Endocrine disruption through oxygen depeletion

Endocrine disruptors are substances, man-made or natural, which can interfere with the hormone systems of humans and wildlife, with potentially adverse effects, for example, on fertility and neural development. They interfere with the hormonal (endocrine) system, e.g. by mimicking the action of a naturally-produced hormone, such as estrogen or testosterone, and thereby setting off similar chemical reactions in the body. They are also known to block hormone receptors in cells, thereby preventing the action of normal hormones, and they can affect the synthesis, transport, metabolism and excretion of hormones, thus altering the concentrations of natural hormones in the body.

Numerous recent studies identify mechanisms and effects of endocrine disruption in invertebrates. Vertebrate steroid hormones (testosterone, estradiol, and progesterone) that are synthesized from cholesterol are found in molluscs, and evidence shows that such sex steroids can serve as ideal biomarkers of endocrine disruption. While the specific function of steroid hormones in molluscs’ endocrine system is still speculative, reports emphasize steroids’ influence on gender differentiation, gametogenesis, gonadal maturation, fertilization and embryonic development, and reproduction. Specifically, testosterone and estradiol concentrations in the gonads vary during different stages in the reproduction process, largely affecting gender determination and gamete growth.

In the 1980s and 1990s the use of tributyltin (TBT) was hotly debated in science and politics. Because TBT was the most effective anti-fouling agent known, it was frequently used in anti-fouling paint. However, TBT disrupts the endocrine system in a variety of animals most prominently in mollusks. The disruption leads to masculinization in females, because the androgen levels are higher than normal. Since less fertile females are available for mating, the population begins to decline and seriously impacts the balance of the ecosystem. After endless discussions and negotiations TBT was worldwide banned in 2008.

But not only specific chemicals can interfere with the hormonal system. There also seems to be a correlation between low dissolved oxygen which you can find e.g. in urbanized watersheds and endocrine disruption. 

A group of researchers from New York did a  five-year bio-monitoring study of the Bronx River Estuary and they found altered hormonal levels, resulting from endocrine disruption, in Geukensia demissa (Atlantic ribbed mussel):

Testosterone, estradiol, and progesterone levels were collected from male and female mussels in the oxygen depleted Bronx River and well-oxygenated Greenwich Cove. Bronx River mussels exhibited higher testosterone levels and lower estradiol levels than Greenwich Cove mussels. The resulting abnormal hormonal ratio seems to indicate that environmental conditions in the Bronx River facilitate an allosteric inhibition of the cytochrome P450 aromatase enzyme, which aids conversion of testosterone to estradiol. Low progesterone levels suggest Bronx River mussels are experiencing a delay in sexual maturation, and morphometric data show a stalling of shell and tissue growth.

In order to confirm that the mussels collected from both sampling sites are the same species, the researchers used DNA Barcoding. Minimal sequence variation confirmed the mussels were from the same species which suggests that there is intraspecific divergence in various endocrine processes, resulting from environmentally induced stress.