The steroids of hexactinellid sponges

The Hexactinellida ('glass sponges') are commonly considered to be the most basic metazoans. Steroids of 20 species from different taxa were studied for chemotaxonomy and biosynthetic implications. All Hexactinellida contain cholest-5-en-3beta-ol (cholesterol) and/or its saturated derivative 5alpha(H)-cholestan-3beta-ol, along with their C-24-alkylated homologues. Where 5alpha(H)-stanols are present, they regularly co-occur with their 3-keto analogues. The steroid concentrations generally decrease with increasing carbon numbers, similar to sterol distributions typically found in marine sediments. These features argue against de novo sterol biosynthesis operating in hexactinellid sponges. Rather, we suggest a dietary uptake of Delta(5)-stenols and their stereoselective transformation via 3-keto intermediates to 5alpha(H)-stanols.