NO. 2 OLGA HARTMAX: SUBMARINE CANYONS 95 



b. On the Replacement of Species Within Canyons: — The replace- 

 ment of species by depth or canyon is an aspect of zonation best 

 illustrated where there is a considerable specific replacement (see 

 Analyses in the APPENDIX). An example is the distribution pattern 

 of two conspicuous, nearly related brissopsid echinoderms, — Brissopsis 

 pacifica and Brisaster townsendi. Because they are large, easily identi- 

 fied and may comprise a considerable part of the animal weight in a 

 sample, they are used to illustrate the change of specific occurrences 

 with latitude, depth and proximity to shore. The canyons are named 

 from north to south and from longshore to ofifshore. Depths are given 

 in meters and range of sampled depths in the right hand column. 



It is shown, therefore, that Brisaster townsendi is most abundant 

 in northern and longshore canyons and attains its maximum abundance 

 in Hueneme canyon, in 397 meters, whereas Brissopsis pacifica is most 

 abundant in Santa Cruz canyon, in 459 meters, as well as in middle 

 and southern longshore canyons. In their median ranges the two may 

 occur in equal numbers. 



Among the ophiuroids, Amphiodia digitata attains its greatest 

 numbers in shallowest depths along shore and diminishes thereafter; 

 in Redondo canyon their numbers are: 388 specimens at 57 meters; 

 39 at 107 m, 109 at 122 m and 21 at 146 meters. 



Amphipholis squomata attains highest numbers in Redondo canyon 

 with 43 specimens in 57 m, 12 in 344 m, 22 in 123 m, and 54 in 

 177 m. Its highest numbers in ofi'shore canyons are in Catalina, with 

 105 specimens in 88 m. 



Amphipholis pugetana, in Redondo canyon, numbers 49 specimens 

 in 542 m; in Santa Cruz canyon there are 10 in 218 m, and 95 in 

 459 meters. In Tanner canyon its numbers rise to 290 in 496 m, 

 and diminish to 23 in 644 m. This species is more characteristic of 

 outer than of longshore canyons. 



Amphiacantha amphacantha in Redondo canyon, numbers 11 in 

 107 m, 24 in 120 m, 25 in 122 m and 5 in 146 meters; its vertical 

 range is thus restricted. 



Amphioplus strongyloplax in Redondo canyon numbers 14 speci- 

 mens in 146 m, with fewer numbers at other depths. 



Amphiodia urtica is one of the most abundant ophiuroids along 



