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THE BIOLOGY OF MARINE ANIMALS 



Boring Animals 



The reefs, rock, coral and wooden materials found in the inter-tidal 

 zone and shallow inshore waters provide niches and crevices in which 

 many animals lodge themselves. Beginning with efforts to enlarge and 

 shape such ready-made cavities, or to burrow into soft shale, animals 

 from several phyla have acquired the capacity to excavate hard materials — 

 stone, wood or shell — and fashion their own burrows or galleries. The 

 specialization of structure and function entailed by the adoption of this 

 mode of life is amply compensated by the protection conferred, and in at 

 least two groups of wood-borers, the gribble and shipworm, a new source 



Fig. 15.13. A Rock-boring Polychaete, Polydora ciliata 



(a) Worm extracted from its burrow; (b) mud tubes erected at the two openings of the 

 burrow, from one of which the tentacles of the worm are extended; (c) diagrammatic 

 section of the burrow with worm in position. Enlarged. (After Caiman (14).) 



of food is also exploited. The following account deals first with animals 

 that excavate stone, shell or coral, followed by wood-borers. 



Excavation of Stone and Shell. Animals that bore into stone include 

 sponges, polychaetes, sipunculoids, crustaceans, sea-urchins and molluscs. 



The monaxonellid sponge Cliona excavates limestone rock and molluscan 

 shells. The borings are rather shallow, seldom exceeding 5 cm in depth, 

 and consist of a series of branching passages which open at frequent 

 intervals to the surface and which accommodate the sponge. Cliona 

 sometimes riddles shells, including those of the oyster, and causes much 

 damage. Actual penetration is ascribed to chemical action by the sponge. 



Several species of polychaetes bore into limestone rock and shell, 

 including Polydora (Spionidae) and Potamilla (Sabellidae). Polydora is 

 a common form which makes U-shaped cavities in shell or rock (Fig. 

 15.13). The excavations are often prolonged by a tubular extension of mud 

 over each of the openings. Since the body of the worm, apart from para- 

 podial chaetae, is soft, it is believed that burrowing is probably accom- 

 plished or aided by chemical means (14, 123). 



