Crown of Thorns in Lutoban
Over recent months our survey teams have been noticing an increase in the number of Crown-of-thorns starfish (COTS) present in Lutoban’s Marine Protected Area (MPA)
Scientifically known as Acanthaster planci (wikipedia), COTS occur throughout the Indo-Pacific regio. They can be easily identified by their many arms (as many as 21!) and covering of long venomous spines. They reach sizes of up to 80cm, makes them one the largest species of sea star.
COTS are corallivores, meaning they eat coral polyps, typically targeting stressed and diseased corals. In a rather bizarre manner, after finding a nice spot to dine using eyespots located at the tip of each arm, (yes, that is in fact where their eyes are) they expel their stomachs out through their mouth and onto the coral. They are then able to digest the coral polyps, by releasing an enzyme and slurping up the nutrients. A single COT can eat up to 10 square metres of coral polyps in a year!
When in sustainable numbers, COTS are a valuable member of the reef ecosystem for a number of reasons. They help to maintain coral biodiversity, because they generally prefer fast growing branching and table type corals (fast growth for a coral is around 4cm a year), which creates space for the recruitment of slow growing, and often more resistant, coral species. As they also prefer weak and diseased coral, COTS reduce the chances of disease spreading through the reef, helping to promote strong healthy reefs.
COTS are also an extremely successful species when it comes to reproduction, a single female could produce up to 100 million eggs a year! In some studies they have even been found capable of larval cloning (a common reproductive method of starfish).
Given the right conditions, however, they can become too successful and this can result in “outbreaks”. Although COTS have natural predators, including Titan Triggerfish, Triton Trumpets snails and Humphead Wrasse. Once they have reached the outbreak level, the populations can no longer be regulated by natural extraction. Unfortunately, coupled with other ecosystem pressures, such as bleaching events, the corals struggle to bounce back. Artificial intervention, therefor, is sometimes needed to bring the population back down to a suitable level where the ecosystem can sustain it.
An outbreak of COTS is said to be when there is more than 15 individuals per hectare, so our first protocol was to measure the number present at the study site, Lutoban MPA. After a morning presentation covering basic COT ecology, biology and the purpose of the survey, the team headed down to the site. All volunteers trained in our monitoring program joined in the efforts to collect data, conducting 5m wide 100m belt transects covering as much of the MPA as possible.
The data collected was extrapolated and the results showed that in the Northern reef of Lutoban 60 individuals were found per hectare in the shallow depth range (3-7m), and 53 per hectare in the medium depth range (9-13m). Although these numbers are well above outbreak level, more replicate surveys and statistical analysis will be undertaken before any action is advised to the LGU.
In the past people have tried to control the populations of COTS by quite literally chopping them up and returning the fragments to the ocean, in the hopes of natural predators consuming the leftovers. However, this method is no longer preferred as many echinoderms, including COTS, have the ability to regenerate organs.n some weird X-Men style fashion, some can even regrow limbs. Instead, the chosen method for many other organisations (including us here at MCP), is to inject the COT with vinegar. This method kills the individual within 24-48 hours and causes little to no harm to the rest of the ecosystem. In fact, it seems predators quite enjoy the taste, as remains of the COTs are rarely found after injection.
Once more surveys have taken place to verify an outbreak, our science team will be able to determine if any intervention is necessary. In the past we have dealt with outbreaks in both Siquijor and Siaton
If you are concerned about crown of starfish outbreaks in your local area, we recommend you take a the Green Fins guidelines on handling outbreaks.