Dive Dry with Dr. Bill

072: Crown-of-Thorns Starfish

'Tis the season I thought as I dropped down into the waters off Half Moon Bay, Isla San Francisco, in the Sea of Cortez with my dive buddy Stephanie. The water temperature was warmer than ours on Catalina, so that wasn't what put me in the holiday spirit. No shivering and shaking like in the Chicago holidays of my youth (81 below one year). However, all around me were reminders that the season was approaching... Christmas tree worms popping back into their tubes as I approached, tinsel squirrelfish darting back under cover, reef coronetfish seeming to play "Little Drummer Boy," and beautifully colored reef fish darting in and out like animated ornaments (or extras from "Finding Nemo"). Truly the joy of the season.

Then Stephanie pointed out something that reminded me of the other end of the story, a crown-of-thorns starfish. Pontius Pilate would no doubt find this invertebrate a worthy substitute for the "crown" he placed on one who only wished to bring peace and joy on Earth. These starfish lack the beauty of the Panamic cushion stars found nearby, but they have their own strange appeal. Looking every bit like a densely-populated pin cushion with their many arms covered in rose-like thorns, these sea stars beg one to keep their distance. Of course I could not resist lifting the starfish off the coral head it was feeding on. Ouch!

This species has toxins (saponins which are related to steroids) that can enter through the holes pierced in my fingers by the spines. Symptoms range from intense pain and swelling to nausea and vomiting which may last hours to days. Some humans have an allergic reaction to the toxin. Fortunately my symptoms were minor swelling but no pain (tough skinned aquaman that I am). Of course Stephanie proved smarter than me, keeping her distance from the starfish (and me! darn). However, I was determined to film the stomach turned inside out to digest the coral polyps externally before taking them into the "mouth."

Crown-of-thorns starfish have a reputation for feeding on coral. The damage they have done in the reefs of Australia and Hawaii has been extensive. That species occurs throughout the Indo-Pacific region from the Red Sea and the shores of East Africa through Micronesia, the South Pacific and according to some scientists to Panama and the Gulf of California (Sea of Cortez). Other marine biologists believe there are several different species, and that the one in the Gulf of California is a close relative of the terror in Australia but not the same species.

In the islands I dived recently it isn't unusual to find these starfish feeding on isolated coral heads. Afterwards, the living polyps are destroyed and the coral head turns from a greenish or reddish color to a stark white. Only the calcium carbonate coral "skeleton" remains. It is reported that when hard corals are scarce, these starfish will subsist on algae, soft corals, snails and even cannibalize one another when things are really tough (an underseas Donner party?).

On the Great Barrier Reef in Australia outbreaks of these seastars occur with up to millions of individuals engulfing the slow-growing coral structures. These outbreaks may result from changes in nutrients, temperature, salinity or other conditions possibly altered by human activity. Some biologists think man may have decreased the numbers of their natural predators causing their numbers to increase unnaturally. Population explosions have also been recorded in the Ryukyu Islands (south-west of Japan), Micronesia, American Samoa, the Cook Islands, Fiji, the Society Islands, the Red Sea and Hawaii with new ones occurring in the Maldives and East Malaysia.

The populations in the Gulf of California are much smaller and do not seem to evidence these devastating increases. In these neighboring waters, temperatures are warmer than Catalina, but still not high enough to allow the growth of reef-building corals except in the southern region at Cabo Pulmo. Rocky surfaces around the islands in the La Paz region have scattered heads of elegant, giant and other hard corals. Because the coral heads are usually separated from one another, the local crown-of-thorns starfish have to search them out. Based on my experiences, the starfish rarely eat an entire coral head.

The hurricane off La Paz last fall resulted in damage to the shallow-water corals around the nearby islands. I could see fragments of coral broken off and lying on the bottom in the coral rubble. I also noticed that there seemed to be changes in the health of the coral itself. Last year the coral heads seemed very healthy. This year many of the intact heads were partially or entirely covered with encrusting growths of algae and invertebrates. I wondered what had happened in less than a year. Perhaps the heavy rains had caused a lot of runoff entrapping soil and nutrients from the islands, resulting in the algal blooms. However the algae and invertebrates would have trouble settling on living coral. Perhaps there had been extensive feeding by the seastars, with the encrusting organisms settling on the newly-exposed non-living skeletal structural. Whatever the reason I noticed there were much larger numbers of young blue-chin parrotfish (hundreds in some areas) which were feasting on the algae growing on the dead coral heads. Perhaps the ecological balance will be reestablished by the next time I dive there.

Unlike most seastars, the crown-of-thorns was apparently not good at math. Perhaps they were exposed to "new math." Instead of the "normal" five arms, it may possess at least 10-15 of them all covered in the protective spines. The arms are short relative to the central disk. The disk of this species may be a foot and a half in diameter. Although the sharp spines and toxin are its primary defense, I frequently watch these starfish roll up into a spiny ball if they are dislodged from the substrate. In this position the soft underparts are completely protected from potential predators. At Los Islotes I watched a young sea lion attempt to pick up one of these and quickly give up. Of course sea lions are not their predators. Most of the predation on these seastars probably occurs on the eggs and young larval stages when their formidable defense system is not fully developed. It is reported that some species of fish (puffers and triggerfish) as well as carnivorous snails feed on them.

Although I left my heart in the warm waters and beautiful islands of Baja, it is good to be home on Catalina for a few weeks to celebrate the season before I head off to Belize and Honduras next month with Lindblad. Happy holidays to all in whatever way you celebrate this season. May the new year bring each and every one of you the joy of family and the possibility of peace on Earth. There must be some Earthly paradise where this last dream can be realized, if only within each individual's own heart, soul and mind. Baja was close! Perhaps it was my mermaid dive buddy.

© 2003 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays.

Crown-of-thorns starfish on a rock; stomach of starfish extruded to feed on corals; spines which serve
as the primary defense; starfish rolled up into a prickly ball, a secondary form of defense

This document maintained by Dr. Bill Bushing.
Material © 2003 Star Thrower Educational Multimedia