Basin and Slope Environment (Bathyal) 



171 



Table 10 



Samples with or without Life from San Pedro and Santa Monica Basins 

 at 50-meter intervals above and below sills 



a reconnaissance way. The best-sampled 

 one is Santa Catalina Basin with ten sam- 

 ples. Only one to three samples have been 

 taken in Santa Cruz, San Nicolas, Tanner, 

 West Cortes, and San Clemente Basins. 

 Photographs have also been taken of the 

 floors of several of the basins, and samples 

 plus photographs serve as general guides as 

 to the findings to be expected after more 

 thorough sampling has been done. Photo- 

 graphs reveal an abundance of polychaete 

 worms, ophiuroids, holothurians, and com- 

 atulid crinoids (probably Florometra sp.). 

 Also frequent in the samples are brachio- 

 pods {Laqueus sp.), siliceous sponges, urchins, 

 and sea whips. Most of the species are 

 endemic, as is well shown by the fact that 

 dredgings by U.S.S. Albatross in 1904 con- 

 tained large numbers of new species. Only 

 2 of 182 species of annelid worms were 

 cosmopolitan. The recent work aboard 

 R/V Velero IV has recovered additional 

 new and endemic species (Hartman, 1955fl). 

 Perhaps the high endemism is a result of the 

 broad area in the bathyal depth zone as com- 

 pared to the narrow belts present on most 

 continental slopes of the world. Another 

 factor doubtlessly is the physical isolation 

 of the basins from other regions and even 

 from each other by the ridges and island 

 barriers. The rich population of the six 

 outer basins as compared to the three im- 

 poverished inner ones is shown by their 

 average biomasses of 9.9 grams/sq meter, 



average of 15 species per sample, and aver- 

 age bioindices of 1.6. The abundant popu- 

 lation is ascribed to the high content of dis- 

 solved oxygen in the bottom water, which 

 in turn is the result of the position of basin 

 sills well below the depth of the oxygen 

 minimum in the open sea. Samples from 

 the side slopes would probably be richer in 

 life and comparable to samples from the 

 slopes of the inner basins, but to date no 

 samples have been processed. 



Among the larger animals mollusks are 

 of greatest interest to geologists because 

 they almost alone have hard parts capable 

 of being preserved in the geological column. 

 Most important in this respect are the three 

 classes, pelecypods, gastropods, and scapho- 

 pods (Fig. 149). Of the two other classes, 

 amphineurans (chitons) are less easily pre- 

 served and cephalopods are rare. Exami- 

 nation of the graph of percentage of sam- 

 ples at various depths containing these 

 mollusks shows that representatives of each 

 class are more frequently encountered on 

 the outer than the inner shelf and that 

 all except amphineurans are less abundant 

 with depth on the slopes of Santa Monica 

 and San Pedro Basins. On the deep floors 

 of these two basins they are absent. Per- 

 haps a better comparison is that of the rel- 

 ative percentage of the total number of 

 individual specimens in various depth ranges 

 (Fig. 150). Such a comparison shows that 

 amphineurans and gastropods are most fre- 



