22 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 27 



The seaward, deeper ends of many of the canyons debouch onto basin 

 floors, especially the biotically impoverished shoreward basins of Santa 

 Barbara, Santa Monica and San Pedro. The frequency of Amphipoda 

 drops to 1.5 individuals/m^ below sill depths, although the slopes and 

 canyon fans above sill depths (475 to 737 m) support between 7 and 21 

 (or as many as 43) individuals/m^. Canyon floors well below sill 

 depths, that empty onto trough floors or onto deeper basins, continue to 

 support 11 individuals/m^. The deeper outer basins (numbers 4-12 in 

 Table 7) support 6 individuals/m^, but apparently these basins, al- 

 though having sill depths well below oxygen minimum layers, have de- 

 pleted biotic frequencies, because two samples on the Patton escarpment 

 above sill depths have an average of 20 individuals/m^. Unfortunately, 

 no samples from nonbasin areas of the deep borderland, except on the 

 slopes of San Pedro, Santa Monica and Santa Barbara basins, have been 

 taken so that these values can not be confirmed. 



Diversity and Dominance of Amphipoda in the Canyons 



About 185 species of Amphipoda occur on the coastal shelf of south- 

 ern California in water depths of 5 to 183 m, according to my records 

 and a manuscript in preparation. The intertidal has not been assessed. 



The most abundant amphipod species on the coastal shelf are shown 

 in Table 11, the most abundant in depths of 92-183 m in Table 12 and 

 the most abundant in depths of 4-10 m in Table 13. Amphipoda of the 

 shallowest 100 m of the canyons (Table 14) are a mixture of species 

 from the above zones. The first two taxa of Table 14, Aoroides and 

 Parapleustes, are phycophilous, the former probably building tubes at- 

 tached to algae, surf-grass and plant debris. These are unusually high 

 rankings for both species, as the latter is almost exclusively an intertidal 

 "form. The remaining species of the canyon heads comprise a high number 

 (9 out of 16) of the common shelf species, as marked with asterisks in 

 Table 14. The high rank of Ischyrocerus pelagops is another indication 

 of the predomination of plants and/or plant debris on substrates of can- 

 yon heads. Little relationship is shown to the upper slope fauna (Table 

 12) except for the presence of Protornedeia articulata, a species that is 

 not necessarily characteristic of slope faunas because it occurs also in 

 moderate shelf depths on silty bottoms. 



Conspicuously absent from the list of important canyon-head Amphi- 

 poda is Paraphoxus bicuspidatus, the most abundant shelf and slope 

 amphipod. 



In depths of 101-200 m, the canyon fauna compares most favorably 

 with that on the upper coastal slopes, 92-183 m (see Tables 12 and 15). 



