Bryopsis

Bryopsis · also called Fern Bryopsis, Hen Pen, Feather Algae

Easy to grow and hard to remove. Bryopsis is a fine, feathery green macroalgae that reef keepers often dread because it grows so fast. In the right conditions it can overrun a whole system and is very hard to remove. It belongs only in a macroalgae tank where you want it long term, never in a reef you hope to keep free of it.

Educational only. Some KinStation entries are reviewed by licensed veterinarians, but none can replace an in-person exam. Always consult a licensed veterinarian or board-certified specialist for diagnosis, treatment, or any decision affecting your pet's health.

Quick facts

Category	Marine & Algae
Family	Bryopsidaceae
Native origin	Coastal seas worldwide
Care difficulty	Beginner
Light	Medium light
Pet toxicity	Toxicity unknown

Light

Grows readily under normal reef lighting, so moderate light is plenty in a dedicated macroalgae tank. Stronger light mainly means more trimming. Keepers do not report any lighting that stops it, so light control alone will not clear an outbreak in a reef tank.

Water

Standard reef parameters: 23-27 C (74-80 F), salinity ~1.025 SG, pH 8.1-8.4. Nutrient control helps slow it. In reefs where it is unwanted, keepers have tried raising magnesium or dosing fluconazole, with mixed results, and fluconazole can harm other macroalgae too. Neither method has controlled studies behind it, and a die-off can spike nutrients.

Soil & potting

Grows from small, root-like holdfasts on rock and other hard surfaces, not from sand. These holdfasts embed in the rock, which is why plucking it rarely works. In a macroalgae tank, tie a tuft to rubble and let it spread from there. Keep that rock out of any reef tank.

Environment — humidity, temperature, placement

Suits a dedicated macroalgae tank or isolated refugium with moderate flow at standard reef temperature, 23-27 C (74-80 F). Do not share rock or equipment with a reef you want free of it, because small fragments travel and settle. Never add it to a display where it is not wanted.

Propagation

Spreads fast from fragments and from holdfasts left in the rock, and pulling it by hand can scatter pieces around the tank. To keep a culture going, simply trim it back and move a tuft to fresh rubble. Bag all trimmings, because this alga regrows from what is left behind.

Toxicity detail

No source covers cats or dogs, so the rating stays unknown. Some Bryopsis hosts bacteria that make kahalalides, toxins that deter generalist fish, though harm to tank animals is not documented. Elysia sea slugs such as E. crispata feed on it, but they rarely clear it. It can overgrow rock and corals if it takes over a reef.

Varieties & cultivars

Natural forms are the wild species; cultivars are selectively-bred colour or variegation forms of the same plant.

Natural forms2

Fern Bryopsis

Bryopsis sp. A delicate, feathery green alga that reefkeepers often dread because it grows so fast. Given the right conditions it can take over an entire system.

Keep it only in a macroalgae system where you want it long term, because it is very hard to remove. Many Elysia sea slugs feed on it.

Hen Pen

Bryopsis plumosa. A delicate, plume-like green alga that reefkeepers often dread because of its speed. It can take over a system when conditions suit it.