The particles produced were recovered by gently washing the rock (and 

 urchin) while inside the bucket, siphoning off the water, and then washing the 

 particles into polyethylene bags. Particles, rocks, and urchins were labelled 

 and then preserved in 70% ETOH. Particles were washed free of salt, oven dried 

 at 95°C, and weighed to 0.01 mg. Twelve free-living urchins were collected and 

 their alimentary tracts removed by dissection, measured, and the length of 

 tract containing sediment estimated. The gut contents were removed, washed, 

 dried, and weighed. The organic content of sediment from the urchin alimentary 

 tracts, experimental buckets, and control buckets was determined by weight loss 

 following H9O2 digestion. Subsequently, inorganic sediment from each sample was 

 wet sieved (0.0625 mm stainless steel screen) to separate mud. Gravel and sand 

 size particles (> 0.0625 mm) were oven dried at 95°C and dry sieved at 0.5 Phi 

 intervals for 5 min using agitation by Vibrapad. Weight of each sieve fraction 

 was recorded to 0.01 mg. Particle size distributions were generated by the 

 Cal-Comp plotter from these weights. 



Attached organisms (mostly algae) were removed from each rock, washed, 

 dried, and weighed. Each rock was then oven dried, and the exposed surface 

 area was measured by the aluminum foil method (Reed, et a! . , 1982). The volume 

 of each urchin bore hole was measured by determining The volume of water 

 contained in a latex sheath placed inside the bore hole. Each rock was dissolved 

 in acid (HC1 + HNO3) to recover the infauna, which was washed, dried, and 

 weighed. Urchins were dried and weighed, and test dimensions and volume were 

 determined. 



RESULTS 



Urchin Population 



The total urchin population at Black Rock was 92.4 x 10^ adults within an 

 area of 2521 m^. Average urchin density was 37 adults m~2 (maximum was 100 

 m"2). Urchins were present between the intertidal zone and 7 m depth. Urchins 

 never were observed to leave their bore holes; during the day they remained 

 stationary at the bottom of the hole, but at night they moved within their 

 individual bore holes. The urchins fed upon both endolithic algae and pieces 

 of algae that apparently drifted into the bore holes. 



Sediment Yield 



For control rocks (without urchins), a significant correlation (r = 0.968, 

 p < 0.05) was found between the total inorganic weight of sediment recovered 

 from buckets and the total surface area of the rocks. Linear regression gave 

 the formula for this relationship: 



Y = 1.133X - 119.71, 



where Y = inorganic sediment weight and X = total rock surface area. The 

 average organic content of sedimentary particles recovered was 23.74% (range = 

 14.99 - 41.98%). The mean weight of attached macro-organisms on control rocks 



154 



