sponges, and bryozoaiis tiiul no stable anchorage : and 

 rock-inliabiting snails and limpets are quickly buried. 

 Clanis, however, tiiid a sandy bottom suitable, if it is 

 hrmly packed, as they are adapted to burrowing and 

 plowing their way through a loose substratum. They 

 are able also to move through a mud bottom, but 

 where silting is Iieavy they close their valves to avoid 

 an accumulation of silt within the mantle cavity and 

 on the gills. The anodontas seem to be the most tol- 

 erant of mud bottoms. 



Some of the mayfly naiads, such as Hexagcnia. 

 are adapted to burrowing in mud, and the surface of 

 the bottom in shallow water is often closely dotted 

 with the openings of their burrows (Hunt 1953). 

 These burrows are relatively permanent in compact 

 mud but would quickly collapse in loose sand. The 

 genus Caenis is peculiar in possessing covers at the 

 anterior end of the abdomen ; they protect the gills 

 from becoming clogged with silt. Midge fly larvae 

 and aquatic annelids exist in mud bottoms : they 

 would be ground to bits among moving sand parti- 

 cles. The pond crayfish will burrow into mud down 

 to water level as a pond dries up, but stream cray- 

 fish will not do so and consequently suffer high mor- 

 tality (Bovbjerg 1952). 



The bottom is important to invertebrates and 

 vertebrates alike for placement of eggs. Some caddis- 

 fly eggs are fastened to smooth rock surfaces in long 

 strings by a cement-like substance. The eggs of other 

 species occur in jelly-like masses and may be secured 

 to plant stems or other submerged objects. Jelly-like 

 masses of snail eggs are often quite common on the 

 undersides of rocks in riffles. Some fish, such as the 

 fantail darter (Lake 1936). make nests in small cavi- 

 ties under stones, but other species, for instance the 

 rainbow darter (Reeves 1907), creek chub (Reig- 

 hard 1908), and river chub (Reighard 1943), build 

 nests in gravel bottoms in the upper parts of riffles. 

 Some of the suckers (Reighard 1920) spawn in shal- 

 low water ; their eggs scatter downstream, finding 

 lodgment in various riffles. 



RESPIRATION 



AND OXYGEN REQUIREMENTS 



Oxygen is usually ample in streams, often 

 saturating the water in turbulent riffles. The oxygen 

 concentration is sometimes low, however, in sluggish 

 streams and standing pools. The difference in oxygen 

 tension of the two habitats is reflected in the respira- 

 tory adaptations of the organisms that inhabit them. 

 The lamelliform gills of the mayfly naiads in- 

 habiting mud-bottom pools are larger in size than 

 those of species inhabiting streams, are doubled in 

 number on the anterior abdominal segments of some 

 species, and are almost continuously flicked back and 



FIG. 5-8 Apparatus for studies of 

 rheotaxis. Right, box for use In streams 

 where water enters at upper end, flows 

 through center trough, and out lower 

 end. Controls nnay be run in side troughs 

 filled with still water. Above, rheotaxis 

 pan in which current is produced artifi- 

 cially with a rod or finger. An organisnn's 

 response is positive when it turns to confront 

 the current; negative, when it faces down- 

 stream; Indifferent, when it orients crossways. 



forth for better aeration. The gills of naiads living 

 in riffles, or in waters in which the oxygen content is 

 high, may have the surface area of the gills reduced 

 by two-thirds in proportion to body weight, compared 

 with mud-dwelling forms. They are never flicked, 

 since the water movement continually brings oxygen 

 to them (Dodds and Hisaw 1924). Other species do 

 not flick their gills at high oxygen tensions, but will 

 do so when tension is reduced. In some swift water 

 species, there appears to be sufficient oxygen diffusion 

 through the general body surface to make gills ines- 

 sential equipment (Wingfield 1939). 



Caddisfly larvae have filamentous gills, and there 

 is some evidence that they increase in number as 

 body size increases and oxygen content of the water 

 decreases. It is probable that oxygen also diffuses 

 readily through the thin skin. A constant current of 

 water is maintained through their cases by undula- 

 tions of the abdomen. Stonefly naiads have poorly 

 developed filamentous gills, located on the thorax, 

 or have none at all. As a result, they are more sen- 

 sitive to variation in oxygen supply than are the other 

 forms mentioned. 



Streams 51 



