Plexaura homomalla
A Caribbean gorgonian coral rich in prostaglandin used as a drug source.
The black sea rod, or Caribbean sea whip, is a gorgonian octocoral from the family Plexauridae. Its range stretches across the Caribbean, from the Florida Keys down to Venezuela’s northern coast. This coral is best known for holding a bioactive lipid called prostaglandin A2 15-acetate methyl ester, which makes up about 3% of its total wet weight—a concentration roughly a million times higher than in most other animals. Before chemists found a way to synthesize it in the 1970s, the coral served as a source for this compound.
The colony grows like a bushy candelabra, branching mostly in a single plane. It has a few dichotomous forks but mainly bears lateral branches, reaching about 35 cm (14 inches) tall. Two forms exist: *P. h. forma kuekenthali* is taller and slimmer, with narrow terminal branches, while *P. h. forma homomalla* is shorter with sturdier terminal branches. The central stalk and branches are black or dark brown, and the polyps are cream, yellow, or pale brown. The calyces that hold the polyps usually don’t stick out, though they may be slightly raised.
The coral lives on reefs in the Caribbean Sea, Gulf of Mexico, Bahamas, and Florida, at depths down to about 55 m (180 ft). The *homomalla* form is more common in shallow water, while *kuekenthali* tends to be found deeper.
Why the coral stores so much prostaglandin is unknown. In mammals, the substance acts as a muscle relaxant, used to induce labor, and can cause nausea and vomiting. In feeding trials, killifish and yellowhead wrasse vomited after eating gelatin pellets laced with prostaglandin, and they quickly learned to avoid those pellets while accepting plain ones. This suggests the prostaglandin serves as a chemical defense, deterring predatory fish.
Quick Facts
- Taxon
- Plexaura homomalla
Facts from the source article.
Lore & Background
Plexaura homomalla forms a bushy or candelabra-like colony of upright branches, tending to branch in a single plane with a few dichotomous forks but mostly bearing lateral branches. It grows to a height of about 35 cm and has two forms: P. h. forma kuekenthali is taller and slimmer with narrow terminal branches, while P. h. forma homomalla is shorter with more robust terminal branches. The central stalk and branches are black or dark brown, and the polyps are cream-coloured, yellow, or pale brown. The calyces from which the polyps protrude are typically not projecting, but may be slightly raised above the surface.
The coral is found growing on reefs in the Caribbean Sea, the Gulf of Mexico, the Bahamas, and Florida, at depths down to about 55 m. P. h. forma homomalla is more abundant in shallow waters, while P. h. forma kuekenthali occurs mostly at greater depths. The function of the large amount of prostaglandin in the coral is unknown, but in mammals it acts as a muscle relaxant, used to induce labour and causing nausea and vomiting.
Reader's Guide
Plexaura homomalla is significant primarily for its exceptionally high concentration of prostaglandin A, a physiologically active lipid that is a million times more abundant in its tissues than in most other animals. This substance was used as a source for the drug until methods for its synthesis became available in the 1970s. The coral contains 2 to 3% by weight of prostaglandin. The ecological role of this compound appears to be chemical defence: in feeding trials, killifish (family Cyprinodontidae) and yellowhead wrasse (Halichoeres garnoti) vomited after being fed gelatine pellets containing prostaglandin, and the fish quickly learned to avoid these pellets while accepting control pellets. This suggests the prostaglandin acts as a deterrent to predatory fishes. The coral's two forms—one taller and slimmer found at greater depths, the other shorter and more robust in shallower waters—show morphological variation across its depth range. Its distribution spans from the Florida Keys to the northern coast of Venezuela, making it a widespread Caribbean species.
Did You Know?
- Plexaura homomalla contains prostaglandin at about 3% of total wet weight, a million times more than in most other animals.
- The coral was used as a source for the drug prostaglandin until synthetic methods became available in the 1970s.
- Killifish and yellowhead wrasse vomit after eating gelatine pellets containing the coral's prostaglandin.
- The coral has two forms: one taller and slimmer at greater depths, and one shorter and more robust in shallow waters.
More in Soft Corals and Leathers, Part 2 1-24
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