Since their heterotrophic origin, animals have been faced with a continual input of foreign compounds, so-called xenobiotics, from their food sources, e.g. polybromomethanes and alkyl halides in seaweeds (Gschwend et al. 1985) and defensive toxins in other animals (Bakus and Kawaguchi 1984); and from the attacks of predators, e.g. venoms of certain gastropods and cephalopods (Fange 1984). For example, “animal-plant warfare” is considered to be a major selective pressure in driving the evolution of some of the gene families of the biotransformation enzyme, cytochrome P-450 (Nebert et al. 1989b). Over time, additional sources of xenobiotics have included erosion of hydrocarbon-containing shales, and oil seepage from natural reservoirs (started at least 100,000 years ago) (Farrington 1985), hostile environments such as the high sulphide levels around deep sea hydrothermal vents (Vetter et al. 1987), and, in more recent years, man’s multifarious industrial and other activities (Mix 1984). Central to the defense against such an enormous and diverse number of potentially toxic compounds is an impressive array of enzymes, which function ideally to detoxify and eliminate xenobiotics from an organism. The biological significance of biotransformation enzymes is increased by the inducibility of some of them by xenobiotics, and by their metabolism of certain xenobiotics to molecular species more toxic, mutagenic or carcinogenic than the parent compound. Elaboration of the qualitative and quantitative aspects of organic xenobiotic metabolism in marine invertebrates, and the function and regulation of the enzymes involved, is important for several reasons, viz. predicting and modelling the fate and toxicity of xenobiotics in marine organisms and ecosystems (e.g. Harris et al. 1984); development of specific indices of biological effect for use in pollution monitoring and impact assessment (e.g. Malins et al. 1985; Kleinow et al. 1987; Payne et al. 1987; Bayne et al. 1988); and understanding the evolutionary relationships of the biotransformation pathways in different phylogenetic groups, and the use of the pathways in invading, or exploiting, ecological time and space to create new niches.

Organic Xenobiotic Metabolism in Marine Invertebrates

Do mollusks use vertebrate sex steroids as reproductive hormones? Part I: Critical appraisal of the evidence for the presence, biosynthesis and uptake of steroids.

1.	
1. Identification of endogeneous steroids in Mytilus edulis (progesterone, androstenedione, testosterone, 5α-dihydrotestosterone, estradiol-17β, estrone, androsterone and androstanediol) in male and female specimens, was carried out by gas chromatography-mass spectrometry.

2.	
2. The levels of progesterone, androstenidione, testosterone, estradiol-17β and estrone were estimated by means of RIA in specimens collected at two different months of the year.

3.	
3. The levels of progesterone and androstenedione are higher than those of the other steroids, and show a different pattern between both samples.

4.	
4. These differences allow us to suggest a possible function of these steroids in the reproduction of Mytilus edulis.

Studies of endogenous steroids from the marine mollusc Mytilus edulis L. By gas chromatography and mass spectrometry

This review provides insights into the distribution and impact of oestrogens and xeno-oestrogens in the aquatic environment and highlights some significant knowledge gaps in our understanding of endocrine disrupting chemicals. Key areas of uncertainty in the assessment of risk include the role of estuarine sediments in mediating the fate and bioavailability of environmental (xeno)oestrogens (notably their transfer to benthic organisms and estuarine food chains), together with evidence for endocrine disruption in invertebrate populations.

Emphasis is placed on using published information to interpret the behaviour and effects of a small number of ‘model compounds’ thought to contribute to oestrogenic effects in nature; namely, the natural steroid 17β-oestradiol (E2) and the synthetic hormone 17α-ethinyloestradiol (EE2), together with the alkylphenols octyl- and nonyl-phenol (OP, NP) as oestrogen mimics. Individual sections of the review are devoted to sources and concentrations of (xeno)oestrogens in waterways, sediment partitioning and persistence, bioaccumulation rates and routes, assays and biomarkers of oestrogenicity, and, finally, a synopsis of reproductive and ecological effects in aquatic species.

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Partitioning, bioavailability and effects of oestrogens and xeno-oestrogens in the aquatic environment

Effects of sex steroids on gonadal development and gender determination in the sea scallop, Placopecten magellanicus

1.	
1. The steroid content of the gonads and hepatopancreas of the marine mollusc Aplysia depilans has been analysed.

2.	
2. The following steroids have been identified in the gonad extract: pregnenolone,∗ progesterone, 17α-hydroxyprogesterone, testosterone, oestradiol and oestrone; pregnenolone and 17α-hydroxypregnenolone have been identified in hepatic tissue.

3.	
3. Incubations of these tissues with acetate-1-14C and cholesterol-4-14C yielded the following metabolites: cholesterol, 20α-hydroxycholesterol, pregnenolone, 17α-hydroxypregnenolone, dehydroepiandrosterone, progesterone, 17α-hydroxyprogesterone, androstenedione, testosterone, cortisol, cortisone and deoxycorticosterone.

4.	
4. These findings support the view that steroid hormones are present in the gonadic tissue of A. depilans and could be produced from cholesterol and acetate, while hepatic tissue contains only the enzymatic systems, which are able to synthesize steroids.

M. Tomasucci

Papers

Identification and biosynthesis of steroids in the marine mollusc Aplysia depilans