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

Because it builds a hard skeleton, it needs stable calcium, magnesium, and alkalinity, the same demands as stony corals, plus moderate light and some flow. Wedge it into rock so it can anchor, and keep reef chemistry steady for good growth. Most herbivores ignore it, since the calcified segments are unappealing to eat.

In the wild

Halimeda tuna is built from chains of flat, coin shaped green segments stiffened with calcium carbonate, giving it a jointed, cactus like look. It grows attached to rock and rubble on the reef and holds its shape thanks to its chalky skeleton. In the aquarium it makes an unusual, sculptural macroalga for the display or refugium.

Good to know

Halimeda lays down calcium carbonate 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.

Did you know

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