Cobalt blue sponge (Acanthancora cyanocrypta), surrounding the cucumber Psolus chitinoides

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Cobalt blue sponge (Acanthancora cyanocrypta), surrounding the cucumber Psolus chitinoides

Once its bright red tentacles have been withdrawn, the cucumber Psolus chitinoides is difficult to see. It is surrounded by the cobalt sponge Acanthancora cyanocrypta, formerly Hymenamphiastra cyanocrypta.