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Crown-of-thorns starfish

2013; Elsevier BV; Volume: 23; Issue: 21 Linguagem: Inglês

10.1016/j.cub.2013.07.080

1879-0445

John S. Lucas,

Ichthyology and Marine Biology

Why ‘crown-of-thorns’? The crown-of-thorns starfish is nearly the largest species of starfish (Asteroidea). It may be 50 cm or more in diameter and can have more than 15 arms. Its ‘crown-of-thorns’ are the long sharp spines that cover its upper surface. What is more, the spines are coated with a saponin toxin, which causes irritation to puncture wounds (a considerable problem when working with the starfish). Coral reefs, where the crown-of-thorns starfish live, have many large predatory fish, so they need to protect themselves. Ironically, it is now the reefs that need protection from the starfish. How so? Crown-of-thorns starfish feed on the living surface tissue of hard corals (Scleratinia). Using hundreds of small, sucking tube feet in each arm, the starfish slowly moves onto the surface of the corals. It pushes its stomach out through its mouth on the underside, spreading the stomach across the surface of the coral beneath it to about its own diameter. It digests the coral tissue with enzymes secreted from the stomach and harvests the digested tissue as it retracts the stomach. The result of this procedure is a substantial area of white coral skeleton cleared of tissue — a ‘feeding scar’ (Figure 1A). The bare skeleton is quickly invaded by algae so that the colour of coral is replaced by a dull algal turf (Figure 1B,C), resulting in a much less attractive reef. Such reefs are still very productive and the complex reef community shifts to more herbivory. The invasion, however, by turf algae and other organisms such as soft corals (Octocorallia) may hamper coral larvae settlement and recovery of hard corals on the reef surface. So, they’re not all that bad, are they? Quite the contrary, the crown-of-thorns starfish are a disaster! They came to prominence in the early 1960s when large populations, ‘plagues’, were found to be causing extensive damage in coral reefs in the Indo-Pacific region, especially to many reefs along the iconic Great Barrier Reef. In the alarming degradation of coral reefs throughout their tropical and subtropical distributions in the Indo-Pacific region, crown-of-thorns starfish continue to be a major factor causing degradation. This is starkly shown in data for the Great Barrier Reef, where hard coral cover on reefs has declined from a mean of 28% to 13.8% between 1985 and 2012. It is estimated that crown-of-thorns starfish were responsible for 42% of this decline. Why are there ‘plagues’? The obvious answer would be that they are due to humans overfishing the large top-predators that prey on crown-of-thorns starfish and control their populations. However, there is no evidence for this hypothesis. Furthermore, the large crown-of-thorns starfish populations on a reef must be preceded by huge numbers of larvae settling onto the reef and metamorphosing to starfish. How do they develop? Early development follows the typical pattern for starfish: egg, gastrula, bipinnaria larva (0.5–0.9 mm), brachiolaria larva (0.8–1.5 mm) settlement and metamorphosis. The bipinnaria and brachiolaria larvae are ciliated and planktonic, feeding on the single-celled algae of the phytoplankton. They develop in the plankton for 11 days or more. The initial starfish settle and feed on the ubiquitous coralline-algae surfaces, leaving tiny feeding scars like the adults. As it grows, the juvenile starfish adds arms to reach its final number and begins feeding on hard corals. How do they reproduce? Unlike many other starfish, the general body surface of crown-of-thorns starfish is soft and flexible, which enables it to swell as it develops huge gonads. Fecundity is obviously related to size, but, as an example, a 40 cm diameter crown-of-thorns starfish may commit about 45% of its total body energy to reproduction and shed an astonishing 50 million eggs. Like many marine invertebrates, crown-of-thorns starfish shed their gametes freely into the ocean and the gametes are wasted if there is no synchrony or proximity in spawning. Crown-of-thorns starfish don’t use precise cues for spawning: they may spawn at any stage of the lunar cycle and even join with other reef invertebrates in multi-species spawnings. They do, however, tend to spawn when the water temperature is about 28°C and often aggregate, apparently due to a spawning pheromone. Proximate spawnings of male and female crown-of-thorns starfish achieve almost 100% fertilisation. Even two crown-of-thorns starfish spawning 60 meters apart can achieve 23% fertilisation, resulting from the vast numbers of sperm released. Does this colossal reproductive capacity explain the ‘plagues’, then? Partially, yes. One must indeed go back through the life-cycle and consider the survival levels of the >108 eggs released by some crown-of-thorns starfish populations. It was observed that crown-of-thorns starfish plagues tended to occur three years after heavy rainfall and terrestrial run-off. Three years is about what it takes the crown-of-thorns starfish to grow to a point where they and their feeding traces become conspicuous. This suggests that the input of run-off nutrients influenced the survival and development of starfish larvae, by promoting higher levels of phytoplankton. So, ultimately, humans do influence the plagues: bad land-use practices in areas adjacent to coral reefs lead to greater terrestrial run-off. In fact, much of the general deterioration of coral reefs internationally is due to these bad land-use practices. How can we get rid of them? Once large populations of crown-of-thorns starfish are observed on a reef it is extremely difficult to eliminate them and, even more, to eliminate them before they eat themselves out of coral (Figure 1). Between 1970 and 1983, almost 13 million crown-of-thorns starfish were removed from the reefs of the Ryukyu Islands, southern Japan, via a bounty for fishers, who changed from fishing to a more reliable income. Despite this huge effort, there are still large crown-of-thorns starfish populations in the Ryukyus. Complete removal is needed because cutting them up in situ isn’t the end: the pieces regenerate to make even more starfish! Successful control programs have only been achieved where there was a relatively small discrete population which was tackled quickly. Comprehensive control of crown-of-thorns starfish must eliminate the sources of nutrient input that promote survival of the larvae. Like all profound environmental problems, it is difficult and expensive to solve. The Australian government invested hundreds of millions of dollars for a Reef Rescue program over the past five years, and the program is now extended for another five years with further funding.