here is a specific note from research paper below, i am highlighting as i think this concept will be important in future discussions on the vitamin D topic as we learn that todays knowledge is not the same as tomorrow's knowledge... vitamin D2 is just starting to be studied ect.....

In mammalian research other vitamin D₃ metabolites, such as 1,25(OH)₂-3-epi-D₃ (Brown et al. 1999), and C23 epimers (Lee et al. 2000), and P₄₅₀ cytochromes involved in their synthesis have been identified (e.g. Bouillon et al. 1995; Tuckey et al. 2008). Understanding the role of these metabolites can, e.g. assist medical research to develop new vitamin D₃ analogues. In research on fishes, the development of specific vitamin D₃ analogues has received no priority. However, for the understanding of the complexity of the vitamin D endocrine system, it is important to realize that many more metabolites of vitamin D₃ than described above may exist in fish and could be of physiological significance. A picture that emerges in other steroids and in thyroid hormones (THs) (Moreno et al. 2008), where multiple TH derivatives have metabolic effects, seems to apply for vitamin D₃ as well.