The Green Hairy Mushroom Coral is a common sight on Indo-Pacific coral reefs and shows up regularly in aquarium exports, but, while it might seem harmless enough, within its slimy tissues lurks something sinister. One could make a strong argument that this is mankind’s deadliest coral scurge, a marine menace of shocking lethality. But for all the danger posed by Rhodactis, surprisingly little attention has been paid to it.

The only published research appeared in the 1960’s by Edgar J. Martin of UC Berkeley, who became aware of this phenomenon while employed with the Dept. of Medical Service on American Samoa. Here, where this coral grows abundantly in the shallows, the local islanders harvested it as a food source, known to them as “matamalu”. When boiled, Rhodactis is apparently quite safe to eat; however, if consumed raw, the result can be an untimely death. The symptoms start shortly after ingestion, with an anesthesia-like stupor that includes the loss of involuntary motor reflexes and pupillary dilation. During this time, blood pressure and heartbeat remain normal, but, after anywhere from 8-36 hours in this condition, patients go into shock, with death coming from pulmonary edema—the filling of the lungs with fluid. During his brief stay in American Samoa, Martin encountered three cases of Rhodactis poisoning, each of which resulted in death. He reports that locals are suspected of using this as a means of suicide.

In the decades since, little else has been discerned on the nature of this toxin. Martin struggled to purify it fully, owing in part to the mucilaginous nature of this organism, which interfered with his equipment. Eventually he arrived upon a technique of centrifugation and acidification that allowed him to deduce that the molecule was a protein… and that’s pretty much all we know about it to this day.

With his Rhodactis extract, Martin showed that this toxin, whatever it might be, was lethal to just about every vertebrate he injected it into: salamanders, toads, mice, rats, rabbits. To kill a mouse within 4 hours requires just 3mg/kg of weight. It caused little in the way of physiological damage, leading to the conclusion that it was acting primarily on the central nervous system. This is similar to the causative agent of Paralytic Shellfish Poisoning, but there are a few differences which suggest the two are not identical. The prolonged stupor which precedes death in Rhodactis poisoning is unlike the rapid lethality of PSP, and the toxicity shows no seasonal variation (whereas the toxicity in shellfish is caused by seasonal blooms of dinoflagellates).

There are so many interesting unanswered questions. Is it just the large Green Hairy Mushroom Coral which causes this, or are all Rhodactis equally toxic? And what about Discosoma and Ricordea and Platyzoanthus and Amplexidiscus? Though Martin identifies the offending species as Rhodactis howesii, in truth, we have no idea what the correct identity of this coral is until the group is taxonomically revised. And then there are the questions of what this toxin is precisely and what its effect is on the nervous system of vertebrates. Does the coral produce this substance itself, or is it made by a symbiotic algae/bacteria in its tissue, or might it be sequestered from something in its diet. And, for that matter, what does a Rhodactis eat in the wild?

Martin posited that the lethality of this coral might simply be a biochemical happenstance, hypothesizing that this nameless toxin might serve some key metabolic role for cnidarians, but, when accidentally introduced into a vertebrate, it instead induces an uncontrollable and unintended reaction. This could very well be correct, but a more plausible explanation may be that this is a potent defensive compound that serves to reduce predation by coral reef fishes. As aquarists already know, there’s not much that will eat a Hairy Mushroom Coral.

• Martin, E.J., 1960. Observations on the toxic sea anemone, Rhodactis howesii (Coelenterata). Pacific Science 14: 403-407
• Martin, E.J., 1966. Anticoagulant from the sea anemone Rhodactis howesii. Proceedings of the Society for Experimental Biology and Medicine, 121(4), pp.1063-1065.
• Martin, E.J., 1966. The macromolecular toxin of sea anemones (Coelenterata). The Galapagos, Proc. Symp. Galapagos Int. Sci. Project, pp.136-138.
• Martin, E.J., 1967. Antitoxin against a coelenterate poison. International Archives of Allergy and Immunology, 32(3), pp.342-348.