Propagating Ulva intestinalis (Gutweed) in Marine Refugiums

How to propagate Ulva intestinalis by dividing its tubular thallus for fast nutrient export, tang food, and lush green growth in marine and brackish refugiums.

Overview

Ulva intestinalis, commonly called gutweed, is a green macroalga whose branched fronds are completely tubular, expanding in width toward the mid-thallus and reaching 15 cm or more in length. Cells are irregularly arranged and carry hood-shaped chloroplasts, typically with a single pyrenoid. It is a cosmopolitan species found across temperate and polar regions and tolerates variable salinity, which makes it well suited to marine, tide-pool, and brackish setups.

In an aquarium it grows fast and exports nitrogen and phosphorus quickly, while the soft tubular sheets are readily grazed by tangs and other herbivores. Because the species reproduces throughout the year and follows an alternation of generations with isomorphic gametophyte and sporophyte stages, you can multiply your stock easily by simple division.

Propagation Method

Gutweed is propagated vegetatively by dividing or tearing the thallus. Each separated piece of tubular frond continues to photosynthesize and elongate on its own, so there is no need for cuttings rooted into substrate. Free-floating clumps tumbled in a refugium grow readily without anchoring.

Step-by-Step

Select a healthy, bright-green clump with firm, inflated tubular fronds.
Gently tear or cut the thallus into several pieces, each a few centimeters across.
Place the divisions in a well-lit refugium or grow-out area with gentle flow.
Let pieces tumble freely or rest loosely; no substrate anchoring is required.
Within days the fragments resume rapid elongation and thicken into new clumps.

Conditions for Healthy Growth

Strong lighting to drive the high growth rate typical of this species.
Stable marine to brackish salinity; the species tolerates variable salinity well.
Temperate-friendly temperatures, reflecting its worldwide temperate and polar range.
A steady supply of nitrogen and phosphorus, which the alga rapidly takes up.
Gentle, continuous water movement to keep clumps tumbling and evenly lit.

Maintenance

Harvest and thin the colony on a regular cycle. Removing a portion of biomass physically exports the absorbed nitrogen and phosphorus from the system, and the trimmings make excellent fresh food for tangs and other grazers. Frequent thinning also keeps the fast-growing mat from shading itself.