ABSTRACT
The sea urchin Echinometra mathaei (Blainville) on Okinawan coral reefs is divided into two types, burrowing and non-burrowing urchins. Burrowing urchins phenotypically designated as Types B and C were agonistic against the intruders and drove them away from their burrows. When 2 urchins of different types were placed in an empty burrow, usually the larger one drove the smaller away from the burrow. However, smaller Type C urchins frequently expelled larger Type B urchins. Recolonization of the empty burrows made by mass mortality of the urchins on the reef flat is shown. Many burrows, 14.5–41.0%, still remained unoccupied till 4.5 yrs after the event, but other benthic organisms such as the corals Goniastrea aspera and Acropora sp., the brown algae Turbinaria ornata and Padina minor, and some gastropods were observed in the burrows. Large amount of sediments was also accumulated there. Ecological meanings of the agonistic behavior and the role of this urchin in the organizing process of the lower intertidal community on the reef flats are discussed.
