300 University of California Publications in Zoology [Vol. 19 



and San Joaquin rivers. During the months of flood water, April 

 and May, the water of the upper part of San Pablo Bay may be fresh, 

 or nearly so, for considerable periods, especially after seasons of heavy 

 precipitation. During the months when the amount of water dis- 

 charged is least, September and October, the salinity may rise as high 

 as 25 parts per 1000. The surface temperature of the water of San 

 Pablo Bay ranges during the year from about 6° to 19° C. 



OCCURRENCE AND HABITS OF THE SPHAEROMIDAE 



Sphaeromidae form a prominent constituent of the isopod part 

 of marine faunas. Fifteen or twenty genera are recognized and two 

 or three scores of species have been described. Of these species at 

 least five in the genus Sphaeroma are reported as boring into wood 

 or stone and several of the genus Limnoria of the closely related 

 family Limnoridae are reported as habitual borers into wood. Rich- 

 ardson (1904, p. 28) regards the Sphaeromidae as extensively dis- 

 tributed within the temperate zones and composed "nearly all of cold 

 water species, though not reaching into the Aretalian Realm." The 

 members of this family are largely shallow water forms, but some 

 marine species have been dredged from a depth of 75 fathoms (Ger- 

 staecker and Ortmann, 1901, pp. 179, 244, and 268). "The list of 

 fresh water Sphaeromidae is large for a marine family" (Richardson, 

 1904, p. 24), nine species being listed from brackish and subterranean 

 waters and even from artesian wells and warm springs, as well as from 

 rivers. 



The free-living forms of the Sphaeromidae "frequent the rocky 

 shores of the ocean . . . and are found under stones and along the 

 muddy banks of estuaries. . . . Others adhere to marine plants float- 

 ing on the surface of the sea. . . . They are able to run with consid- 

 erable agility" (Bate and Westwood, vol. 2, p. 400). Some species 

 are said to swim only on their backs, and the specimens of Sphaeroma 

 pentodon from San Pablo Bay which were kept in the laboratory 

 frequently darted aboiit the aquarium in this way when taken out of 

 their holes. They also moved about on the smooth glass floor of the 

 aquarium by walking and by a combination method of walking with 

 the legs and of propulsion with the swimmerets. 



In feeding habits most of the isopods are carnivorous, living as 

 scavengers, and a few are parasitic, but Sphaeroma is said to be a 



