Do mollusks use vertebrate sex steroids as reproductive hormones? II. Critical review of the evidence that steroids have biological effects.

In assessing the evidence as to whether vertebrate sex steroids (e.g. testosterone, estradiol, progesterone) have hormonal actions in mollusks, ca. 85% of research papers report at least one biological effect; and 18 out of 21 review papers (published between 1970 and 2012) express a positive view. However, just under half of the research studies can be rejected on the grounds that they did not actually test steroids, but compounds or mixtures that were only presumed to behave as steroids (or modulators of steroids) on the basis of their effects in vertebrates (e.g. Bisphenol-A, nonylphenol and sewage treatment effluents). Of the remaining 55 papers, some can be criticized for having no statistical analysis; some for using only a single dose of steroid; others for having irregular dose-response curves; 40 out of the 55 for not replicating the treatments; and 50 out of 55 for having no within-study repetition. Furthermore, most studies had very low effect sizes in comparison to fish-based bioassays for steroids (i.e. they had a very weak 'signal-to-noise' ratio). When these facts are combined with the fact that none of the studies were conducted with rigorous randomization or 'blinding' procedures (implying the possibility of 'operator bias') one must conclude that there is no indisputable bioassay evidence that vertebrate sex steroids have endocrinological or reproductive roles in mollusks. The only observation that has been independently validated is the ability of estradiol to trigger rapid (1-5 min) lysosomal membrane breakdown in hemocytes of Mytilus spp. This is a typical 'inflammatory' response, however, and is not proof that estradiol is a hormone - especially when taken in conjunction with the evidence (discussed in a previous review) that mollusks have neither the enzymes necessary to synthesize vertebrate steroids nor nuclear receptors with which to respond to them.