NO. 2 OLGA HARTMAN : SUBMARINE CANYONS 37 



and some other amphfodids which are well represented along the edge 

 of the shelf with hundreds of individuals in a sample, are sparse or 

 nearly absent in most canyons. The same is true of some of the 

 commonly occurring polychaetes, Pectinaria calif orniensis, Prionospio 

 spp., nephtyids, Chloeia pinnata, and a small brown ostracod typically 

 associated with Ainphiodia urtica in shelf depths. Listriolobus pelodes, 

 a tongue-worm, is very abundant on the Santa Barbara shelf and 

 occurs also in shallower depths of Hueneme canyon, but thereafter 

 is replaced by another echiuroid, Arhynchite. Sea whips and sea pens 

 which exist over large areas of the shelf and slope lands, are nearly 

 absent from canyon depths. 



All canyon walls are characterized by a change in kinds and num- 

 bers of animals, changing with depth or sediment to such an extent 

 that the majority of species at one station or depth class will be 

 nearly or entirely replaced by the next depth (see also charts of step- 

 down effects). Where sediments differ grossly the change in specific 

 entities is nearly complete. This sliding scale effect is noted also in 

 numbers of species and specimens, with the shallowest or near shelf 

 depths having the highest numbers, and the deepest and lowest depths 

 having the least. Exceptions are noteworthy, chiefly in those samples 

 containing much detritus or with gravelly sediments, or where normal 

 salinity conditions are believed to be disturbed. At the lowest levels 

 of the longshore canyons the bottoms are impoverished or dead, as in 

 the basins with which they merge. 



As the number of kinds and specimens diminishes with depth, 

 so also do biomass values but in a different way. Where there are a 

 few large individuals of brissopsid urchins, or echiuroids, or nemer- 

 teans, the total values per sample may be high, with these large 

 individuals comprising most of the weight. When these individuals 

 are lacking or. excluded from total weights, the decline is normal, with 

 increasing depth. 



STANDING CROP 



Brissopsid echinoderms, thalassemid echiuroids and polychaetes 

 comprise the largest part of the biomass values in canyons, from 

 shallow to moderate depths. Where muds and silts prevail the first 

 two are best developed, in median to low median depths. At shallowest 

 and deepest stations the polychaetes occur in greatest masses, accom- 

 panied by mollusks in upper, and by ophiuroid echinoderms in lower 

 levels. 



