The effects of ocean acidification, elevated temperature and herbivory on tropical crustose coralline algae.

The coral reef ecosystem supports the greatest taxonomic diversity in the ocean and is shaped by a suite of biotic and abiotic factors. One ecological interaction that is important in structuring coral reefs is herbivory. A series of field surveys found that sea urchin grazing on crustose coralline algae was a frequent occurrence on the fringing reefs of Nanwan Bay, southern Taiwan, where more than 50% of grazing scars had been inflicted by sea urchins. Grazing assays conducted in southern Taiwan in August 2009 showed that the closely related sea urchin species <i>Echinothrix diadema</i> and <i>Diadema savignyi</i> had similar grazing effects on the tropical coralline alga <i>Hydrolithon onkodes</i>, where each species grazed similar amounts of coralline algal surface area. <i>Hydrolithon onkodes</i> is an abundant species of tropical coralline algae throughout the Indo-Pacific, and on the northshore of Moorea, French Polynesia <i>H. onkodes</i> accounted for up to 11% of the benthic cover on the shallow backreef. A series of grazing assays conducted in Moorea in January 2010 showed that the sea urchin <i>D. savignyi</i> had a greater effect on the cryptic species of coralline alga <i>Goniolithon improcerum</i> by grazing more surface area than the heavily calcified species <i>H. onkodes</i>. These results suggest that sea urchins are important grazers of tropical coralline algae, and that the extent of thallus calcification may impact susceptibility to grazing.