24 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 27 



Hartman, 1959) was brought up in a deepwater sample in Hueneme 

 Canyon (177 m). The distribution of this species had been rather thor- 

 oughly investigated, and it was known to be replaced on the upper coast- 

 al slope by other genera of spoon-worms. Because Listriolobus lives on 

 sediments that are very finely particulate for the coastal shelf, its perim- 

 eter is limited to a small area of silt on the Santa Barbara coastal 

 platform. Other areas of suitable sedimentary texture for Listriolobus 

 would lie somewhere on the low-gradient fans at the base of the coastal 

 slope and canyons; these presumably would be in waters too deep for 

 Listriolobus. 



Continental slopes have been poorly studied, although Emery and 

 Terry (1956) reported on a slope sediment with a median diameter of 

 22jii, approximately the same as that found in the Listriolobus beds. But 

 presumably the average slope sediment is coarser than the average outer 

 shelf sediment (75-100 m), hence restricting Listriolobus. The sedi- 

 ments of submarine canyons are patchy and extend from black muds 

 bearing coarse organic matter to coarse sands pouring down the canyon 

 heads; but generally as the gradient decreases the sediments become 

 finer and merge with basin or trough muds and clays. Some canyon bot- 

 toms support sediments of extremely high organic content, producing 

 hydrogen sulfide and methane. 



The find of Listriolobus in Hueneme Canyon indicated that some 

 shelf species descend to greater than normal depths wherever suitable 

 sedimentary texture exists. Perhaps other species descend regardless of 

 texture ; they may be dependent on factors such as the availability of 

 organic matter. The two variables, grain size and organic content, are 

 usually complementary but the supply of organic matter to patches of 

 fine sediment trapped on the coastal slope far from shore may be too 

 low to support feeding by various organisms. Some species may ignore 

 the steep thermal inclines in favor of adequate food supplies or ab- 

 sence of competition. 



The lack of slope biological samples hinders the detection of other 

 shelf species that descend only down canyons, but presumably a num- 

 ber of taxa listed in Table 19 have descended to greater than normal 

 depths in the canyons. Their maximum depths (within 10 m) on the 

 coastal shelf are based on 348 samples. Some marked with asterisks are 

 known to be associated with algae and may have been rafted down the 

 canyon slopes. Insular shelves and slopes of the offshore islands have 

 not been adequately sampled but a few samples from those places re- 

 vealed species living at greater depths than on the mainland shelf, ap- 



