Invertebrates endocrine disruption

Our present meagre understanding of normal endocrine processes in invertebrates makes the assessment of chemical endocrine disruption in the field difficult (LeBlanc 1999). Steroid roles differ between species and sometimes sexes, and their influence may vary at different developmental stages. In most studies of invertebrates, endocrine disruption appears to involve androgenization rather than oestrogenic effects (see Box 7.2). Arthropods (crustaceans and insects), annelids and molluscs use ecdysteroids, terpenoids and vertebratelike sex steroids for endocrine control. For example, the ecdysteroid ecdysone is naturally converted to 20-hydroxyecdysone (Fig. 7.10), which induces moulting (ecdysis) in both insect larvae and crustaceans. 

In addition to reproductive effects, fish exposed to endocrine disrupters may have a decreased response to stress or decreased growth and metabolism which can affect their ability to survive, or to defend themselves against predators. All of these factors can affect the ability of the species to survive and to reproduce itself in sufficient numbers to maintain the stocks on which our commercial and sport fisheries are based. Not all fish species will be equally susceptible to the effects of endocrine disrupters. Selective sensitivity to such effects, especially those affecting reproduction, may well lead to major changes in the flora and fauna of some of our major aquatic ecosystems as the balance between fish, mammals, invertebrates and plants, and between predators and prey, is destabilised... 

Whilst the impact of xenoestrogens has been the most widely studied effect in vertebrates, it is unclear whether endocrine disruption in invertebrates proceeds... 

Table 2 Chemicals suspected of causing endocrine disruption in invertebrate species...

Exposure to estrogenic compounds through diet will differ for herbivores and carnivores, the latter being most likely to encounter endogenous steroids in their prey. Efficient uptake of steroids in mammals is illustrated by the use of the contraceptive pill, but routes of absorption in invertebrates remain to be determined. The relationship between endocrine disruption and metabolic toxicity, with reduced reproductive viability a secondary consequence of metabolic disturbance, also merits further study in invertebrate species. 

Many pesticides cause endocrine disruption in vertebrate and invertebrate species at concentrations that are not overtly metabolically toxic. The insect growth inhibitor diflubenzuron can affect the reproduction, development and behaviour of estuarine crustaceans at concentrations of just lOmgP (reviewed... 

Several recent expert reviews and workshops have discussed the effects of endocrine disruption on wildlife and especially invertebrate species. These include the EU workshop on the impact of endocrine disrupters on human health and wildlife (Weybridge, 1996), the lEH workshop (Leicester, May 1997), the Environment Agency Consultative report (January 1998) and the Tyndall Forum at the Royal Institution (February 1998). They have concluded that endocrine disruption may have far-reaching adverse consequences for biodiversity and the sustainability of natural ecosystems. More comprehensive bioassay systems are required to identify and assess chemicals alleged to produce endocrine modulating effects. 

Culture a range of invertebrate species from the major phyla, preferably species with short life cycles. The effects of potential endocrine disrupting chemicals on growth rate, reproductive output, viability of offspring and sex ratio, and the vulnerability of different stages of the life cycle, can then be determined. 

Of particular interest in the present context is that TBT can inhibit cytochrome-P450-based aromatase activity in both vertebrates and aquatic invertebrates (Morcillo et al. 2004, Oberdorster and McClellan-Green 2002). The conversion of testosterone to estradiol is catalyzed by aromatase, and so inhibition of the enzyme can, in principle, lead to an increase in cellular levels of testosterone. The significance of this is that many mollusks experience endocrine disruption when exposed to TBTs,... 

Oehlmann, J. and Schulte-Oehlmann, U. (2003). Endocrine disruption in invertebrates. Pure and Applied Chemistry 75, 2207-2218. 

Endocrine Disruption in Invertebrates Endocrinology, Testing, and Assessment (EDIETA). Amsterdam, The Netherlands, 12 to 15 Dec 1998. Published by SETAC, 1999. 

Barata C, Porte C, Baird DJ. 2004. Experimental designs to assess endocrine disrupting effects in invertebrates — areview. Ecotoxicology 13 511-517. 

Hutchinson TH. 2002. Reproductive and developmental effects of endocrine disrupters in invertebrates in vitro and in vivo approaches. Toxicol Lett 131 75-81. 

Hutchinson TH. 2007. Small is useful in endocrine disrupter assessment — four key recommendations for aquatic invertebrate research. Ecotoxicology 16 231-238. 

Human health biomonitoring may also use animal surrogates in the environment to assess potential health hazards to humans - the proverbial canary in the coal mine . For example, chemical and biomarker analysis of bodily fluids or tissue biopsies from family pets, especially dogs, are sometimes used to assess potential chemical exposure and effects in children. This is because dogs often accompany children in the outdoor environment, and both have a tendency to (accidentally or intentionally) consume environmental media such as soil and surface water There has also been an increasing trend to use native animals as sentinel species, that is, fish, wildlife, or invertebrates that are indicators of possible human health risks from environmental hazards. For example, increased incidences of tumors or endocrine disruption in fish may indicate the presence of compounds in the water that may cause cancer or reproductive dysfunction in humans. Concern has also been raised over the increased incidence of deformities in frogs, because these may indicate an increased level of chemicals in the environment, which can cause birth defects in humans. 

Invertebrates in Ecotoxicological Studies of Endocrine-Disrupting Substances... 

Tributyltin (TBT) is one of the best-studied endocrine disrupting compounds in invertebrates (Oberdorster and Cheek 2000). Concentrations as low as 1 ng/1 of TBT can lead to the development of male sex organs in female snails. This imposex response has been identified in approximately 150 species of gastropods and is clearly due to an interference with some part of the molluskan endocrine system. The toxicity of TBT to gastopods has caused its regulation. 

LeBlanc G.A. (1999) Steroid hormone regulated processes in invertebrates and their susceptibility to environmental endocrine disruption. In Environmental Endocrine Disrupters an Evolutionary Perspective (ed. Guillette L.J., Crain A.). London Taylor Francis. 

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