INTRODUCTION
The Acanthaster planci (commonly known as Crown-Of-Thorns Starfish or COTS) is one of the major cause of coral reef degradation (De’ath et al. 2012). The COTS feed on coral tissue and cause extensive coral mortality especially during the population outbreak. Manual collection and land buried is a widely used and effective method to reduce the number of COTS during an outbreak. However, chemical injection is an alternative method to effectively induced mortality in the COTS. Many type of inorganic chemicals (such as hydrochloric acid and formalin) has been used for the injection. However, the usage of organic chemical in the injection is less explore (Rivera-Posada and Owen, 2014). Environmental friendly organic chemical, if at the correct dosage, could be an effective option to replace inorganic chemical for the injections (Boström-Einarsson and Rivera-Posada, 2015). The objective of this study is to examine the physiological response and mortality rate of COTS toward organic and inorganic chemicals.
Figure 1. Acanthaster planci feeding on scleractinian corals, leaving exposed (white) skeleton (left). Underneath the arms are podia and tentacles help in movement and feeding (top).
METHODOLOGY
To conduct the experiment, four 96L plastic cage with messed wired door were built then deployed on the INOS platform (depth= 12m) in front of UMT Bidong Island Research Station (5°37'17.28"N, 103°3'24.40"E) (Figure 2). Each cage was served as a chemical treatment for the study. Twenty adult Crowns-of-Thorns starfish (more than 20 cm in diameter) were collected from surrounding reefs and placed evenly in the cage. Total of five COTS per cage. Experiment started after 3-days of incubation. To test the effects of difference chemicals, COTs in the cage were injected each with 400g/L sodium chloride (NaCl), 15% Hydrochloric acid (HCl) and citric acid extracted from lime. No injection was performed on the control treatment. To examine the physiological response of COTS, observation was carried out using SCUBA every two hours for the first 24 hours of post injection period. After that, observation interval was increased to every four hours. The experiment was terminated after 120 hours of the injection.
Figure 2. Four plastic cages with messed wired cover were built and deployed underwater for the experiment (top left). Different colour codes indicating different chemicals in the syringes (bottom). Chemical was injected in to the stomach of the Acantasher planci via central disc (top right).
RESULT AND DISCUSSION
The mortality rate of COTS varied with different chemicals. All COTS that injected with HCl were found dead with in the first 24 hours of post-injection. In contrast, only 40% of COTS injected with citric acid died within first 24 hours. The lethal effect of NaCl on COTS was slower, where mortality occurred after 48 hours of post injection (Figure 3). No mortality was observed on control treatment throughout the five days of experiment. The physiological responses observed in the study were blister, lesion, necrosis or expose of organ, matting and loss of spines, loss of turgor and swelling, mucus production and hyperactivity (Figure 4.) Blister, lesion, necrosis or expose of organ were most likely and common to observe in the study from 48 hours to 120 hours of the experiment. Citric acid may served as an ideal organic chemical to control COTS number during the outbreak. All physiological response was observed in citric acid treatment except hyperactivity. It gave osmotic shock toward COTS body (Rivera-Posada and Owen,2014) and also slower the regeneration of body part (Pechenik, 2010). More over, citric acid was extracted from lime therefore the cost to prepare this chemical is low compare to the inorganic chemicals, and would not cause adverse impact to the marine environment.
Figure 3. Variation in mortality rate of Acanthaster planci in response to the different type of chemicals after 24 hours (left), and 120 hours (right) of the injection.
Figure 4. Examples of physiological responses of the Acanthaster planci post injection: matting and loss of spines (left) and lesion (indicate by red circle in left photo), and exposed of organ on the oral side (right).
ACKNOWLEDGEMENT
This research was supported by School of Marine and Environmental Sciences, UMT. Thank you the laboratory officers from Biodiversity Lab, PPSMS UMT in providing field assistant. Also, we would like to thank Dr Hii Yi Siang to allow us to use the INOS platform for the experiment.
REFERENCE
Boström-Einarsson, L., Rivera-Posada, J. (2015) Controlling outbreaks of the coral-eating crown-of-thorns starfish using a single injection of common household vinegar. Coral Reefs DOI 10.1007/s00338-015-1351-6
De’ath, G., Fabricius, K. F., Sweatman, H., Puotinen, M. (2012) The 27-year decline of coral cover on the Great Barrier Reef and its causes. Proceedings of the National Academy of Sciences 109, 17995-17999
Pechenik, J. A. (2010) Biology of the Invertebrate (6th Ed). McGraw-Hill Companies, Inc
Rivera-Posoda, J., and Owens, L. (2014) Osmotic shock as alternative method to control Acanthaster planci. Journal of Coastal Life Medicine 2014 2,99-106
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