SUBTIDAL ENVIRONMENTS AND ECOLOGY 



123 



Fig. 22 Heads of Goniastrea retiformis with deep grooves formed through the 

 activities of alpheid shrimps. 



wrasse Thalassoma lutescens readily took bivalves exposed 

 during collecting. Bivalves were not common borers of 

 Enewetak massive corals. Highsmith (1981a) found only 

 four bivalves in more than 100 coral heads and contrasts 

 with other areas where they produce significant boring. 



Polychaetcs are also significant borers of corals 

 (Highsmith, 1981a) but are often believed to occupy 

 empty sponge chambers. He found 280 polychaetes in a 

 single Pontes lutea head; the diversity of polychaetes 

 exceeded any other infaunal organisms. Although they arc 

 common, they are probably not as important borers as are 

 sponges. Sipunculans were imp>ortant borers of coral rub- 

 ble, rather than live coral (Highsmith, 1981a). 



Highsmith (1981b) discussed the role of endolithic 

 algae, Ostreobium spp., in several species of Enewetak 



corals. They occur as one or more dark green bands in the 

 upper few centimeters of the coral skeleton. He found 

 Ostreobium in every coral sampled from the surface to 

 30 m depth. No significant effect by the filamentous algae 

 on the integrity of coral skeletons was detected. In some 

 species of corals there was an inverse correlation, with 

 considerable variation, between water depth of a coral and 

 the depth of its outermost algal band. Algal bands are 

 believed to occur where and when conditions are suitable 

 for vigorous growth. 



DiSalvo (1969) isolated bacteria from within the skele- 

 ton of the coral Pontes lobata. Bacteria were cultured from 

 light brown discolored regions revealed when the corals 

 were split of>en. Attempts to culture bacteria from adja- 

 cent, nondiscolored skeleton were not successful. Some of 



