>This is the peculiar but typical behavior of one of the most influential invertebrate predators found on Pacific coral reefs, the crown-of-thorns starfish (Acanthaster planci). The starfish is doing what its ancestors did for millennia — eating the corals with which they evolved millions of years ago. Like many other species that aren't widely accepted as aesthetically pleasing or don't fit neatly into popular concepts of a healthy ocean, crown-of-thorns starfish (COTS) are subject to vilification. Their recent history has been tarnished by some massively destructive population outbreaks. These days the large, coral-eating echinoderms are familiar inhabitants of tropical Pacific reefs, and virtually every diver and snorkeler knows what they are. At the same time, however, few people appreciate the species' functionality and its role in the overwhelmingly complex coral reef ecosystem.
>The hungry starfish on top of the Acropora coral is a mature female and will soon spawn. Unlike some other echinoderms, COTS are not known to reproduce asexually via arm autonomy; sexual reproduction through synchronized spawning usually coincides with seasonal spikes in water temperatures. This female, being large and therefore quite fecund, can release close to 60 million eggs per spawning season. Broadcasting her eggs into the current while nearby males release sperm at the same time should produce fertilized eggs that rapidly metamorphose into planktonic larvae. Widely distributed, COTS larvae normally spend three to four weeks drifting at the whim of ocean currents before settling on a reef.
>The evolutionary history of COTS is still something of a mystery, but the speciation from herbivorous ancestors undoubtedly coincided with the success of reef-building corals sometime between 1 million and 3.7 million years ago in the southwestern Pacific. Recent genetic studies have indicated that the species is still diverging, and A. planci is, in fact, a complex of four sibling species, or clades. Similar to all starfish, COTS are pliable and have no rigid skeleton. But uniquely shaped calcium-carbonate spicules provide some structural support and are the only evidence of COTSs that can be found in fossil records. Interestingly, COTS spicules have been discovered in ancient soil strata, which indicates that large numbers of COTS existed at one time and then died off quickly. These layers of spicules suggest natural boom and bust cycles on coral reefs that occurred well before humans ever reached many Pacific islands.
>Crown-of-thorns starfish have apparently acquired their sinister name within the last 50 years. Understandably, divers and snorkelers who explore and appreciate coral reefs for their aesthetic value aren't happy to watch their underwater playgrounds being decimated by starfish outbreaks. Under ordinary circumstances, COTS are found on the order of 1-15 per 100 square meters of reef. During outbreaks, abundances can be 400-600 percent higher than normal, and coral mortality on a given reef may range from 50-100 percent. Though the effects of the outbreaks are well known, the triggers remain somewhat mysterious.
>An outbreak occurs when the number of COTS on a reef rapidly increases so that they consume coral faster than the corals can grow. They prefer particular coral species such as table and staghorn corals, but in general they simply act like a living, slow-moving and devastating wildfire. Outbreaks radically modify reefs; duration and severity vary greatly, which means some reefs and regions are affected much more significantly than others. After COTS devour all of the edible corals in an area, algae swiftly colonize most of the newly available space. The fish community changes, and sessile invertebrates such as sponges and soft corals grow over areas once covered by hard corals. In the aftermath of an outbreak it may take decades for a reef to recover its original biodiversity, and some reefs never fully recover.
>Over the past several decades researchers have posed a number of hypotheses about how and why COTS outbreaks happen. The hypotheses are split into two basic categories: those that consider outbreaks natural and recurring, and those that presume outbreaks are recent, novel events. The two categories are not necessarily mutually exclusive; while the roots underlying outbreaks are still debated, it is likely that they are caused by an ever-changing amalgamation of natural and anthropogenic factors.
>In the early 1980s Charles Birkeland, now professor of biology at the University of Hawaii, proposed the terrestrial runoff hypothesis, stating that nutrients in freshwater runoff, especially from large storms, cause phytoplankton blooms that boost the food supply for COTS larvae. Assuming that under normal conditions most COTS larvae starve from lack of food, increased larval survival and recruitment leads to subsequent outbreaks. With increases in human populations throughout the Pacific and more nutrient input, often from agricultural sources, phytoplankton blooms are occurring more often than in previous decades and creating optimal conditions for COTS larvae.
>One of the popular hypotheses for why outbreaks have become more common is the reduction of COTS predators. At least 12 different species, including crabs, shrimp, a polychaete, gastropods and fish, are known to prey on juvenile and/or adult COTS. But most predators will not feed on COTS larvae or adults if given a choice of other food options. COTS eggs and larvae are protected by toxins, and the spine-laden adults can resist all but the most effective predators; the cryptic juveniles are probably the most vulnerable, but there is virtually no evidence that predation controls COTS population sizes.
>When disturbance is removed, species richness decreases as competitive exclusion increases. In other words, species competing for the same resources cannot coexist in the same niche. By feeding on fast-growing corals, COTS in normal abundance help maintain robust reef systems. Fast-growing corals, if not kept in check, have the potential to overgrow a reef, creating a monotypic habitat that is susceptible to collapse under any number of environmental stresses. The more diverse a reef system is, the more capable it is of withstanding various environmental pressures and changes. On the other hand, all species on a reef are at risk of going extinct when there is too much disturbance. Therefore COTS, in ordinary conditions, play a vital role in Pacific reef ecosystems.
>Outbreaks appear to be natural reset buttons as well as recurring anthropogenic sources of devastation. It is time for a paradigm shift, or maybe a reality check, with regard to how we think about COTS and their influence on reefs. We must begin looking at the ways in which humans are involved in the outbreak process: nutrient input along coastal regions and sea surface temperature rises caused by anthropogenic climate change. Let's look at COTS not as a cancer on Pacific reefs but as a naturally evolved and integral player in healthy reef environments. It is time to focus less on managing COTS and more on managing human activities, human populations and human ignorance.
>© Alert Diver — Q1 Winter 2015