The effects of oil well drilling on coelobite communities (reef cavity-dwellers) were evaluated 15 months after the completion of drilling the second well at Matinloc Field, some 50 km west of Palawan Island in the Philippines. Drilling discharges (mud and/or cuttings with iron
flakes) are trapped in coral rubble cavities and stain the cavity wall brown. Staining is correlated with affected cavity-dwellers. Discharges have accumulated in cavities and the underside of coral rubble up to a radius of 100 m from the well head. The coelobite community was largely
disturbed within 40 m of the well site along north–south and east–west transects in 26-m depths. Minor changes in community structure were detected out to 75 to 100 m from the well site. The heavily damaged area coincides with the position of the drilling ship where debris and
iron scraps were observed and drilling mud/cuttings had accumulated. The presence of barium, clay and iron flakes in trapped mud in rubble cavities suggests that the drilling mud and cuttings have stained the cavity wall, and seriously affected the coelobite community. The visual effect
of drilling on bottom surface dwellers (non-cavity-dwellers) was less apparent. This study showed that coelobites can serve as sensitive indicators of environmental stress.
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