retro-
control is reinforced because TBT inhibits conversion of testosterone into estradiol, and
thus the negative estrogenic retrocontrol network.
Moreover, several other factors, not shown in Figure 9.2, take part in the establishment and
development of male sexual organs in gastropods. Among them is the “de-differentiation
penis factor” (DPF) produced by the pleural ganglia. This factor causes penis regression
in the absence of its antagonist MPF. Both males and females, independent of the pres-
ence of a penis, produce DPF. Development, growth, or regression of the penis depend on
the interplay between these two antagonistic hormones. And this balance is influenced by
environmental factors such as pollution by TBT.
Fluctuation in MPF and DPF levels in correlation with age or season may explain why
females are refractory or, on the contrary, responsive to the masculinizing effects of TBT.
These seasonal variations probably explain the penis loss observed in males of Littorina
littorea by Streiff and Le Breton (1970). Oberdörster et al. (2005) reported that TBT pollution
caused a MPF/DPF equilibrium change leading to the absence of seasonal penis regres-
sion in males of Ilyanassa obsoleta .
Steroid levels are not only controlled by an aromatase, a member of the large cyto-
chrome P450 superfamily, which transforms testosterone to estradiol. A second control
mechanism implicates steroid conjugation. Conjugated steroids are less hydrophobic and
therefore easier to excrete. In L. littorea , TBT was shown to inhibit this steroid conjugation,
which leads to higher free testosterone levels (Ronis and Mason 1996).
9.1.4 Temporal Evolution of TBT Contamination
Dissemination of TBT into the environment started in the late 1960s. At the beginning
of the 1980s, a link was established between the observed decrease in oyster spat-fall in
Arcachon Bay, France, and the presence of TBT. His and Robert (1980) demonstrated the
mortality of larvae of the pacific oyster, Crassostrea gigas , with environmentally relevant
concentrations of TBT. TBT was also suggested as the agent responsible for the “chamber-
ing” of the oyster's shell (Alzieu et al. 1981). This aberration of calcification diminishes the
commercial value of the oysters. This, together with the reduced spat-fall, endangered
the oyster industry, and a French law was quickly passed in 1982 that limited the use
of TBT. Subsequently, several other countries reacted by establishing similar regulations.
The International Maritime Organisation (IMO) banned its use on small vessels (<25 m) in
1987. In 2003, new applications of the substance to ships hulls were banned, and finally, in
September 2008, a total ban of TBT use was brought into force by the IMO.
9.1.4.1 Case Study: Brest Harbor
As legislations differ from one country to another, temporal changes in the levels of
imposex vary locally. Since 1992, a survey of N. lapillus imposex has been undertaken on
a yearly basis in Brest, France, and the surrounding area. Follow-up results from 34 field
stations under the heavy influence of the Brest harbor area (Huet et al. 2004) are shown
in Figure 9.3. The temporal trends of three groups of imposex intensities are shown: stage