Biology Reference
In-Depth Information
by the settlement of plants and animals on submerged surfaces, with deleterious con-
sequences. Friction forces on the hulls of marine vessels are enhanced, thus decreasing
speed and causing much higher fuel costs. Fouling also accelerates surface erosion and can
block subsurface ducts by colonization of their walls.
Many toxic substances, such as copper or mercury, have been used in marine antifoul-
ing paints (Alzieu et al. 1987). The search for more active compounds led in the late 1960s
to the use of TBT. It proved highly effective, and as a result was used for this purpose
worldwide.
9.1.2 Imposex: A Bioindicator Specific for TBT Contamination
9.1.2.1 TBT Is Responsible for Masculinization of Female Gastropods
Female masculinization in marine gastropods was called “imposex” by Smith (1971),
derived from the words “impose” and “sex.” In the Plymouth area, Blaber (1970) had
observed a penis-like structure in females of the dogwhelk
Nucella lapillus
. Sub sequently,
Féral (1974) encountered the same phenomenon in another snail species,
Ocenebra erina-
cea
. She showed that imposex was not linked to sequential hermaphroditism: all spe-
cies affected by imposex are gonochoristic, meaning that they have separate sexes. She
also observed imposex-free populations (Féral 1976a, b), and by means of transplants she
showed that the phenomenon has an environmental origin (Féral 1980b). The implication
of TBT was established by Smith in
Ilyanassa
(
Nassarius
)
obsoleta
. He first linked the pres-
ence of affected animals to the vicinity of harbor areas (Smith 1981a, b). Subsequently,
he showed that the causative agent originated from the boats themselves (Smith 1981c).
Among all compounds tested, only TBT was able to induce imposex (Smith 1981d).
9.1.2.2 Relation between Imposex Development and TBT Contamination Levels
Imposex consists of the superimposition of male sexual organs on female gastropods; a
penis, a prostate, and in between them, a spermatogenic duct called the
vas deferens
(Gibbs
et al. 1991). The development of these male organs in a female is directly proportional to
TBT concentrations in the seawater and the tissues (Gibbs et al. 1987; Oehlmann et al. 1992).
In instances of high contamination, female sterilization is caused either by loss of integrity
of the “capsule gland” (Gibbs et al. 1990) or by obstruction of the genital opening through
uncontrolled proliferation of the
vas deferens
(Gibbs and Bryan 1986).
An imposex development scale has been proposed by Gibbs et al. (1987) and is presented
in Figure 9.1. Stage 0 represents the uncontaminated condition: no signs of masculiniza-
tion. The first masculinization stage shows an outline of a penis behind the right eyestalk.
At stage 2, a
vas deferens
starts developing near the genital papilla. The penis has already a
similar form to the male, although it is reduced in size. Stage 3 is characterized by a longer
penis, which prolongs into the proximal portion of the
vas deferens
. The distal portion of
this duct is also longer than in stage 2. Stage 4 shows the fusion of the two portions of the
vas deferens
, and the penis has become even longer. In highly contaminated conditions,
the
vas deferens
proliferates around the genital papilla, which can lead to obstruction of
this its opening. The female is unable to copulate and cannot eject her eggs anymore; she
has become sterile (stage 5). When aborted egg capsules are present in the capsule gland,
stage 6 is reached. At stages 5 and 6, the female penis is similar in size and form to that of
normal males. In extreme cases, the ovary is reversed into testis (Gibbs et al. 1988). This
progress of masculinization led to the definition of two bioindices of TBT contamination.
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