CONTROL OF CORAL REEFS BY BORING SPONGES 31 



ous pores open into each incurrent papilla, and a single oscule 

 terminates each excnrrent papilla (Fig. 8), The presence of the 

 openings for papillae at the surface of the substrate provides the 

 only indication of the presence of boring sponges in calcareous ma- 

 terials under natural conditions. 



The Mechanism of Boring by Clionid Sponges 



The unique quality of clionid sponges is their capacity to exca- 

 vate calcareous material. The mechanism bv which thev accomplish 

 this feat has been the subject of considerable investigation and 

 much discussion on the part of zoologists since Grant (1826) de- 

 scribed the first clionid, Cliona ceJata, living in oyster shells. The 

 best observations of the boring mechanism are those of Nassonow 

 (1883), who studied Cliona stationis (= C. vastifica) living in oys- 

 ters in the Black Sea. He collected the sponge larvae in an aquarium 

 and allowed them to settle on thin transparent flakes of ovster shell. 

 Upon settling, the larvae flattened out and immediately began to 

 bore into the calcareous substratum. The first evidence of this was 

 the appearance of a group of etched lines outlining elliptical areas 

 on the surface of the shell (Fig. 6). Nassonow said he could see 

 cellular processes burrow into the shell following the course of the 

 etched lines on the surface. Eventually small chips of shell were 

 freed and voided to the exterior of the sponge. Bv the end of the 

 first day of their existence as metamorphosed sponges, each of Nas- 

 sonow's Clionas had excavated from 11 to 15 such particles from 

 the shell fragment on which it had settled. These observations were 

 confirmed bv Warburton (1958), who planted fragments of Cliona 

 celata on transparent calcite crvstals and also allowed lai"vae of 

 this sponge to metamorphose on the same substrate. He, too, noted 

 the appearance of etched lines on the calcite underlying the sponge. 

 The lines surrounded more or less elliptical areas, 35 to 45 microns 

 in greater diameter, from which calcite fragments were eventually 

 removed. Each fragment was bounded by several curved faces, 

 convex or concave, meeting in sharp edges. Warburton also noted 

 that if sponge fragments which had been allowed to attach to cover- 

 glasses were removed gently, the cells left behind "showed a re- 



