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In the new citizen science project (see: Sea Floor Explorer), numbers of seastars, scallops, crustaceans and other animals are counted. Already one can see a heavy bias in favor of seastars, both the fat and brittle kind.

I would be interested in why this creature is so succesful, especially if it is poisonous to e.g., crustaceans, and what poisons exactly are responsible.

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it may not be that they are poisonous. seastars have extraordinary reproductive capabilities – ParaChase Sep 15 '12 at 22:54
That is the other factor of success. But they are really poisonous, see the answer. – rwst Sep 16 '12 at 6:38
up vote 4 down vote accepted

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.

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