ROCK-BORING ORGANISMS 3 



acids produced in metabolism (Newliouse, 1954; Newell, 1955, 1956). 

 Less eas)' to understand is the boring activity of the green algae 

 which penetrate into living coral rock, where thev form a green zone 

 up to 3 mm within the skeleton. They may also occur in dead coral 

 fragments. The commonest genus appears to be Acht/Ia, but others 

 are listed by Utseumy (1942). The little that is known about the 

 manner of boring indicates that this is due to the combined action 

 of acid production and growth pressure. There is also the suggestion 

 that entrance may be associated with the presence of nutritive 

 matter, presumably derived from the small organic content within 

 the skeleton (Duerden, 1902, 1905). Odum and Odum (1955) have 

 recently suggested that there may be some symbiotic relation be- 

 tween the corals and these algae, but, in the absence of any experi- 

 mental evidence, this is difficult to accept. 



A full understanding of the mode of boring and of the source of 

 material needed for protein svnthesis in these algae is badly needed. 

 But the problems are complex and investigation is difficult.* 



Animals. Possible direct association. Certain animals which bore 

 into coral rock may possiblv gain all or a significant part of their 

 food from the boring algae and the almost equally common and 

 widely ramifying sponges (described later). This could apply to 

 the sipunculid worms, such as the numerous species of Aspidosiphon, 

 which commonly bore into coral rock. The small cuticular plates 

 present in the skin of sand-burrowing sipunculids are here fused to 

 form massive shields with which the animal bores. Since the primi- 

 tive habit is burrowing into sand, which is swallowed in great quanti- 

 ties with digestion of the small amount of contained organic matter, 

 change to a rock-boring habit presents no major problem. It is cer- 

 tainly possible that the rock into which the worms bore may contain 

 as much organic matter, in the form of boring plants and sponges, 

 as the sand. 



What applies to these sipunculids may also, although less prob- 

 ably, apply to some of the coral-boring polychaetes. Gardiner ( 1903) 

 has listed the more important families. According to Crossland 



* Since the above was written, knowledge has been significantly advanced by 

 Wain Wright (1963). 



