I am not a real knowledgable fellow regarding seastars, all I could comment is that your one (Pentagonaster duebeni) was/is probably feeding on the coral polyps. Most seastars are predators or scavengers, so I think it is quite possible that this little red beast got the better of the coral.
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For as long as I can remember I wanted to SCUBA dive. I remember being fascinated by my dadís cylinders of air with yellow flaking paint and belts of lead weights that lay around the house, and the promise of what lay beneath the surface of the sea. As luck would have it, by the time I seriously started thinking about diving I had been diagnosed with adult asthma.