Colonial hydroids

Identification

Colonies of many small stalked polyps joined by a creeping brown, root-like base (stolon / hydrorhiza). Individual polyps are a few millimeters tall with fine tentacles, packed into fuzzy brown mats or feathery, branching plumes — the Myrionema type forms brown pom-pom-like tufts. They stay out in the light (unlike night-only digitate forms), spread visibly across rock faces, and commonly engulf zoanthid mats and coral bases.

Often confused with

Digitate hydroids — those are lone fingers with no shared mat or branching colony structure. Also confused with bryozoans (rigid, lacy and stingless), with feather duster worms (a crown that vanishes into a hard tube and no creeping mat), and with brown algae films, which have no individual stalked polyps under magnification.

What it targets

No fixed host. The colony overgrows rock and coral bases and stings whatever it contacts; zoanthids and palythoa engulfed by Myrionema-type colonies and Acropora bases are frequent casualties.

How it spreads

Hitchhikes on live rock and frags as tiny, easily missed colonies, then expands locally by creeping stolons and colony growth; fragments scrubbed loose in the tank can resettle. Some hydroids also disperse through a swimming medusa stage.

Control (safest first)

Manual and mechanical removal outside the tank works best: pull the rock, scrape, brush or chisel the colony and its creeping base off, and siphon loosened material rather than brushing it loose in the display where fragments resettle. Keepers report colonies rooted into the rock regrow from anything left, so abrade below the surface layer. Spot killing in place: boiling RO water from a syringe, kalk paste smeared over the patch with pumps off, or covering the patch completely with reef epoxy or putty. All are reported to work on small patches; colonies routinely grow around a partial covering and re-emerge at the edges. Rock sacrifice for bad cases: removing and torching, drilling out or discarding heavily infested rock is the method behind most permanent success stories, including one keeper who drilled half an inch into the rock, repeated the treatment when it regrew, and stayed clear afterward. Fenbendazole treatment (see digitate entry for the full warning set): reported effective against hydroids generally, but it persists in rock and sand, threatens snails and some corals and other invertebrates, and is a last resort with livestock relocated — NOT a casual reef-safe dose. Predators and helpers are thin for colonial forms: Halloween hermits and limpets are reported picking at some hydroid colonies (WetWebMedia notes limpets eating Myrionema), and bumblebee snails get occasional mentions, but no predator is reliable enough to plan around.

Natural predators

• Halloween hermit crab→
• Keyhole limpets→
• Bumblebee snail→

Field notes

A WetWebMedia keeper ran both hydroid types in a 29-gallon tank for over a year: the colonial form spread over much of the rockwork and engulfed zoanthids and palythoa despite biweekly water changes and careful feeding, leading the keeper to conclude eradication looked impossible without replacing rock and sand — and the crew knew of no dip that kills the hydroid while sparing the coral. An Ottawa forum keeper ground a colonial patch off an Acropora base with a Dremel, watched it regrow within about a week, and planned to cut the coral higher, remount it and throw the infested base away; in the same thread another keeper reported boiling water from a syringe kept a patch gone for a month — the two reports together sum up how depth in the rock decides the outcome. Not every keeper fights them: an Ultimate Reef poster left half a dozen similar patches alone for years because they preferred low light, stayed put and did not bother corals much, while another in the same thread warned a patch growing over zoas justified removing the rock before it spread everywhere.