AquaticCare difficulty: IntermediateLegal complexity: Low

Black sea rod

Plexaura homomalla · also called Golden sea rod

A bushy Caribbean sea rod with brown to black branches that turn soft and pale once its cream to yellow-brown polyps open. Most of its energy comes from the algae living in its tissue, so it suits a well-lit reef with good flow rather than a dim, heavily fed system. The alternate name golden sea rod is used loosely, so check the animal and not just the label.

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Quick facts

Size	Bushy colony, mostly spreading in one plane. Mature colonies are reported up to about 35 cm tall, and one colony in a spawning study reached about 65 cm.
Lifespan	—
Social needs	solo
Native region	Florida, the Bahamas, Bermuda, the Gulf of Mexico and the Caribbean
Origin	New World
Climate	Tropical
Water type	Marine
Family	Plexauridae
Genus	Plexaura

Part of the Soft Corals

Soft corals such as leathers, colt, cloves, Anthelia, gorgonians and Sympodium. Octocorals with flexible, often swaying colonies and eight-tentacled polyps, and no stony skeleton. Many are hardy, beginner-friendly reef corals driven by photosynthesis and tolerant of a wide range of light, flow and nutrients, but some gorgonians have no algae of their own and depend on regular feeding, so check eac

Anthelia (Waving Hand Polyps)Blueberry sea fan Cabbage Leather Coral Clove Polyps Colt Coral Devil's Hand Leather Finger leather coral Gorgonian Sea Fan Green star polyps Grooved-blade sea whip Isoxya sea fan Kenya tree coral Knobby sea rod Mushroom coral +7 more →

Color & pattern variants

Natural variants occur in the wild; selectively bred (man-made) variants were developed in captivity.

Natural

Black Sea Rod →

The dark form, with brown to black branches and pale yellow-brown to cream polyps.

Golden Sea Rod →

A golden tan colony known by this hobby name. The name is applied loosely, and other tan, tree-shaped rods also carry it.

Habitat & enclosure

Place it in the mid to upper reef with moderate to strong light and moderate to strong flow, enough to keep the branches moving and free of film. Wild colonies grow on nearshore hard bottom, lagoon patch reefs and outer reefs from the shallows to about 30 m, and a slimmer form reaches about 50 m.

Keep stable reef water: SG about 1.025, 24-27 °C (75-81 °F), pH 8.1-8.4, alkalinity 8-11 dKH, calcium 400-450 ppm and magnesium 1250-1350 ppm.

Substrate

Fix the base to live rock or a plug with reef-safe gel glue, or wedge it firmly into a rock crevice so it stands upright in the current. It needs a hard attachment point, not loose sand.

Equipment & setup

Use reef lighting strong enough for a light-dependent coral, with powerheads that give varied, turbulent flow instead of one fixed jet. A protein skimmer and regular water changes keep nutrients down and discourage film on the branches.

Diet

Relies on light. It keeps zooxanthellae, and the sugars and amino acids they pass across are its main energy source, so steady lighting comes first. Its polyps also catch zooplankton in the wild, which makes fine plankton foods a useful extra a few times a week. Food supplements light here. It does not replace it.

Behavior & temperament

Open polyps give the colony a fuzzy outline and closed polyps leave it dark and smooth, so the look can change within a day without anything being wrong. Two forms are recognised: forma homomalla, with stiffer, thicker branches nearer the surface, and forma kuekenthali, with slimmer branches in deeper water. The species is well known to chemists for carrying prostaglandins at around 2-3% of its dry weight, and what they do for the animal is still unclear. Wild colonies may last a few decades, though no firm lifespan is on record.

Health

The main risk is algae or cyanobacteria settling on the branches where flow is too weak, which can kill the tissue underneath. Peeling tissue, bare axis or polyps that stay shut for days all point to too little flow, too little light or poor water. Inspect new colonies closely and correct flow before anything else.

Tips, DIY & hacks

Sort out light and flow first, and treat food as the extra. Because the branches mostly spread in one plane, turn the broad face of the colony toward the main current. If a piece is labelled golden sea rod, look at it before assuming it is this species, since that name also lands on other tan, tree-shaped rods.