Basin and Slope Environment (Bathyal) 



173 



located at the mouths of burrows of large 

 worms. These hillocks probably remain for 

 centuries after the worm that made them 

 dies — until they become buried under the 

 slowly deposited sediment or perhaps erased 

 by a rare turbidity current. Similar features 

 were observed by Peres and Piccard (1956) 

 during dives in the bathyscaphe in the Med- 

 iterranean Sea; plowing through the hillocks 

 uncovered no inhabitants, as though the 

 burrows had long been abandoned. Hil- 

 locks of related origin are present on the 

 shelves, where the concentration of animals 

 is greater than in the basins, but there the 

 hillocks are probably eroded away by the 

 long-period swells entering the region from 

 the Southern Hemisphere each summer. In 

 the still shallower water of marshes, where 

 animals and evidence of their activities are 

 even more abundant, most of the burrow 

 heaps are destroyed daily by the current 

 and wave erosion at high tides. 



Not to be overlooked are bacteria, which 

 live in great abundance in the sediments of 

 the basin floors — more abundant there than 

 in the sediments of the shelves because of 

 the higher percentage of organic matter in 

 the fine-grained basin muds. The highest 

 bacterial counts occur at the sediments sur- 

 face where both organic matter and dis- 

 solved oxygen are most abundant. Plate 

 counts of 100,000 to 100,000,000 cells per 

 gram of wet sediment are typical, far greater 

 than in the basin water. At depth in the 

 sediment the counts decrease sharply at first 

 and then more gradually; nevertheless, sig- 

 nificant numbers have been found at all 

 depths that have been sampled. Counts are 

 usually made either by direct visual enum- 

 eration under the microscope or by a growth 

 method. The direct count method is time 

 consuming, cannot usually distinguish be- 

 tween dead and living cells, overlooks small 

 cells, and is compHcated by the presence of 

 grains of sediment. More commonly used 

 is successive dilution of samples, incuba- 

 tion, and counting of colonies. Dilution 

 may be carried to extinction, dilution so 

 great that not a single cell is present in a 

 diluted fraction, or dilution may be less ex- 



Figure 1 50. Graph showing frequency at various depths 

 of dominant living mollusks in terms of percentage 

 number of specimens. Averages were computed for 

 following depth ranges: 7-14, 14-36, 36-73, 73-146, 

 146-220, 220-366, 366-550, 550-732, and 732-915 me- 

 ters. Graph by O. L. Bandy (1958) from data of Wilson 

 (1956). 



Pelecypod faunas: 1, Compsomya subdiaphana, Luci- 

 noma annulata, Macoma yoldiformis, Protothaca tener- 

 hma, Tellina buttoni; 2, Nuculana taphria, Parvilucina 

 tenuisculpta, Solamen columbianum, Solemya panamen- 

 sis, Solen rosaceus, Thyasira barbarensis; 3, Axinopsis 

 sericatus, Lyonsia californica, Nemocardium centhfilo- 

 sum, Nucula carlottensis; 4, Adontorhina cyclica, Aligena 

 sp., Amygdalum pallidulum, Yoldia scissurata; 5, Acila 

 castrensis, Cyrilla munita, Tellina carpenteri; 6, Cardita 

 redondoensis, Saxicavella pacifica; 7, Dacrydium sp., 

 Nuculana conceptionis; 8, Xylophaga washingtonia; 9. 

 Kellia suborbicularis, Rochefortia sp., Xylophaga mexi- 

 cana. 



Amphineuran fauna: Chaetoderma sp., Leptozona 

 catalinensis, Limifossor sp., Lepidopleurus nexus, Neo- 

 meniinid. 



Scaphopod fauna: Cadulus fusiformis, Dentalium neo- 

 hexagonum, Dentalium rectius, Cadulus tolmiei. 



Gastropod faunas: 1, Halistylus subpupoides, Odo- 

 stomia sp., Turbonilla sp.; 2, Balcis rutila, Cyclichna 

 diegensis, Epitonium tinctum, Micranellum crebricinctum, 

 Volvulella californica, Volvulella tenuissima; 3, Bittium 

 catalinensis; 4, Amphissa bicolor; 5, Leptogyra sp., 

 Nitidella permodesta. 



