of undersea habitation: the orientation of empty pelecypod valves; 

 the significance of Malacanthus plumieri (sand tilefish) as a 

 geologic agent; the rate of surface and internal biogenic modifi- 

 cation as a function of environment: the rate and depth of bio- 

 turbational burial of pebbles as a function of environment and of 

 size, shape, and composition of the clasts; nocturnal as compared 

 with diurnal bioturbation; the duration of surface integrity; the 

 degree of lateral sand transport by organisms; and the effects of 

 specific organisms. 



ORIENTATION OF EMPTY PELECYPOD VALVES 



Empty pelecypod valves traditionally have been thought to lie pref- 

 erentially in their most hydraulically stable position, concave 

 side down. This attribute has been used as a criterion for deter- 

 mining the tops of beds in sequences of deformed sedimentary rocks. 

 Observations made during Tektite I indicate that empty valves in 

 iinagitated water lie predominantly concave side up. This orien- 

 tation theoretically could result from either the action of pre- 

 dators and scavengers or bioturbation (Emery, 1968, p. 1268) . An 

 objective during Tektite II was to determine the effect of bio- 

 turbation on shell orientation and the function of shell size and 

 shape of bioturbational rotation. 



In order to determine the effect of bioturbation on shell orienta- 

 tion, nearly 1200 shells were placed near a coral reef on unvege- 

 tated sand at a depth of about 10 m and left for 40 days . The 

 shells were divided among four plots, two 2 meters distant from the 

 reef, two 6 meters distant. Each contained an identical assemblage 

 of seven shell species in five different size classes. In two of 

 the plots, one at 2 meters, one at 5, all shells were placed concave 

 side up; in the other two, all were placed concave side down. As 

 the bottom at the experiment site was not disturbed by waves or 

 currents during the 40 days of observation any changes in shell 

 orientation that occurred during this time resulted from bioturbation. 



A tabulation, at the end of the experiment, of shell orientation 

 by size, species, and whether or not the shells are buried provides 

 considerable data concerning rotation of shells by bioturbation. 

 Most of the shells (61.4 percent) lay concave-side up, demonstrating 

 that bioturbation is capable of producing a surficial assemblage of 

 dominantly concave-up shells . The concave-up tendency in general 

 increased with increasing shell size, both overall and within indi- 

 vidual species. The orientation also depends on shell shape, the 

 rounder, more gibbous valves showing a greater rotation to a concave- 

 up position than flatter shells. Of the shells that were buried 

 beneath the sediment surface, 51.5 percent lay concave-up, of those 

 remaining on the surface, 70.6 percent; the difference is probably 

 related to a difference in the net rate of rotation to a concave-up 

 position between buried and exposed shells . A more complete descrip- 

 tion of the shell orientation studies is given in Clifton Cin pressl. 



VI-23 



