OVERVIEW: CORAL REEF COMMUNITY STRUCTURE AND FUNCTION 



Mark A. Hixon 



Department of Zoology and 



College of Oceanography 



Oregon State University 



Corvallis, Oregon 97331 



This paper summarizes an open discussion session which took place during 

 the "Coral Reef Ecology Workshop" held at the annual meeting of the American 

 Society of Zoologists in Philadelphia on December 27, 1983. Due to either 

 sampling error or a genuine bias in current research emphasis, most of the 

 participants in this session studied herbivory and grazing patterns on reefs. 

 Consequently, most of the discussion centered around two related topics: (1) 

 the recently discovered mass mortality of urchins in the Caribbean and (2) the 

 relative importance of urchins vs. fishes in structuring benthic reef communities 

 in areas facing different fishing pressures. A third more general topic was 

 discussed briefly toward the end of the session: approaches to research on the 

 community structure of reef systems. I will cover each of these topies in 

 turn, extending discussion on the final topic with an overview of future needs 

 in the study of coral reef communities. 



MASS MORTALITY OF CARIBBEAN URCHINS 



One of the major herbivores on Caribbean reefs is the long-spined urchin 

 Diadema antillarum . In November 1983, a letter published in Science reported 

 unprecedented mass mortalities of this species that were sweeping the Caribbean 

 (Lessios, et al . , 1983). Haris Lessios (Smithsonian Tropical Research Institute) 

 attended the session and provided both a description of the effects of the 

 apparent pathogen (the origin or nature of which was currently unknown) and 

 an updated account of the spread of the calamity. 



The apparent disease runs a 4- to 5-day course, beginning with the urchins 

 emerging from coral cover and climbing up any available substrate. The urchins 

 then discolor, gradually lose their attachment capabilities, and ultimately 

 lose their spines and die. Up to 98% mortality has been reported in some 

 populations, with apparently resistant individuals remaining. Curiously, no 

 other species of urchin has been affected. 



The outbreak was first noted near the Caribbean entrance of the Panama 

 Canal in January 1983. Urchin mortalities were observed subsequently in 

 Colombia, Costa Rica, and Grand Cayman by June; in Jamaica, Belize, Cancun 

 (Mexico), and Key Largo (Florida) by July; in the Bahamas by August; in the Dry 

 Tortugas and Bermuda by September; in Grand Turk and Haiti by October; and 

 throughout the Greater and Lesser Antilles, including Curacao, Bonnaire, and 

 part of the coast of Venezuela, between November 1983 and January 1984. 



Initially, the spread of the die-off appeared to follow prevailing ocean 

 currents. However, certain exceptions, such as the outbreak in Barbados in 

 October (a full month or two before the remainder of the Antilles), suggested a 



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