Bob I'm not sure why they would disappear over time unless they are being predated.  I've always considered 'pods standard fare in any healthy reef.
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-Quinn
Man, n. ...His chief occupation is extermination of other animals and his own species, which, however, multiplies with such insistent rapidity as to infest the whole habitable earth, and Canada. - A. Bierce, Devil's Dictionary, 1906
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