Overview
Myrionema hydroids (Myrionema spp.) are one of the more annoying “specific” hydroid pests reef keepers run into—especially if you keep zoanthids. In the tank they often show up as tiny, starry or fuzzy polyps that creep along surfaces near zoa mats, frag plugs, and rockwork. They’re small enough to ignore at first, but they can spread into patches that cause persistent irritation: closed zoas, poor extension, and that “my colony just won’t open on that side” problem.
What makes Myrionema stand out from generic “hydroids” is how often hobbyists notice them in and around zoa gardens, where they behave like an irritating stinging carpet rather than a random fuzz on a rack.
If you’ve got zoas that refuse to open in a localized area and you see tiny hydroid polyps nearby, Myrionema is a very real suspect.
Quick Care Snapshot
Common Name: Myrionema Hydroids
Scientific Name: Myrionema spp.
Reef Safe: No (stings/irritates, especially zoanthids)
Difficulty: Easy to “keep,” frustrating to eliminate completely
Temperament: Stinging colonial polyps
Lighting: Tolerant (often thrives in moderate reef lighting)
Flow: Moderate flow zones are common; can adapt
Diet: Micro-foods, suspended particles, reef “soup”
How They Spread: Creeping colony growth + new polyp settlement
Main Risk: Zoa irritation, persistent closure, spreading on plugs/mats
Natural Background
Myrionema are hydrozoans—small cnidarian polyps that grow as colonies on hard surfaces. In the wild, they occupy reef substrates where they can capture microscopic food drifting past.
In aquariums, they exploit the same things as other hydroids:
• stable conditions
• lots of hard surfaces
• steady particulate foods
• limited predation
The difference is that Myrionema often presents as a creeping, persistent irritant right where we keep soft coral mats and frag plugs.
Tank Requirements
If you’ve got a reef tank, you’ve got potential Myrionema habitat.
They tend to thrive when:
• there are lots of frag plugs, racks, and mat-forming corals
• you feed coral foods or fine particulate regularly
• there are protected areas with moderate flow
• colonies are left alone long enough to establish
Where you’ll most often find them
• On frag plug tops/edges near zoas
• Along the rock around zoa colonies
• On the underside of plugs and shelves
• On frag racks and overflow walls
Feeding
Myrionema feed on tiny suspended foods:
• fine coral foods
• micro-particulates
• detritus dust
• general dissolved/particulate organics
This is why they often show up in systems where you’re doing “everything right” for coral growth—stable tank, consistent feeding, lots of surfaces. Unfortunately, that’s also great for hydroids.
Compatibility
With corals (especially zoanthids)
This is the big one.
Myrionema can:
• irritate zoa mats and cause persistent closure
• sting soft tissue and keep polyps retracted
• creep into spaces where zoas would otherwise expand
• create a “dead zone” where nothing seems happy
Zoas often show the clearest signal because they’ll stay closed around the affected area even when everything else in the tank looks normal.
With fish and inverts
Most fish ignore them. Cleanup crews are unreliable because:
• the polyps are tiny
• they sting
• they often grow in protected, hard-to-reach zones
Common Mistakes
1) Treating it like a parameter issue
People chase light, nutrients, flow, and dosing because the zoas are closed—meanwhile the real issue is localized stinging irritation on the mat.
2) Ignoring the first patch
Small patches turn into creeping carpets if left alone.
3) Scrubbing inside the tank
Scrubbing or brushing colonies off plugs/racks in the water can spread fragments and worsen the issue.
4) Only treating the visible area
Hydroids often exist:
• on undersides
• around glue lines
• in crevices behind the colony
If you only treat the top surface, they come right back.
5) Letting frag plugs become hydroid farms
This is how they persist and re-seed your display.
Notes & Variations
What they look like (practical ID)
In the hobby, Myrionema is often described as:
• tiny starburst polyps
• short fuzzy “tufts” on a creeping mat
• patches that look like “micro grass” or “micro fuzz”
• concentrated around zoa colonies or plug edges
Because many hydroids look similar, the most useful ID combo is:
• small stinging polyps
• creeping spread
• localized zoa irritation/closure
Why they’re so persistent
They’re small, fast to regrow, and able to live in tiny protected spots. You might remove the “visible” part and still have the colony rooted in a crack.
Final Thoughts
Myrionema hydroids are the kind of pest that doesn’t always look dramatic, but they can ruin your enjoyment of zoas because they cause chronic “why won’t this open?” frustration.
The biggest win is spotting them early and treating them as a plug/rack management problem, not a tank-wide chemistry mystery. If you keep your intake/frag infrastructure clean, you prevent most Myrionema from ever establishing.