Sea lettuce

Geographic Origin and Habitat: Cosmopolitan par excellence, Ulva lactuca (or Sea Lettuce) is ubiquitous in oceans worldwide. It tolerates any type of climate zone, from the cold North Seas to the equator. It thrives in very shallow coastal waters, brackish estuaries, river mouths, and intertidal zones subject to heavy urban organic pollution (eutrophication). It is often found torn from the bottom, floating freely in vast surface agglomerations moved by the waves.

Taxonomy and Genetics: It belongs to the Ulvophyceae class. It is a simple multicellular green alga with an alternating life cycle (visually identical haploid and diploid generations). Its incredible adaptability stems from the fact that every single cell of its tissue is capable of photosynthesis, nutrient absorption, and reproduction, making it essentially a living, essential cellular sheet without any true specialized organs.

Physical Structure: It has no stems or branches. The alga consists of a single large, extremely thin foliaceous blade (laminar thallus), composed of just two superimposed layers of cells. The edges of the blade are often wavy, crumpled, or irregularly torn. It anchors weakly to rocks via a tiny basal holdfast, but can survive indefinitely adrift (free-floating) or entangled around submerged structures.

Color and Texture: The color is a typical bright pea green or emerald green. The texture is impalpable and translucent: if held up to the light, you can literally look through it. To the touch, it is silky but extremely fragile and slippery, similar to wet tissue paper. It is the only algae commonly used in the aquarium hobby that is also employed in human nutrition and haute cuisine.

Lighting and CO2: A photosynthetic monster. To proliferate, it requires brutal lighting; the stronger, the better. PAR values exceeding 300 or direct sunlight push it to exorbitant growth rates. Under weak light, it rots rapidly, disintegrating into mush. There is no need to supplement CO2, but due to its immense photosynthetic rate, the aquarium must have a strong reserve of carbonates and buffers, as the Ulva will significantly raise the pH during the day.

Nutrition and Substrate: It is the nightmare of pollutants. It is used in bioreactors to break down nitrates and ammonia. To keep it intact and green, it needs an astonishing load of organic nutrients. If the aquarium's nutrients (nitrates and phosphates) drop to optimal reef levels, the Ulva will turn white, become riddled with holes, and die of starvation. It needs no substrate other than water turbulence.

Water Chemistry: Totally indifferent to thermal or salinity fluctuations, which it tolerates from light brackish to hyper-saline, and from 10 to 30°C. The only precaution is the replenishment of microelements (especially Iron), which it burns through at impressive speeds due to its primitive, hyper-accelerated metabolism.

Space Management and Placement: Aesthetically, it is messy and difficult to anchor. In the display, it will tend to tear and tangle its transparent fragments among the corals and in the wavemaker grates. Its primary use is in the sump, inside a large open refugium or in special cylinders with rotational flow (tumbling) that keep the sheets constantly in suspension under blinding light.

Trimming: Pruning is not only necessary but frequent (sometimes weekly). No tools are needed: simply tear away the excess leaves. Removing Ulva means extracting enormous masses of nitrates from the tank. The removed blades are the perfect, natural, zero-cost food for any tang, rabbitfish, or blenny species, which will devour it eagerly.

Risks and Diseases: If grown in refugiums lacking adequate flow, it tends to compact into layers. Because the blade is two cells thick, the layers that do not receive light rot within a day, releasing phosphates and creating dangerous black pockets of hydrogen sulfide. The key to Ulva lactuca is keeping it vigorously tumbling to prevent anoxia of the underlying tissue.