Worms in the genus Eunice, sometimes called "bobbit" worms are, if anything, more impressive than their Palola cousins. Eunice is a large genus, with well over 150 species, and it is hard to generalize about them. As with the Palola, they have five antennae, including one in the center of the forehead and they all have jaws, in some cases wicked, scimitar shaped hooks with accessory spines and hooks. These are not jaws for chewing the prey or food item, but rather are jaws designed to ensure the food that, once seized, never gets away. Some of these worms get very large; the largest I have seen reported from reef tanks was in excess of 6 feet long, and individuals of Eunice aphroditois may be much larger. Eunice individuals tend to live in mucus-lined borrows in rock or sediments and may have several entrances to their tubes. Two distinct kinds of worms seem to be represented in this genus. One kind, which is benign in reef tanks, seems to be mostly scavenging its food. It lives in a burrow in the sediment, or more rarely, in a burrow in the rocks. When feeding, one of these worms will slowly extend from its burrow. They typically have four to six eyes and are quite capable of detecting motion outside the aquarium, and across the room. It will slowly search the surrounding area for food and if bothered by a fish, or its own shadow, will retract into its burrow with a velocity that has to be seen to be believed. Contraction back into a burrow has been clocked in excess of 20 feet per second, and if only a couple of feet of the worm are visible while it is foraging, that worm can disappear, quite literally, in the blink of an eye.
Figure 4. This is "Max," the large Eunice individual that I have had in my aquaria for about five years. A. The head, note the characteristic five head tentacles from the head, and specifically note the one arising directly from the center of the "fore head." The worm is about half an inch across. Note the complete lack of visible setae or bristles; eunicids do not have the white protective bristles found in fireworms, and they often keep their other bristles withdrawn. B. In August of 2002, I had to move Max from one tank to another, and he broke into three fragments. His total length was over four feet long. He has recovered from this and is living in my present tank. He is shorter, but I anticipate he will grow to his former length. C. Note the gills and compare them to the ones on Eurythoe in Figure 1.
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.
This is taken from
http://reefkeeping.com/issues/2003-04/rs/index.php
