Hypnea · also called Blue Hypnea, Purple Hypnea, Tattered sea moss
Hypnea is a red macroalgae that grows as a tight, branching clump. Blue Hypnea (H. pannosa) is prized for a blue-violet sheen made by reflected light rather than pigment, so it fades under actinic lighting. It is delicate, and new pieces often look dull grey before they color up.
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Quick facts
Category
Marine & Algae
Family
Cystocloniaceae
Native origin
Tropical and subtropical seas; H. pannosa is common in the Indo-West Pacific
Care difficulty
Intermediate
Light
Bright indirect
Pet toxicity
Toxicity unknown
Light
Wants bright full-spectrum reef light. The blue sheen is reflected light, so actinic-only lighting hides it and deep blue settings can mask branch detail. Wild H. pannosa grows in brightly lit shallows. A grey new piece usually colors up once it settles, so do not write it off.
Water
Standard reef parameters: 23-27 C (74-80 F), salinity ~1.025 SG, pH 8.1-8.4. Keepers report best results with stable salinity and measurable nitrate and phosphate rather than stripped water. Avoid sudden swings in calcium, alkalinity or magnesium. Some keepers dose iodine, though that advice is anecdotal.
Soil & potting
Grows on hard surfaces, not rooted in sand. The simplest start is to tie a clump onto a chunk of rubble with monofilament line and let it fasten itself. Purple forms also attach to substrate and stay compact. Wild H. pannosa favors hard bottoms in shallow water.
Environment — humidity, temperature, placement
Broad moderate-to-strong flow keeps debris off the branches; a brown coating usually means detritus is settling in weak flow. It suits a well-lit display or refugium at standard reef temperature, 23-27 C (74-80 F). Keep it out of dead spots and avoid moving it once placed.
Propagation
Grows slowly, so heavy pruning takes a long time to replace. Snip small sections from an established clump and tie them to fresh rubble. Trim lightly and handle it as little as possible, because it is fragile when moved. Give each cutting good light and steady flow while it settles.
Toxicity detail
No source checked reports harm to fish or invertebrates, so the rating stays unknown. It is not a strong grazing deterrent: tangs, rabbitfish and some angelfish may eat it, which can strip a display piece. No smothering of corals was reported for kept forms, but keep growth trimmed back. A relative, H. musciformis, is invasive in Hawaii, so never release any Hypnea or tank water.
Varieties & cultivars
Natural forms are the wild species; cultivars are selectively-bred colour or variegation forms of the same plant.
Natural forms2
Blue Hypnea
Hypnea pannosa. A red alga whose surface reflects blue and violet light, so it reads as an iridescent bluish purple. The color comes from reflection rather than pigment, which is why it vanishes under actinic lighting. Fresh pieces often look dull grey until they settle under good light.
Keep it under full-spectrum light, since actinics alone will not show the blue. Notably fragile, and a grey cast at first does not mean it has died.
Purple Hypnea
Hypnea sp. A Hypnea that fastens to substrate and forms a compact, tightly branched clump, much like Blue Hypnea in habit. Fresh pieces often look grey before coloring up under good light.
Care matches Blue Hypnea: give it full-spectrum light, and do not mistake early grey color for a dead plant. Somewhat fragile when moved.
KinStation
Bright indirect
Keep it under full-spectrum light, since actinics alone will not show the blue. Notably fragile, and a grey cast at first does not mean it has died.