are disc-shaped and pressed close upon tlie sul)- 

 stratiim, to wliicli they cHng with tiieir legs and 

 backward-chrected spines. They are sometimes called 

 water pennies. When ready to pupate the larvae 

 crawl out of the water. 



■iiiiidinp the current 



Diminutive body and appendage sizes and as- 

 sumption of a stream-line shape keep the amount of 

 surface exposed to the full impact of the current at 

 a minimum. The conical shape of the limpet Fcrris- 

 sia and the flatter cone of water pennies offer little 

 resistance to water flow. Flat bodies, such as arc 

 found in many swift-water animals, appear to be not 

 only an adaptation lowering resistance to current but 

 also to escape it by enabling the animals to seek 

 shelter in crevices and underneath stones (Dodds 

 and Hisaw 1924, Nielsen 1950). Most species, even 

 those with specialized means of clinging to the bot- 

 tom, are more abundant on the undersides of rocks 

 in riffles than they are on the uppersides. Some spe- 

 cies, however, such as the free-living caddisfly lar- 

 vae, rotifers, tardigrades, water mites, and proto- 

 zoans, find shelter within the mass of algae that may 

 cover the top of the rocks. The hellgrammite, tabanid 

 fly larvae, and stream crayfishes possess no special 

 structures for withstanding currents and only occur 

 in riffles providing protection or lodgement under- 

 neath and between rocks. Even swift-water fishes, 

 strong swimmers, take maximum advantage of what- 

 ever protection is available. 



The clams avoid the full force of stream current, 

 and at the same time retain position, by lodging their 

 bodies between stones. In pools, they bury them- 

 selves in an oblique position in the gravel, sand, or 

 mud. Their posterior ends are directed upstream (ac- 

 cording to Ur. Max Matteson), and their siphons 

 usually maintain contact with open water so there 

 can be circulation through the mantle cavity, for gain- 

 ing food and oxygen. Clams occurring in pools one- 

 half to one meter in depth may remain more or less 

 sedentary, but those occurring in shallower waters 

 move around considerably, especially in res|)f)nse to 

 changes in water level and temjjerature. 



S,i, 



Locomotion of swift-water invertebrates is, in 

 the main, restricted to short-distance crawling. May- 

 fly naiads that occur in riffles do not swim, although 

 related sjiecies frequenting quiet waters do so, regu- 

 larly. Only the more vigorous fishes can maintain 

 position in swift currents by swimming, and many of 

 them do so only when feeding. At other times they 

 congregate in the pools that occur between riffles. 

 Salmon and trout are well known for their ability 

 to swim against strong currents, an accomplishment 

 of sheer force of powerful, muscular, tails. The sub- 

 family of darters, Etheostominae, which contains a 

 variety of brightly-colored small fish, are especially 

 adapted to live in the riffles. The air bladder of the 

 darters has become very degenerate, even absent, so 

 that the specific gravity of the body is increased. 



TABLE 5-3 Rheofacti 



ic responses 



of Invertebrates from riffles and oools (from Shelford 1914). 



Averages 



Streams 47 



