was 4.6 g dry weight (range = 1.3 - 13.9). The average yield of inorganic 

 particles produced by infauna of the control rocks was 0.50 +_ 0.07 mg cm _ 2 dayl 

 (X + 1 SD; table 1). This rate of sediment production was equivalent to 5.0 g 

 m-2~~ dayl, and for the area of 2521 m^ in which these rocks occurred, the yield 

 was 12.6 kg day*. The average dry weight of infauna recovered by acid dissolu- 

 tion of each control rock was 8.6 g (range = 4.5 - 13.8). Dominant infauna 

 included eunicid polychaetes, sipunculids, boring sponges, Lithotrya barnacles, 

 pelecypods, and other microborers. 



For experimental buckets (rock + urchin), the sediment recovered was a 

 mixture of particles produced by the rock infauna and by the urchin inside its 

 bore hole. We assumed that the amount of sediment passing through the mesh 

 from outside sources was the same for control and experimental buckets. Because 

 our underwater observations showed that visibility was 12-18 m, we further 

 assumed that the possibility of mud contaminating the buckets was insignificant. 

 The average organic content of particles recovered was 15.83% (range = 11.75 - 

 20.30%). The average weight of attached macro-organisms was 6.7 g dry weight 

 (range = 1.0 - 13.1). To calculate the weight of sediment produced solely by 

 the urchin, we used the regression formula (above) and subtracted the weight of 

 sediment made by the infauna of the experimental rock from the total inorganic 

 weight of sediment recovered. The calculation for each experimental bucket was 

 made as follows: 



[total organic-free wt] - [1.133 x area of experimental 



rock - 119.71] 



mg urchin - ! day - * = x 24. 



duration of experiment in hours 



These calculations indicated that the average weight of inorganjc sediment 

 produced by adult urchins was 242.2 + 146.0 mg urchin -1 day* (X + 1 SD, table 

 1). The total urchin population produced 22.4 kg day - l or an average of 8.9 g 

 m - 2 day -1 within the 2521 m^ area containing urchins. Urchin bioerosion was 

 64% of the total 13.9 g m - 2 dayl measured by our bucket experiments. In 

 nature, physical processes of abrasion and other bioeroders (fish) would 

 contribute additional particles. 



Particle size analysis showed that the percentages of sand (2.00 - 0.0625 

 mm) and mud (< 0.0625 mm) were similar for sediment from alimentary tracts of 

 uncaged urchins and from control and experimental rocks. The alimentary tracts 

 of the urchins contained mostly pellets (fig. 1C) which readily disintegrated 

 into sand and mud. No gravel (> 2.00 mm) was found in alimentary tracts of 

 urchins. Sediment from control and experimental buckets contained - 28.5% 

 gravel. Sand fractions from alimentary tracts of uncaged urchins contained 

 particles most abundantly between 0.125 - 0.177 mm (fig. 2, top) and particles 

 of this size were also the most abundant in sediment recovered from experimental 

 buckets which contained eolianite and a living urchin (fig. 2, middle). As 

 determined with a low-power microscope, the grain size of sand particles in the 

 eolianite is the same as the modal grain size of sand from urchin alimentary 

 tracts (0.125 - 0.177 mm). However, sediment from urchin alimentary tracts was 

 50% mud, which greatly exceeds the mud content of the eolianite. These 

 observations suggest that urchins cause erosion at Black Rock by breaking 

 individual eolianite grains away from the substrate, yielding sand, and also by 



155 



