COMMUNITIES OF THE SEA BOTTOM 339 



North Pacific Communities 

 Pandora- Yoldia Community (Biome) 



This community has been found on mud bottom in 3 to 75 meters 

 of water on shoreward protected spots among the islands south of 

 the Frazer River. The bottom is characterized by fine mud and a deep 

 layer of plankton detritus. The forms generally present are the asym- 

 metrical bivalve {Pandora filosa Carp.), the yellow bivalve {Yoldia 

 Umatida Say), and the thin-shelled clam {Marcia subdiaphana Carp.). 

 Certain annelids, such as species of Sternaspis and Amphicteis, are 

 usually present and also one snail {Phacoides tenuisculptus Carp.). 

 The fragile starfish {Luidia foliolata Grube) is always found in some 

 numbers, and the large 20-rayed star is represented by scattered indi- 

 viduals. The biome is divisible into two associations, one of which 

 has been studied much more than the other; however, it appears that 

 the Alaska shrimp {Crago alaskensis [Lock]) and the small bottom- 

 inhabiting fish called red devil {Lyconectes aleutensis Gilb.) are com- 

 mon in both. In general, crabs and hermit crabs, especially the 

 latter, are few. The two associations may be recognized through out- 

 standing differences in about half the constituent species. 



Cucumaria-Scalibregma Association. This community occurs in 

 areas of somewhat higher salinity and with less silt than the other 

 association described below. The white cucumber {Cucumaria popu- 

 lifera (Stimp.) and the expanding worm {Scalibregma inflatum Rath) 

 are usually found in great abundance, together with the forms listed 

 as characterizing the biome. Among the brittle stars, Ophiopholis 

 acideata var. kennerlyi (Lyman) was present in varying numbers dur- 

 ing the several years of study. The short-finned eelpout {Lycodes 

 brevipes Beau.) and the Pacific eelpout {Lycodopsis pacificus Coll.) 

 were usually infrequent. A number of faciations have been described 

 by Shelford and by Weese (Shelford et al., 1935), and the latter dis- 

 cusses succession from this association to land. 



Clymenella-Yoldia Association. This occurs near the mainland 

 where the fine mud of the bottom contains more silt and less organic 

 material than the other association. The bamboo-worm {Clyynenella 

 rubrocincta John) and the giant nudibranch {Dedronotus giganteus 

 O'Don) are very abundant. The brittle stars are represented by 

 Amphiodia urtica (Lyman) ; Scalibregma is sparse, but usually pres- 

 ent. All the species characteristic of the larger community are to be 

 found. 



