Chondrus

Chondrus · also called Irish Moss, Carrageen

Chondrus is a genus of cool-water red macroalgae with flat, lobed, branching fronds. C. crispus, Irish moss, is a purple alga whose frond tips can shimmer blue from layered structures in the cuticle. It is the traditional source of carrageenan, and it needs chilled water, so it does not suit a tropical tank.

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Quick facts

Category	Marine & Algae
Family	Gigartinaceae
Native origin	North Atlantic for C. crispus; other species in the North Pacific
Care difficulty	Advanced
Light	Medium light
Pet toxicity	Toxicity unknown

Light

Moderate full-spectrum lighting suits it and shows the blue-violet shimmer. The color is structural, comes from a layered cuticle at the frond tips, and appears only in the gametophyte stage, so not every plant will show it. It needs water to work, so the shimmer vanishes when a frond dries.

Water

Keep it well below 21 C (70 F). Growth falls away by 21-24 C and stops near 26 C, so it needs a chiller and does not suit a tropical tank. Wild plants in New Hampshire grow in water of -1 to 19 C. Salinity ~1.025 SG and pH 8.1-8.4 are fine.

Soil & potting

Grows from a small holdfast on hard surfaces, not in sand. Tie a frond base to rock or rubble with line, or leave it on the stone it came on. Avoid burying the base, and let it settle before you move it. Wild plants grip rock firmly, so a secure footing matters in moving water.

Environment — humidity, temperature, placement

A cool-water alga for a chilled macroalgae tank or temperate system, not a tropical reef. Hold temperature well below 21 C (70 F), because growth falls away by 21-24 C and stops near 26 C. Moderate flow keeps debris off the fronds. Do not plan to mix it with tropical corals, whose needs sit well above this range.

Propagation

Trim lobed fronds and tie a section to rubble to start a new plant. Keepers have little record of growth rates in tanks, and cool conditions are the main limit. The life cycle includes separate gametophyte and tetrasporophyte stages, and only gametophytes show the blue shimmer.

Toxicity detail

No toxicity data for tank animals was found, so the rating stays unknown. It is eaten by people and used for carrageenan, but that is no proof of safety for fish or invertebrates. No grazing deterrent or coral smothering risk is recorded. Remove failing plants from warm water.

Varieties & cultivars

Natural forms are the wild species; cultivars are selectively-bred colour or variegation forms of the same plant.

Natural forms1

Irish Moss

Chondrus crispus. A purple alga with thin, lobed, branching fronds that shimmer blue-violet when lit with full-spectrum light. It comes from cooler water.

A cold-water alga that does not suit a tropical tank. Keep it chilled, well below 21 C (70 F).