Coral-boring spionid worms
Family Spionidae, boring Polydora-type species (order Spionida)
The worms tunnel through living coral skeleton, weakening the colony, deforming growth and stunting or slowly killing SPS and LPS from inside.
Identification
Almost all you ever see is a pair of thin, pale, worm-like feeding strands — the palps — waving from a pinhead hole in coral skeleton, coral base or rock, retracting fast when touched or shadowed. Heavy infestations show dozens of paired strands across a colony base, sometimes with fine mud-like tubes at the holes. Colonies may encrust oddly, stall for months or lose tissue around the base while water quality checks out fine.
Often confused with
Vermetid snails, which build a raised, hard, coiled tube and cast a mucus net; a spionid hole stays flush with the skeleton and shows exactly two palps. Also confused with harmless spionid and chaetopterid worms living in sand or rock away from coral, and with feather duster worms, which open a fan crown rather than two single strands.
What it targets
Living stony corals: Acropora and other SPS are the classic victims, with Montipora, torches, hammers, chalices and Duncan corals also reported. The worms bore interconnected tunnels in the skeleton, which research quoted on Reef2Reef notes weakens colonies and raises breakage risk; they also colonise snail shells and rock.
How it spreads
Arrives inside infested coral colonies and frag bases, and spreads within a system as larvae or young worms settle new skeleton — keepers have watched numbers climb from three worms to around sixty on a colony over a year, and found them turning up on neighbouring frags and even turbo snail shells.
Control (safest first)
Frag it out: cut away and discard the infested section of skeleton and remount clean tissue on a fresh plug. Keepers rate this the most dependable fix for frag-sized colonies, because the worms live protected deep inside the skeleton where dips cannot reach. Superglue the holes and seal the base: works as a holding move on light infestations, but keepers report worms boring a fresh exit beside the glue, so inspect sealed colonies weekly. Experimental ivermectin dip, outside the tank only: Reef2Reef keepers report a 1% ivermectin dip at roughly 8-9 drops per 473 mL of water for 1 hour killing spionids on dipped frags (reported in the Reef2Reef spionid treatment threads). Results are mixed, the protocol is experimental, and ivermectin is lethal to many invertebrates and some fish — never dose the display tank with it; one in-tank attempt killed about 98% of the worms but the keeper then could not clear the medication from rock and sand and lost other livestock. Potassium chloride dip: a keeper in the same threads reported worms appearing lifeless after 1 tablespoon per gallon for 20 minutes (Reef2Reef report). Also experimental and stressful to coral; test on a single frag first. Standard coral dips (Coral RX, iodine, peroxide and similar) repeatedly failed in keeper trials against worms sealed in skeleton, and discarding a heavily bored colony before it seeds the rest of the system is often the honest end point.
Natural predators
No natural predators are listed.
Field notes
One Reef2Reef keeper traced two years of a prized Acropora doing nothing to spionids in its skeleton, and found the worms in nearly every local system they later inspected, arguing most tanks have them and management style decides whether they matter. Tolerance splits the threads: a keeper with worms only in a single Acropora reported the colony deforming around them, another with worms under the encrusting bases of two thriving colonies left them alone, and a third counted four of six infested corals stunted once the worms were inside the skeleton itself rather than under the base. A keeper treating turbo snails reported spionids bored into the snails, and cleared both snails only after covering the shells with superglue twice — a measure of how many hiding places these worms find beyond coral.