Which poison makes seastars inedible to possible predators?
Asterosaponins are the class of compounds - they have a cholesterol like organic core.  

Apparently, these saponins make pore-forming complexes with Δ5-sterols of cell membranes, and so are deadly to all usual kind of life, including bacteria and fungi. Quote:


  Starfish and sea cucumber cell membranes are resistant to their own
  saponines due to the presence of Δ7- and
  Δ9,11-sterols, sulfated Δ5-sterols, and
  β-xylosides of sterols instead of the free Δ5-sterols.