Written By Danielle Moloney
The crown of thorns starfish (COTS) has long been observed alongside corals in the Indo-Pacific region. These unique echinoderms prey on corals, and while they will eat most types of coral, their preferred meals are Acropora. Acropora, also known as table or branching corals, make up an important group of corals; they are considered the one of the key building blocks of many reefs world-wide. Though COTS are naturally occurring in low levels on many reefs, combined effects from anthropogenic and other stressors can cause outbreaks of high COTS numbers. These outbreaks often lead to high mortality levels for corals in the area, as the starfish consume corals faster than they can grow. On the Great Barrier Reef alone, there have been four waves of COTS outbreaks documented since the 1960s. Previously, little was known about the movements of COTS as they relate to coral levels. A new study from the University of Tasmania in Australia has linked coral availability to COTS movement patterns. A better understanding of COTS behavior can then help scientists predict and control outbreaks in order to protect reefs.
The authors of this study hypothesized that the movement of COTS would be dependent on the availability of Acropora, the preferred coral prey. In order to test this hypothesis, the researchers used a time lapse photography setup to track and quantify the movement of individual COTS over time on the reef. Individuals were identified by flagging spines with tape in unique arrangements. They set up cameras focused on each individual and included a plastic frame used to calibrate their measurements. Surrounding cameras were set up to track starfish as they moved away from their original starting points.
The researchers also took other measurements such as percentage of coral cover, feeding scars left behind by COTS, and number of COTS individuals in the study area. The authors note that 15 or more COTS per hectare is the point at which COTS numbers are considered an outbreak.
Based on the quantified movement of individuals, the researchers found that COTS move in two patterns: localized movement (staying largely in the same area) and roaming (moving more than 10 meters from the starting point in the span of 24 hours). The type of movement was not random: it correlated with coral cover. Starfish in areas where coral cover was plentiful stayed close to their original location, whereas starfish in areas with low coral cover roamed further away, likely in search of more food. Aggregations of COTS caused by their tendency to roam to areas with high levels of Acropora, paired with stressors that cause coral bleaching, can lead to synergistically-induced declines in reef health.
The study also uncovered a newly observed COTS pattern: homing behavior. In this behavior, an individual exhibits ‘outward’ movement to areas with Acropora for feeding, followed by a retreat to ‘home-site’ or ‘shelter’ areas. Homing behavior has been observed in other echinoderms before, but this is the first time that it has been noted in COTS. Dr. Ling, the lead author of this study at the University of Tasmania, equated this behavior to that of a teenager: “The crown-of-thorns starfish often partied all night, slept-in, and only those with a well-stocked larder found their way home- so it’s very much a teenager model of behavior,” A better understanding of COTS homing patterns may help clarify their population dynamics, and therefore, the effect of their presence on coral assemblages.
The findings of the study reviewed here will aid in making ongoing culling (selective killing of animals to reduce a population) more effective along the Great Barrier Reef. Detectability is one of the most significant factors in the effectiveness of culling. Experiments performed on COTS locomotion at different light levels will help divers optimize their culling efforts by pinpointing the time of day when they will be best able to locate individuals or interrupt their feeding.
The breakdown of COTS movement into homing, prey-available activity versus roaming, prey-searching activity helps to explain the aggregations of individuals during times of population outbreaks. The authors hope that by clarifying the behavioral ecology of COTS, including their movement patterns and possible behavioral indicators, that researchers will be better equipped to control COTS outbreaks in the future, thereby mitigating the destructive impact that the starfish have on reefs.
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Featured Image: Photo courtesy of Deeper Blue