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All of the large Eunice individuals that I have heard about in reef aquaria seem to be scavengers. However, the largest Eunice individuals seen in nature are impressive predators. Individuals have been reported to strike upward from the sediment surface, grab a four-inch long fish swimming above the sediment, pull it under the sediment and presumably snack on it at its leisure. Such worms are also reported to be an inch in diameter and about thirty to fifty feet long, making them a bit larger than most home aquaria could accommodate.
There are also smaller species of Eunice, and these seem to be reported from time to time in aquaria. They generally appear to be harmless scavengers, however, even I, a self-proclaimed vermophile, would consider them amongst the "usual suspects" if some small fish such as fire fish or small gobies disappeared without a trace.
Regardless of the size of its individuals, the major characters for identification for the identification of eunicid species would be the absence of white tufts of setae, and the presence of five large and visible antennae (large relative to the worm, not the aquarist), such antennae are typically about two to three body diameters in length. Colors are secondary characters with these animals, but the Palola worms are often dark colors, while the Eunice individuals are, typically, shades of brown