Changes in steroid profiles of the blue mussel Mytilus trossulus as a function of season, stage of gametogenesis, sex, tissue and mussel bed depth

This paper describes changes in the content of free steroid hormones e.g. testosterone (T), estradiol-17β (E2), estrone (E1) and estriol (E3) of Mytilus trossulus from the southern Baltic Sea as a function of season, stage of gametogenesis, sex, tissue (gonadal and somatic) and depth. The highest levels of T, E2, E1 and E3 were found in mussels sampled in spring and summer while the lowest levels were found in winter. This pattern was stable and was seen in both sexes and tissues in mussels from both mussel beds. The spring and summer peaks in steroid levels (SL) coincided with advanced levels of gametogenesis (the highest gonadal index, GI) of our model species. But, the lowest GI (autumn) and the lowest steroids content (winter) did not overlap. Instead, water temperature increase was followed by increase of SL and vice versa. This suggests that steroids may not be actively involved in the early stages of gamete development and does not preclude them from potentially being involved as endogenous modulators in the final stages of reproduction (e.g. spawning). Hence, observed fluctuations in SL in our model species are unlikely to be caused by reproductive cycle but are rather of unknown nature, likely linked with environmental conditions. Sex-related differences in steroid content included estrogen domination in females and androgen domination in males. A trend towards higher level of steroids in gills than in gonads was found, supporting the hypothesis about an exogenous origin of steroids in bivalves. However, based on the present results, we cannot exclude the possibility that these steroids have both an endogenous and exogenous origin.