Green Star Polyp

Green star polyp (GSP) is one of the corals most often recommended to those starting a reef aquarium. The colony consists of a thin, flexible purple mat attached to the rock surface; when the lights come on, hundreds of small, bright green polyps emerge from this mat and sway in the current like a meadow. Each polyp has eight feathery tentacles, a trait that places it among the eight-armed corals (Octocorallia). It has no hard skeleton.

Its main problem in the aquarium is the price of its hardiness: under suitable conditions it spreads nonstop and covers every surface in its way. For this reason its spot should be planned from the start. Because it is not a stony coral, it is not on the CITES list; most pieces for sale are aquacultured colonies.

It lives in the warm, shallow waters of the Indo-Pacific, on reef flats, in lagoons and on fringing reefs with strong wave action. It forms wide carpets covering dead coral skeletons, rubble and hard bottoms. Being used to an environment that occasionally turns turbid and where light and flow change frequently is the main reason it is so tolerant in the aquarium.

• Light: It adapts to everything from low light to strong reef lighting; it shows its fullest color under moderate light (roughly 100–250 PAR). If the light is increased suddenly, the polyps may stay closed for a few days.
• Flow: It needs moderate to strong flow that changes direction. In still water detritus settles on the mat and the polyps do not open.
• Placement: Put it not on the main rock structure but on a stone standing alone on the sand (an island); since it cannot cross the sand, its spreading stays limited.
• Water parameters: Salinity 1.024–1.026; KH 7–11 dKH, calcium 400–450 mg/L, magnesium 1250–1350 mg/L. It is better for nitrate and phosphate to stay low but measurable and stable rather than dropping to zero.

Symbiotic algae called zooxanthellae in its tissue produce food from light and cover most of its needs. The polyps also take in very small particles and dissolved organic matter from the water. No target feeding is needed; the fine leftovers from fish feeding that disperse in the water are enough.

It has no stinging tentacles or sweeper arms; it does not directly harm its neighbors. The danger is that it spreads its mat under and over neighboring corals, shading them and eventually smothering them. Leave at least 10–15 cm of space between it and neighboring corals; the safest solution is an island rock.

It causes no problems with clownfish, gobies, blennies and cleaner shrimp. Dwarf angelfish and butterflyfish may pick at the polyps; a colony that is constantly picked at stays closed for days.

It is very easy to propagate. A strip of the purple mat is cut with a sharp knife or scissors and attached to a small stone or frag plug with coral glue or a rubber band. It attaches to the new surface within a few days. Pieces that spread on their own onto the glass or a pump can be lifted off with a razor-blade scraper and moved to another stone.

• Polyps not opening: Relocation, flow or KH swings and detritus settling on the mat are the most common causes. The colony occasionally closes and sheds the thin waxy film on its surface; once the film lifts off in the flow, the polyps open again.
• Algae covering the mat: When weak flow and high nutrients come together, filamentous algae wrap around the purple mat. Increase flow and lower phosphate.
• Uncontrolled spreading: Cut back and remove the spreading edges while they are small; once it gets into the pores of the rock, cleaning it off becomes very difficult.