Watercress algae

Halimeda tuna

Also known as Coin algae, Calcified green algae, Watercress alga

SaltwaterMacroalgaeFragmentation

Watercress algae (Halimeda tuna) — Diego Delso · CC BY-SA 4.0

Care at a glance

Light: Medium light
CO2: No CO₂ needed
Temperature: 73–81°F; 23–27°C
Max height: 5.9 in; 15 cm

Growth rate: Slow
Placement: Midground
Type: Macroalgae
pH: 8.0–8.4
Spread: 7.9 in (20 cm)
Nutrient uptake: Water column
Growth region: Submersed
Difficulty: Medium
Family: Halimedaceae
Origin: A calcified green macroalga of tropical and subtropical reefs in the Indo-Pacific, Caribbean, and Mediterranean, anchored to rock from shallow water down to deep reef slopes.

Overview

Watercress algae is a calcified green macroalga with a jointed, cactus-like look, an unusual sculptural piece for a reef display or refugium. It is rated medium in difficulty.

Did you know

When watercress algae dies, its skeleton crumbles into fine white carbonate sand. Over long stretches of time, these algae are major builders of tropical reef sand and sediment.

Caring for watercress algae

Wedge watercress algae into the rock so it can anchor. Give it some flow, and keep it at 73 to 81°F (23 to 27°C) and pH 8 to 8.4.

It builds a hard skeleton, so it needs stable calcium, magnesium and alkalinity, the same demands as stony corals. Keep reef chemistry steady for good growth. Medium light suits it, and it needs no added CO2.

Growth is slow. Pieces that break off keep growing, so a fragment can start a new plant. Most herbivores ignore it, since the calcified segments are unappealing to eat.

Good to know

Watercress algae, Halimeda tuna, is a green macroalga in the family Halimedaceae. It is built from chains of flat, coin-shaped green segments stiffened with calcium carbonate. It lays that carbonate down as aragonite crystals inside its segments, so the alga is partly mineral. The living tissue is a single continuous cell system threaded through the calcified skeleton.

In the wild

Watercress algae grows on tropical and subtropical reefs in the Indo-Pacific, the Caribbean and the Mediterranean. It anchors to rock and rubble from shallow water down to deep reef slopes, and its chalky skeleton lets it hold its shape there.