Marine & AlgaeAdvanced🌤️ Bright indirect

Mermaid fan

Udotea sp. · also called Mermaid's fan, Mermaids fan, Fan algae, Udotea

🐾 Pet-safe

Generally non-toxic to cats and dogs.

Udotea is a calcified green macroalgae that grows as a single fan-shaped blade on a stalk rooted in sand, resembling a small mermaid's fan. It is decorative but more demanding, needing a mature deep sand bed and stable calcium/alkalinity.

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Quick facts

Category	Marine & Algae
Family	Udoteaceae
Native origin	Tropical western Atlantic and Caribbean reef and seagrass flats; sandy lagoon bottoms
Care difficulty	Advanced
Light	Bright indirect
Pet toxicity	Pet-safe

Light

Moderate-to-strong reef lighting (medium-high aquarium light / good PAR) supports its calcified blade. Display reef or strong refugium LEDs work; insufficient light leads to decline. Mapped to bright lighting needs.

Water

Reef parameters: 23-27 C (74-80 F), salinity ~1.025 SG, pH 8.1-8.4. As a calcifier it draws on calcium (~400-450 ppm), alkalinity (8-11 dKH), and magnesium (~1300 ppm); keep these stable. Prefers established systems with low-to-moderate nutrients. No CO2 (marine).

Soil & potting

Unlike most macros, it roots into the substrate with a buried holdfast/rhizoid system, so it NEEDS a deep, mature live sand bed to anchor and draw nutrients. Plant the stalk base into the sand; do not leave it loose. No aquasoil/root tabs - it uses the live sand bed.

Environment — humidity, temperature, placement

Submersed only. No CO2 (marine). Gentle-to-moderate flow; too much current can shred the delicate fan. Best in a display or refugium with an established deep sand bed. Foreground/midground placement on the sand. A slower, more sensitive species suited to experienced keepers.

Propagation

Reproduces by growth of new fans from the rooted base and can spread via rhizoids through the sand bed; does not propagate easily by simple cutting because of its calcified single-blade structure. Generally allowed to grow and spread on its own rather than fragmented.

Toxicity detail

Reef-safe and non-toxic; its calcified tissue deters grazers, so it survives in herbivore tanks. Not invasive or regulated in the hobby. Dying fans bleach pale and should be removed to prevent nutrient release.

Growth stages

How this plant changes through its life — each stage often has its own care, diet and space needs.

Photo coming soon

Spore / recruit

Aquatic plants and macroalgae establish from spores, seeds, or drifting fragments that settle and attach to substrate or rock. Many freshwater aquarium plants and marine macroalgae also spread readily from a detached piece that takes root or holdfast.

Photo coming soon

Young growth

Young growth puts out its first blades, fronds, or leaves and anchors with roots or a holdfast. Submersed plants may look different from their emersed form, and growth speeds up as the plant adapts to the water's light and nutrients.

Photo coming soon

Mature

A mature aquatic plant or macroalga reaches its full size and characteristic shape, forming the dense growth, runners, or fronds typical of the species. Established plants spread to fill space and can be divided or trimmed to propagate.

Sources

Udotea - Wikipedia (encyclopedia)
Udotea (Mermaid's Fan) Calcified Macroalgae - Reef2Reef (care guide)