662 THE BIOLOGY OF MARINE ANIMALS 



mandibles: these are dissimilar in shape, the right one having a sharp 

 point and roughened edge, which fits into a groove with a rasp-like surface 

 on the left mandible. The density of infection sometimes reaches 45-60 

 animals per square centimetre. 



Other crustacean wood-borers are Sphaeroma, a large isopod which 

 burrows into timber and soft rocks, and the amphipod Chelura (Fig. 

 15.16). The latter usually occurs in association with the gribble. It works 

 closer to the surface than the gribble, and appears to require the pioneer 

 assistance of the latter before it can effectively attack the wood (14, 

 137). 



Coral-reef Dwellers 



Coral reefs are inhabited by a great variety of boring animals. Many 

 species bore into dead coral or coral limestone, including sponges (Spiras- 

 trella), polychaetes (Eunice), molluscs (Lamellibranchia, Amphineura), 

 sipunculoids, cirripedes (Lithotrya) and sea-urchins (Cidaris, et al). 

 Reference has already been made to the rock-boring habits of bivalves 

 such as Lithophaga, Gastrochaena and Petricola, which also attack reef 

 coral. The most remarkable of reef-boring bivalves is Tridacna. Small 

 species bore downwards into calcareous rock by grinding action of their 

 valves, until they are flush with the surface. Since the shell has rotated 

 with reference to the body in these animals so that the hinge now lies on 

 the under-side, the free edges of the valves face upwards at the surface. 



A reef chiton Acanthopleura rasps out shallow burrows with the radula, 

 and various polychaetes excavate burrows in calcareous rock by means of 

 hard jaws, aided perhaps by stiff bristles. Reef-burrowing sipunculoids 

 are provided with bands of hard skeletal material about the anterior and 

 posterior ends. A rock-boring barnacle Lithotrya excavates burrows in 

 coral rock, into which it withdraws when the tide is out. Boring is accom- 

 plished by means of studs on the peduncle : the studs consist of a chitinous 

 core overlain by a calcareous covering, and they are shed and renewed at 

 intervals. These various animals assist in eroding and destroying coral 

 reefs. 



Living coral colonies are not attacked to the same extent as dead coral 

 masses by boring animals. Borers found in living coral include sponges 

 (Cliona), polychaetes, sipunculoids, cirripedes and various bivalves 

 (especially Lithophaga). 



There is a great variety of animals which settle on coral when young, 

 and grow with the colony. Some of these coral dwellers are highly special- 

 ized. Various species of terebellids and sabellids settle on growing coral 

 colonies and increase in size with the latter. Crabs of the family Hapalo- 

 carcinidae live commensally with reef corals. These curious creatures 

 settle on the coral which grows up around them. The mechanical activity 

 of the crab and the respiratory currents which it creates in some way 

 influence the growth of the coral, resulting in the formation of pits or 

 galls. Other decapods with similar commensal habits are Paratypton, a 



