The lovable crown-of-thorns

If bottlenose dolphins are among the most adored forms of marine life, the crown-of-thorns starfish (Acanthaster planci, family Acanthasteridae) might miss the lowest popularity ranking only because not everyone is familiar with them. First of all, they occur solely in the Indo-Pacific, from eastern Pacific outliers near Central America, west to Polynesia, all the way to the Red Sea, and usually in close association with their favorite food, corals. Second, they are largely nocturnal, except in cases of extreme population explosions, when the competition for food seems to drive them to feed actively in the daytime as well as at night.

For some years the devastation of significant coral areas by hordes of these starfish alarmed the public to the extent that dive expeditions sought to intervene in the carnage, and all of the fuss even made international news. Some scientists speculated about whether pollution and other human-induced environmental changes were exacerbating the situation. Further research indicated that these population booms were probably within the bounds of normal reef-ecosystem dynamics and did not warrant interference. In fact, the cropped-back coral areas grew back vigorously and with renewed diversity.

But that still didn’t get the crown-of-thorns completely off the hook. Wendy and I spotted our first one during the early part of our Pacific voyage, while scuba diving at Cocos Island off Costa Rica. Later that year we saw them regularly while assisting a charter dive operation at Christmas Island (Kiribati), usually under ledges and hiding in holes along the reef during daylight hours. One day I decided to extract one from a hole to show a visiting diver.

The familiar blast of a venomous sensation hit me the moment the spines pricked through my cloth and rubber gloves, and I quickly abandoned my plan before the situation got serious. A brief consultation of our textbook library later aboard élan revealed that the skin covering the thorns is what produces the venom, and that serious punctures can cause extreme pain, swelling, redness, nausea and vomiting, and victims may go into shock. The consensus was to wash the wounds thoroughly with antibacterial soap, then plaster them with drawing salve (e.g., ichthymall or a similar product, now commonly available over the counter); cover with a sterile bandage, and then settle in for an evening of lively sensations in the afflicted area.

Despite such nasty protective armor, crown-of-thorns starfish have several natural predators, including the triton trumpet (family Ranellidae, Charonia tritonis) and two large reef fishes, the humphead wrasse (Cheilinus undulatus) and the titan triggerfish (Balistoides viridescens). Otherwise they appear to perish in large numbers after they have decimated local food sources in the course of a population explosion, apparently from starvation. We never encountered more than the odd individual or two while diving in various parts of Polynesia, Micronesia, Australia and briefly in Melanesia.

Wandering seafarers can add this interesting echinoderm to their observation list quite easily with no more than a dinghy snorkeling expedition to an area of well-developed hard coral reef. The first impression, especially in profile, is of a large, spiky half-sphere. Viewed from above, multiple short arms radiate from the rounded main body. These are robust, obvious invertebrates that can measure 20 inches across. Color is variable, ranging from gray or brown to deep red. One good way to suffer venomous thorn punctures is to grope blindly under a hole or ledge in the reef, a practice for which a crown-of-thorns encounter is only one of many dangerous possibilities. Night divers will more often observe these starfish out on top of the coral, actively feeding on polyps.

The dazzling display of sea life on tap around tropical and subtropical islands seduces many into focusing on the big picture, developing the habit of moving along while mentally registering larger images like crown-of-thorns starfish. I invite you to stop and focus on the close-up view, either fine details of a big organism or tiny inhabitants living in or on the reef. One memorable such magnification is to put your dive mask a few inches from a crown-of-thorns. The multitude of sharp venomous spines, frequently over an inch long, forms a dense thicket based on a smooth, surreal template, as though someone poured a thick, creamy batter onto an oversized pincushion.

Crown-of-thorns starfish represent no imminent threat to coral reefs, and their venomous spines are easy to avoid. This leaves us simply with another magnificent reef creature to observe and enjoy.

By Ocean Navigator