that were completely buried. On the algae flat, none of the pebbles 

 were completely buried, and this plot contained the only pebbles in 

 the experiment that showed no sinking whatever. 



The experiment regarding biotrubational rotation of shells described 

 provides further information on bioturbational burial . After 40 

 days, nearly half of the shells were buried, originally concave-up 

 shells and originally concave-down shells in nearly equal numbers. 

 A much greater amount of shell burial took place in the 2 plots 5 m 

 from the reef than in the 2 plots 1 to 2 m from the reef. As with 

 the pebbles , the number of shells buried depends on their size and 

 shape; small or flat shells are likelier to be buried than larger 

 or more gibbous shells. The density or weight of the shell may 

 also be a factor; of two species of approximately equal gibbosity 

 used in the experiment, distinctly more of the relatively thick- 

 shelled Anadara lienosa f loridana were buried than the much 

 thinner-shelled Laevicardixim laevigatum . 



OTHER ASPECTS OF BIOTURBATION 



Nocturnal and diurnal : A thin layer of dyed sand was dusted over 

 an area of approximately a square meter of bottom at each of the 

 experimental sites at dusk one evening. At dawn the following 

 morning, each surface was broken by 6 to 12 small cones produced 

 by burrowing organisms. In the sand strip, small gobies were 

 disturbing the sand around an abandoned conch shell, and in the 

 patch reef small trails 1-2 mm wide crossed the fresh surface. 

 After 3 hours of daylight each plot showed considerably more 

 disruption than it had at dawn. By dusk, the surface in the 

 channel mouth was much pocked by small craters, crossed by a 

 trail, and disrupted in general. In the sand strip the sand 

 was thoroughly stirred within 5 cm of the conch shell; but 

 the surface was otherwise less disrupted than at the canyon 

 mouth. On the algae flat, the surface was broken by numerous 

 small shallow holes and by sand cast up from burrows. In the 

 patch reef, small mounds and burrows interrupted the dyed sand 

 surface. The observations indicate that the sediment surface 

 is more disrupted by organisms during daylight than it is at 

 night . 



Integrity of sand surface: The dyed sand surfaces were moni- 

 tored daily to determine how long the integrity of the surface 

 would persist. The surface became undistinguishable in the 

 patch reef after 4 days and in the channel mouth after 5 days . 

 At the other sites, small (2 by 5 cm) areas of the original 

 surface remained undisturbed, but even these had a scattering 

 of undyed sand on the surface. Observations of this experiment 

 attest to the rapid bioturbational alteration of the sediment 



VI-30 



