Most divers have seen the occasional brightly colored, many-limbed, spiny rubbery-looking Crown-of-Thorns starfish. They appear quite quirky and can be a good subject for macro photography. However, as predators, they also team up to form an extremely efficient coral killing machine.
The crown-of-thorns starfish, or sea star (Acanthaster planci) have, as most organisms on a reef, their place in the food chain and fulfill a role in a balanced and healthy reef system; they are however, voracious predators. I dive from Pemba Island, Zanzibar regularly and towards the end of a visit in late September 2009, I was told that one of the dive sites, home to leaf coral, lattice coral, and a soft-looking porite knuckle coral was under attack from a plague of crown-of-thorns starfish.
Crown-of-Thorns Starfish Facts
Young crown-of-thorns starfish feed on algae encrusted on the coral, common amongst rocks and rubble on the reef. At approximately six months of age, they start to eat coral and growth rates increase from 1 cm (0.39 in) to 25 cm (9.94 in) in the subsequent two-year period. Crown-of-thorns starfish spend about half their time feeding. When there are few crown-of-thorns starfish, they are elusive and hide in the reef and under corals during the day. Larger starfish (more than 40 cm (15.74 in)) usually feed during the day while smaller starfish (less than 20 cm (7.87 in)) usually feed at night. Extruding their stomachs onto the coral, they literally suck the life out of it.
Crown-of-thorns starfish feed mainly on faster growing table coral species, particularly from the genus Acropora, and may only eat a portion of the entire coral colony. As a result, the reef is able to recover quite rapidly from low levels of predation by crown-of-thorns starfish. Some reefs seem to support small populations of crown-of-thorns starfish for many years, with only a small reduction in coral cover. Scientists estimate that a healthy coral reef with about 40-50 percent coral cover can support 20-30 crown-of-thorns starfish per hectare (2.47 acres).
However, a prevalence of crown-of-thorn starfish in large numbers can result in intense competition for food with most types of corals being eaten, including species such as slow-growing Porites spp., which are not usually eaten by the starfish. During a severe outbreak, the crown-of-thorns starfish can number 20 per square meter (1076 sq ft) or more, piling on top of each other three or four deep at times. They are able to devour so much that they can kill most of the living coral in that part of the reef, reducing hard coral cover from the usual 25 – 40 percent of the reef surface to less than 1 percent. Such a reef can take over 10 years to recover.
What Causes Outbreaks of Crown-of-Thorns Starfish?
Scientists support three theories on the causes of outbreaks of the crown-of-thorns starfish. These theories have neither been proved nor disproved. Firstly, fluctuations in crown-of-thorns starfish populations are a natural phenomenon. Just like any other organism, populations vary. Secondly, human exploitation of the coastal zone has increased the flow of nutrients to the sea and has resulted in an increase in planktonic food for the larvae of crown-of-thorns starfish. The improved survival of larvae has led to an increase in the number of adult starfish which results in outbreaks.
Crown-of-thorns starfish spawn over four to five months a year when water temperatures are around 28°C (82° F). The starfish release eggs and sperm into the water through pores on the top of their central disc and when the eggs are fertilized, these develop into larvae that drift like plankton for two to four weeks. The one to two millimeter juveniles settle onto the reef and live among rocks and rubble on the reef, remaining almost invisible until they are about six months old. They first breed when two to three years old and continue to reproduce for five to seven years. Each female can spawn up to 60 million eggs during a single season. By gathering together to spawn to increase the chance of fertilization, fertilization rates for crown-of-thorns starfish are, in fact, the highest measured for any invertebrate. Thus, with even more favorable factors, their proliferation could be increased considerably.
The final theory is that the removal of the natural predators of the crown-of-thorns starfish has allowed populations to expand. Although these starfish have few predators, one theory suggests that predators play an important role in keeping starfish populations balanced. Predators of adult crown-of-thorns starfish include the giant triton snail (genus: Charonia), the napoleon or Maori wrasse (Cheilinus undulatus), the starry pufferfish (Arothron stellatus) and the titan triggerfish (Balistoides viridescens). The giant triton snail is highly prized and heavily collected for sale to landlubbers (and ignorant divers); the shells for sale in tourist shops on Unguja Island (Zanzibar) and in Dar-es-Salaam have come from somewhere. However, the triton snail can consume relatively few adult crown-of-thorns starfish per week and thus its capacity to prevent starfish outbreaks seems limited, although it plays an important role when the starfish numbers are stable.
Predation by other reef fish on juvenile starfish might also limit crown-of-thorns starfish populations. Juvenile starfish are most likely to be eaten aged around six months, when they start to feed on coral. If numbers of the predator fish were depleted by fishing activities, this might allow an abnormally large number of starfish to survive to maturity. There is no substantial evidence at this time to show that commercially exploited fish eat significant numbers of juvenile crown-of-thorns starfish.
The Dive Site
There is little human habitation near the dive site in question, nor any rivers to bear new nutrients from further a-field. There is subsistence fishing in the area, including some illegal dynamite fishing, and fishermen do not seem at all bothered about the size or type of fish caught. Given that there is no data available on past fish populations, it is impossible to state that their numbers are in decline, but it would not be beyond the realms of possibility. The numbers of starfish larvae that usually survive and then settle is unknown and it is difficult to estimate feeding rates of predators required to keep them in check.
Whilst the causes are therefore impossible to define categorically, the existence of abnormal numbers of crown-of-thorns starfish is easy. On returning in early December, armed with what I hoped to be a solution to the problem, I dropped onto the site to gauge the progress of the preceding two months. Sections of the dive site, roughly 10 m (32 ft) wide and 20 to 30 m (65 ft – 98 ft) long, had been destroyed by the slowly creeping underwater zombie-like horde, leaving tracts of coral completely white and, critically, dead. The crown-of-thorns starfish appeared in patches, seemingly moving from the shallower sections of the reef (10m) to the edge of the wall (17m), advancing in an underwater phalanx, in parts stacked four high.
Dealing With the Outbreak
In the past, crown-of-thorns starfish would be physically removed from the reef and buried on land. This method has several drawbacks though. Firstly, the starfish cling on to the reef and yanking them off damages the reef. Secondly, they are covered in sharp, toxic spines (the toxins come from the coral that they eat) that deliver very painful and sometimes dangerous stings, which necessitates them being handled with tongs or sharp sticks. This makes removal slow and labor-intensive. Another method tested in the past was cutting them up under water, but this too damages the coral, there is still a considerable risk of being stung and crown-of-thorns starfish that have been cut in four were observed still alive two weeks later!
Research on the Great Barrier Reef in Australia provided the solution that I was going employ. An injection of sodium bisulphate solution into the center of the crown-of-thorns starfish has been found to kill them in a couple of days, and has no effect on other marine organisms. Thus armed with this knowledge, I purchased some 60ml syringes, some long needles through eBay and ordered the cheap and readily available chemicals from a pool cleaning company, then crossed my fingers that I would not be required to do some explaining at Dar-es-Salaam airport.
Once in Pemba and over the dive site, we mixed 200 grams of sodium bisulphate with 720 ml of seawater and filled six syringes. Accompanied by two other divers, I dropped down. Each starfish was to be injected with 2 ml, and thus one syringe would deal with 30 starfish. Working mainly head down and feet up, it took about 15 to 20 minutes to empty our syringes. The skin of the crown-of-thorns starfish is quite thick, but it has a sort of anal-like aperture at the center of its topside and the needles slipped in easily. One diver collected the syringes, surfaced, refilled and came down again. In total, 12 syringe-loads were emptied in less than an hour, totaling around 360 injections. Of course, some recipients may have received a bit more than 2 ml to begin with and no doubt, some specimens were done twice in the disorientating process!
Over the next week, we returned twice to tend to neighboring patches and the initial site was checked. The results were nothing short of spectacular – after a day, the crown-of-thorns starfish were starting to become flaccid and their skins were not as tough (and thus injected specimens were easily identified) and a couple (that probably received an extra dose) were splitting down the middle, but still moving. After two to three days, the specimens were dead and in various states of liquefaction. It resembled a sci-fi scene; a patch of gooey white substance with the spines sprinkled on top.
On returning in November 2010, the starfish were no longer evident, other than the odd one here or there, though the track of their passage across the reef clearly was. Paths of coral rubble ran down the top of the reef to the edge of the wall, as if cut by an underwater lawnmower. Fortunately, the dive site is part of a long stretch of wall and the affected area only covers three small sections and the rest is unharmed.
It was a very effective way, in terms of both time and cost, of restoring balance to the reef.