Impairments of Endocrine Functions: Causes and Consequences - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 203

of biological variables directly under endocrine regulation, such as oocyte growth, molt,
and fertility, have been proposed as tools based on the weight of evidence to diagnose and
identify compounds suspected of being ED in crustaceans. For example, Tatarazako and
Oda (2007) proposed using the production of males in Daphnia magna as a specific indica-
tor of exposure to compounds having a similar effect to that of MF. Indeed, MF antagonist
activity can be measured by exposing crustaceans to MF to stimulate male offspring pro-
duction and assessing the ability of chemicals to block this action (Wang et al. 2005). Thus,
dieldrin appeared antagonistic at low, endogenous levels of MF, but agonistic in the absence
of MF (LeBlanc 2007). However, this approach cannot be applied to crustaceans with sexual
reproduction. Similarly, Mu and LeBlanc (2002) proposed using the intermolt cycle dura-
tion in D. magna as a specific response to exposure to ED antagonists to ecdysteroids, by
comparing the effects obtained in organisms after an exposure to a compound alone and in
combination with the 20-hydroxyecdysone, known as the active form of molting hormone
in crustaceans. Finally, Geffard et al. (2010) developed a biotest that specifically tests the
impact of pollutants on the molt cycle, embryonic development, and oocyte growth (second-
ary vitellogenesis) and production in G. fossarum . As previously demonstrated, all of these
biological variables are perfectly synchronized during the reproductive cycle of this spe-
cies under uncontaminated conditions; consequently, for each molt stage there is a specific
oocyte surface and embryonic stage. Any disruption of the synchronism of these endpoints
by a chemical contaminant would highlight a possible endocrine disruption mechanism.
8.6 Endocrine Control and Disruption in Mollusks
In mollusks, neuropeptides produced by neurosecretory ganglia have been described to
play a role in several physiological processes such as reproduction, energy, and ion regula-
tion. Among the best-known neuropeptides—gonadotropic, egg-laying hormones, insulin-
like and FMRFamide peptides—the gonadotropic and egg laying hormones are the main
neuropeptides identified in gastropods to play roles in sexual differentiation, gonad matu-
ration, egg laying, and behavior (Joose 1988; Nassel 1996; LeBlanc et al. 1999; LaFont 2000;
de Jong-Brink et al. 2001; López-Vera et al. 2008).
Although their role in invertebrates is still under discussion, vertebrate-like steroids, includ-
ing 17β-estradiol (E2) and testosterone (T) have been described in bivalves and gastropods
as well. Janer and Porte (2007) reviewed studies in which sexual steroids were detected in
invertebrates, and key steps of steroidogenesis leading to androgens or estrogens described
in species from different phyla. Similar to vertebrates, a combination of cytochrome P450 and
steroid dehydrogenase catalyzes the conversion of cholesterol to sex steroids in molluscan
species (Porte et al. 2006). Nevertheless, the transformation rates of some metabolites could
be low (Gottfried and Dorfman 1970; De Longcamp et al. 1974). Some temporal variations in
their levels have been linked to possible roles in the reproductive cycle in bivalves (reviewed
by Lafont and Mathieu 2007) and gastropods (Sternberg et al. 2008; Gust et al. 2011).
Steroid conjugation and esterification have also been described in mollusks. Only a small
quantity of testosterone and estradiol is free in Marisa cornuarietis , whereas the major part is
esterified (Janer et al. 2006). Other steroids have been found in the form of esterified prod-
ucts in bivalves (e.g., 17β-estradiol in Mytilus galloprovinciallis , Janer et al. 2005; and Mytilus
edulis , Labadie et al. 2007). The physiological role of the esterification of steroids is not eluci-
dated; nevertheless, it seems to be a factor in the regulation of homeostasis of the free active