Testing the mechanism and benefit of coral-killing in Terpios hoshinota

• Main Authors: Siang-tai Syue, 薛翔泰
• Other Authors: Ker-yea Soong
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/31383445722583888364

碩士 === 國立中山大學 === 海洋生物研究所 === 101 === An outbreak of Terpios hoshinota population first occurred at Orchid Island and Green Island in 2006. The grayish to blackish encrusting sponge reduces light reaching the underling corals by more than 98.5%. T. hoshinota covered and killed all kinds of stony corals in shallow waters causing decline of living coral coverage. Here, I examined how T. hoshinota killed corals. Three hypotheses were proposed, i.e., 1)by toxicity only; 2)by shading only; and 3)both toxicity and shading, and tested. Field experiments using sponge fragments, grinded sponge tissues, or supernatant of grinded sponge tissues, as well as black vs. transparent caps, were applied on live corals in paired designs. We used the image analysis software, ImageJ, to compare the changes in color intensities of RGB channels as an index of damage to the corals among treatments. The results showed that the toxicity of sponge tissues with shading had stronger effects on corals than those of single factors applied, suggesting that besides toxicity, shading is also a tool T. hoshinota used to kill corals. In addition, I tested the ultimate factors, i.e., for substrate, or for nutrients, that the sponge kill corals by using stable isotope tracers 13C and 15N. The results showed that stable isotope signature in sponges covering labeled corals were higher than control sponges covering natural corals, suggesting that T. hoshinota probably gained nutrients from corals. However, the contribution from underlying corals represented only small percentages, i.e., 13C: 9.5±0.8%, 15N: 16.9±0.7%, of the composition of new sponge growth. Besides, there was no evidence that T. hoshinota transplanted 13C or 15N to the back part of the sponge. Based on these results, I suggest that T. hoshinota kills corals primarily for the substrate.