32 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 27 



about 125 to about 20 g/m^. This marked decline corresponds with the 

 occurrence of the oxygen minimum layer. 



4. Four decreasing steps in the density of amphipod individuals 

 occur in canyons, the first at 0-100 m (588 individuals/m^) , the second 

 at 101-500 m (54/m2), the third at 501-1000 m (14.7/m2), and the 

 fourth at 1001-1600 m (2.8/m2). Below sill depths of the borderland 

 basins, the average number of amphipods is 6.0/m^, except in the shal- 

 low nearshore basins where only 1.2 individuals/m^ are found, ap- 

 parently in relationship to low dissolved oxygen in subsill waters. 



5. There is little change in the faunal composition between the 

 deeper ends of the submarine canyons and the subsill parts of the basins. 

 So far only five amphipod species have been found in the basins that have 

 not been collected in the canyons, although, because several basin sys- 

 tems have greater bottom depths than the canyons, more species are to 

 be expected when trawling with fine nets is undertaken. 



6. Widespread and consistently deleterious effects of sediment 

 movement within canyon axes have not been detected. Not all canyon 

 samples have produced amphipods, but all have produced faunal elements 

 of one kind or another. Hartman ( 1963) has reported on the occurrence 

 of specifically impoverished, brackish and pollution-tolerant canyon 

 faunas that probably result from the emergence of aquifers. 



7. Specifically impoverished topical faunas, dominated in part by 

 the amphipod Protomedeia articulata in Monterey Canyon, appear to be 

 related to large quantities of organic debris that have settled en masse. 

 The poor diversity indicates that a slump may have demolished the prior 

 fauna and that Protomedeia and several species of polychaetes represent 

 an early succession. 



8. About half of the known coastal shelf Amphipoda have been 

 collected in the shallow depths of the canyons and probably more are 

 present. No faunal disparities except those mentioned in paragraphs 1 

 and 7 above have been detected that would indicate that canyons com- 

 prise a special or a restrictive environment. This statement is supported 

 by the broad spectrum of sediment types collected in the canyon axes. 



9. Bathyal indicator species, especially the harpinias, occur at mini- 

 mum depths of approximately 300 m in the canyons. 



10. Several sublittoral Amphipoda seem to be abnormally displaced 

 to great depths via canyon pathways. In some cases this displacement may 



