Outbreaks of coralivores Drupella snails have been on the rise throughout the pacific over the last several decades. Koh Tao experienced it’s first documented Drupella snail outbreak following the mass coral bleaching event of 2010. During and following that event, the NHRCP was active in the monitoring of Drupella populations, and targeted removal efforts. During 2010 and 2011 our team spent over 3,170 diver minutes to collect over 17,000 Drupella snails. Interestingly, we found that there had been a shift in the food preference of the snails following the bleaching event, whereby they started to eat the mushroom corals, which had never been observed before. A publication based on the findings of our studies was published along with Dr. Bert Hoeskema in the journal Coral Reefs.
By 2013 we felt that there had been a noticeable decline in the number of Drupella snail aggregations in some of our removal areas such as Chalok Ban Kao. We were unsure if there truly was a decline in their numbers, or if the recovery of corals following the bleaching had just made the cryptic Drupella snails less obvious. We also wanted to know if the Drupella stopped eating the mushroom corals after recovery of the faster growing branching and bushy corals. In order to follow up on the studies and work completed in 2010-2011, we hosted Mahidol University International College student Tawin Kim for his bachelor’s degree thesis project.
Tawin repeated some of the data collection that we had performed in 2010 and 2011, along transect lines laid in the same regions as the previous study. He evaluated the types of corals being consumed by Drupella, and also took note of the life stage of the Drupella in the area (Adult, Juvenile, Recruit).
In Chalok Ban Kao, Drupella snails were still eating Fungia corals, although they are also found on the branching (Acropora and Poccilopora) corals. It would appear that the shift in their food preferences has continued long after the bleaching event. This is significant because mushroom corals are an important frontier species, moving into areas other corals could not survive and then laying the foundations for new reef growth. Mushroom corals are much slower growing then branching corals; “Acropora can grow quickly and therefore Drupella aggregations on Acropora will cause fewer damages because of the coral’s fast growth rates.”
In terms of number of Drupella on the reef, it does seem that populations have fallen to below outbreak levels in most regions surveyed. According to the accepted literature, and outbreak of Drupella is defined as more than 3 individuals per square meter of reef. In Chalok Ban Kao, Tawin found there to be an average of 1.18 individuals per square meter, and in Taa Chaa the total average was about 1.17 individuals per square meter. But looking at individual depths there may be some areas that are still at outbreak levels, such as the 6 meter depth in Ao Taa Chaa, with 3.5 individuals per square meter.
Our group will continue to monitor this situation over time, both through the EMP program and through directed studies conducted by our interns such as this one by Tawin. Based on this study, we will also continue our removal efforts, with a focus on the areas found to be of ‘high’ or ‘outbreak’ levels during this study.
You can download Tawin Kim’s Full report titled “Determining the Abundance, Density, Population Structure, and Feeding Preference of Drupella Snails on Koh Tao, Thailand” on our Publications and Student Papers page.