Plexaura homomalla is a coral similar in appearance to gorgonians, belonging to the order Alcyonacea and family Plexauridae. In this family of Anthozoans there are 43 different genera, one of them, Plexaura, that contains together with Plexaura homomalla another 42 species. This species is widely distributed in the waters of the Caribbean Sea and the Gulf of Mexico. It grows at depths of up to 55 meters.
This colonial coral adopts an arborescent or candelabra shape, with vertical branches. Plexaura homomalla tends to branch in a single plane with dichotomous branches, although it is also possible to observe lateral branching. The colony can reach a height of up to 35 cm. The central stem as well as the branches are black or very dark brown and hence their colloquial names, black sea cane or Caribbean sea whip. Completely covering the branches are small cream, yellowish or pale brown polyps. These polyps arise from small calyxes embedded in the branches themselves, so that the calyx does not protrude from the branch or does so very slightly.
There are two variants of this species, Plexaura homomalla kuekenthali whose colonies are taller and thinner, with narrower branch ends and generally more abundant at greater depths; and the second variant or form is Plexaura homomalla homomalla with less tall colonies, with more robust branch ends and found predominantly in shallower waters.
A curiosity of Plexaura homomalla is that its tissues contain an unusually high level of a lipid called prostaglandin. Various derivatives of this compound have been widely used as anti-inflammatory drugs in medicine. Plexaura homomalla has levels of this compound up to 1,000,000 times higher than what we would find in any other animal, with the weight of prostaglandin present in a colony representing between 2% and 3% of the total colony weight. In fact, it was not until the appearance of the first methods of obtaining prostaglandins by chemical synthesis during the 1970s that the compound was no longer obtained by harvesting this species. The role of these very high levels of prostaglandins is not known for certain, although the most plausible and widely accepted hypothesis is that they are used by this coral to induce vomiting in any predator that tries to engulf part of the colony.
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