Jenny (CC BY 2.0)
Eunicid worms (bobbit worms)
Family Eunicidae (Eunice spp., including Eunice aphroditois)
Large predatory species ambush small fish and invertebrates at night and can take chunks out of corals; a big specimen can work through livestock for months before it is seen.
Identification
A long, thick, segmented worm, iridescent pink-brown to dark brown, from a few centimetres to well over 30 cm in tanks, with reports of far larger. The head carries five banded antennae around powerful retractable jaws, and many keepers note a pale band just behind the head. It lives in a burrow in rock or sand and darts out at night to snatch prey, retreating with lightning speed when lit or touched. Signs include missing small fish, bitten corals and frags dragged toward a hole.
Often confused with
Common bristleworms, which are thinner, visibly bristly along every segment and lack the five-antennae head and jaws. Small dorvilleid worms in the same order are tiny detritus feeders often found under corals and are harmless. Peanut worms have a smooth, unsegmented body and no antennae.
What it targets
Small sleeping fish, shrimp, crabs, snails and other worms; large individuals also bite zoanthids and other soft corals and drag frags to the burrow. Many small eunicid species stay harmless scavengers, and risk rises steeply with size.
How it spreads
Hitchhikes inside live rock, sometimes in coral bases, and can stay hidden for a year or more while it grows. It does not spread between tanks except on moved rock or coral.
Control (safest first)
Remove the home rock: watch at night with a red light to learn which rock it lives in, lift that rock out whole and flush or prod the worm out over a bucket. Most keepers rate this as the only dependable method; wear gloves and never grab the worm by hand. Baited bottle or commercial traps at night next to the burrow: worth trying for smaller worms, but experienced keepers report large eunicids are trap-shy and fast enough to back out, so treat traps as a first attempt rather than a plan. Do not chop or tear the worm during a failed grab: a broken worm retreats deep into rock and the problem continues, and bristles and jaws can injure fingers. If a grab fails, switch to rock removal. Freshwater or soda-water flushing of the removed rock, outside the tank, can drive the worm out; keep the rock out of the display until the worm is confirmed gone, accepting some die-off on that rock.
Natural predators
No natural predators are listed.
Field notes
A Reef2Reef keeper only identified the culprit after setting up an overnight time-lapse camera: a large Eunice rose from the rockwork in the small hours to hunt, after a year of a zoa frag being dragged nightly to plug a hole and two fish vanishing. An Ultimate Reef keeper dismantling a 6 ft tank found a worm estimated at 8-9 feet long that had never been seen in normal viewing, and blamed it for eating fist-sized pieces of coral per night before removal. Reports split on small specimens: one keeper found tiny metallic-green eunicids grazing coralline algae and left them alone with no fish losses, while others insist any confirmed Eunice comes out immediately because of what it becomes at size. A wrasse was credited with eating the smallest ones in one sump report.