Sediment Production by Echinometra mathaei in the Shallow Littoral Zone of Coral Reefs, the Ishigaki Island, Ryukyus, Japan

Sea urchins graze on limestone and at the same time, they also produce sediments by discharging various intakes into the sea bottom of the coral reefs. This study was done on the Shiraho and Arakawashita reefs, the Ishigaki Island, Ryukyus. There are some differences in reef topographies and sediment composition between the 2 reefs. In this study, the author mainly analyzed the gut contents of Echinometra mathaei distributed predominantly within the 2 study reefs. The result are as follows; 1) E. mathaei produces the fine sediment of less than 1 mm in diameter into the sea bottom of the coral reef. The grain size distribution of the gut contents of E. mathaei is different from that of bottom sediments near the area inhabited by E. matheaei. It is thought that the fine sediment ingested by E. mathaei moved from the inhabited area to the other locations by wave and tidal currents. This indicates that E. matehaei discharges calcium carbonate accumulated by reef builders and moves to another place. 2) Comparing the results obtained from 2 coral reefsites, the contents ingested by E. mathaei change according to geographical zone. The intake of E. mathaei contains not only fragments eroding coral limestone but also sediments from the nearby sea floor. 3) There has been no research on the characteristics of intake materials to quantify erosion by E. mathaei. The erosion rates reported in the previous research seem to be over estimats than the actual rate. 4) E. mathaei yields fine sediments by crushing and discharging coral limestone produced by reef builders. Moreover, it is thought that the sediments from E. mathaei have a little influence on the total amount of sediment produced by other organisms in the whole coral reef.