The following three papers in the volume deal with damselfishes and/or 

 urchins on reefs. The territorial activities of damselfishes are known to be 

 significant for the structure and productivity of reef communities (e.g., 

 Brawley and Adey, 1977; Hixon, 1983; and others). Sadovy studied the detailed 

 dominance relations among individuals of bicolored damselfish in the field. 

 Her results showed a linear, size-dependent dominance hierarchy in these fish 

 and revealed some interesting similarities and differences between field and 

 otherwise comparable laboratory studies. Robinson and Williams compare population 

 characteristics of threespot damselfish and urchins ( Diadema ) in shallow back 

 reef and fore reef environments. They found that urchins are larger, occur in 

 lower densities, and are found higher in coral branches on the back reef than 

 on the fore reef; in contrast, damselfish live at higher densities, have larger 

 lawns, and exhibit less aggression toward urchins on the fore reef than on the 

 back reef. Urchins may remain small and close to the substrate on the shallow 

 fore reef because high wave action inhibits their grazing; thus interference 

 between urchins and damselfish is less strong when mediated by disturbance on 

 the fore reef than in more benign conditions on the back reef. Examining 

 another species of urchin with a different but very significant impact on the 

 structure of reefs, Hoskin and Reed use carefully controlled experimental 

 methods to estimate the rates of bioerosion of carbonate substrates by urchins 

 ( Echinometra ) and other burrowing infauna. These urchins excavate approximately 

 8.9 g substrate/m^/day (9 tons/year for the population), while the remaining 

 infauna produces about 5 g sediment/m^/day (6.5 tons/year). This study illus- 

 trates the tremendous amount of sediment that is produced and transported in 

 benthic reef communities. Scouring and burial by this sediment must represent 

 one of the most important agents influencing the structure and dynamics of reef 

 communities, yet the significance of these processes for community organization 

 are almost unstudied. In addition, this study provides new data on the sources 

 and rates of degradation of reef substrates, a significant aspect of long term 

 morphogenesis and diagenesis of reefs. This paper therefore provides a link 

 between the papers on community organization and those on community metabolism. 



Adey and Steneck show how reef productivity is related to substrate 

 complexity and surface area, light and water flow, and abundance of turf algae. 

 They demonstrate that all of these factors can be understood in the context of 

 geological development of the reefs. Using upstream/downstream flow respirometry 

 techniques on reefs in three different stages of geological development in St. 

 Croix, these authors provide good evidence for some of the highest rates of 

 primary productivity reported for any marine environment. Dustan explores a 

 different method of assessing primary productivity. He provides initial data 

 on the spectral intensities of light impinging on, absorbed, and reflected by 

 different substrates. Dustan suggests that much can be learned about the photo- 

 biology of reef organisms using these nondestructive bio-optical sampling 

 techniques and indicates that we may be able to estimate primary productivity 

 over vast tracts of previously inaccessible reefs if optical signals from these 

 reefs can be detected from orbiting spacecraft. Although we are in the initial 

 stages of interpreting these data, such new technological approaches offer the 

 promise of measuring and understanding productivity on a global scale. Seitzinger 

 and D'Elia present data on another new and potentially important aspect of 

 community metabolism on reefs. While nitrogen fixation on reefs has received 

 well deserved attention, Seitzinger and D'Elia show that some reef habitats, in 

 particular dead coral heads and sediments, may be the sites of extensive denitri- 



