NO. 2 HARTINIAX : QUANTITATIVE SURVEY 251 



SAN PEDRO SHELF 



The San Pedro (SP) shelf, in depths to 50 fms, supports a diversi- 

 fied and complex fauna, changing from one location to the next, accord- 

 ing to kinds of sediments, location, and other physical factors. Some shal- 

 lowest depths, ofif Seal Beach, characterized as reddish brown beach-sand, 

 have a unique association of animals; similar deeper sands, in 17 fms, 

 have different kinds and numbers of animals. Vertical zonation is simi- 

 larly demonstrable in gray to black silts, in hard-packed sands, and in 

 rocky or mixed sediments — all of which are represented in the San Pedro 

 area. Changes of specific units with depths, kinds of sediments and other 

 factors, are indicated on the charts of species; they show not only eco- 

 logical groups of species, but vertical zonation, and relative abundance 

 of the more numerous kinds. Proportionately few species occur in all 

 depths, from shallowest to greatest; most are limited to one or few sta- 

 tions ; some other species show replication by association with other bio- 

 logical units. 



The charts demonstrate further the repeated occurrence of species 

 within genera ; in polychaete genera it is shown for species of Luinbri- 

 neris, Pherusa, Prionospio, Glyccra, Spiophanes, and others. These oc- 

 currences may be more than incidental, for it can be assumed that the 

 "rain of larvae" is present over all; their settlement and growth may be 

 restricted by factors concerned with peculiarities of larval or planktonic 

 adaptability. 



Another process of some interest is the frequent recurrence of non- 

 reproductive individuals, especially in outer, slope areas ; here shelf-species 

 are present but do not attain maturity ; they show the same patterns of 

 distribution and growth as do those below sill depths of offshore basins. 



1 he changes of species with depth, kinds of sediments, faunal com- 

 position, and ecological associations are shown for the polychaetes, echi- 

 noderms and mollusks, in the charts below. 1 he amphipods, which were 

 equally numerous in some samples, and other crustaceans are omitted from 

 the tabulations because they have been incompletely named, although 

 more than 200 species have been named or identified in reports by Dr. J. 

 L. Barnard (1959-1963) and many identified collections are consultable 

 in the collection of the Allan Hancock Foundation. 



A comparison of numbers of species and specimens, in depths of 6.5 

 to 48 fms, shows no decline or rise with depth of bottom, except in rocky 

 areas, where numbers appear lower ; this is due, in part, to the ineffective 

 sampling methods, for when samples are hand-picked, as by diving, the 

 numbers rise remarkably. The values of specific diversity, on the San 

 Pedro shelf, by increasing depth, are illustrated : 



