Their fan-shaped pectoral fins are enlarged, and 

 project at right angles from the lower side of the 

 body. When at rest they maintain position by con- 

 tact with the bottom, fins lodged between pebbles or 

 the body partly buried. They never float suspended 

 in the water, as do other fish ; when disturbed, they 

 dart swiftly from one anchorage to another. The 

 Etheostominae are confined to North America east 

 of the Rocky Mountains. 



Stream fishes are in general quite sensitive to cur- 

 rent, and the discontinuous distribution of a species 

 within the same stream may be closely correlated 

 with gradient (Trautman 1942, Burton and Odum 

 1945). The smallmouth bass, for instance, is mostly 

 absent in southern streams of gradient less than 40 

 cm/km (2 ft/mi) : is of moderate abundance in 

 gradients up to 135 cm/km (7 ft/mi) ; is very abun- 

 dant in gradients of 135-380 cm/km (7-20 ft/mi) ; 

 and becomes less common again, until it disappears 

 altogether, in gradients above 475 cm/km (25 ft/mi). 

 Perhaps streams with very slow current do not pro- 

 vide suitable gravel nest-sites for spawning, and in 

 streams with very fast currents they are unable to 

 maintain position. 



Salamanders that live in swift mountain streams 

 generally have short limbs and toes, reduced size of 

 fins, smaller lungs in the adult and shorter gills in 

 the larvae, and relatively few large eggs, which they 

 fasten to the underside of flat rocks (Noble 1931). 



Orientation behavior 



Structural adaptations for withstanding or 

 avoiding current are of no avail without appropriate 

 behavior responses to make use of them. The rheo- 

 tactic responses of animals may be tested either in 

 the field or in the laboratory by means of special ap- 

 paratus. 



When animals from riffles and those from pools 

 are compared (Table 5-3), it is apparent that, at low 

 current velocities, responses of the two groups of 

 animals are nearly the same. The elongate body, 

 notably of stream animals, brings an automatic turn- 

 ing into the current much as wind directs a weather- 

 vane. As the velocity of current is increased, how- 

 ever, there is a marked increase in the percentage of 

 riffle animals that face into or move against the cur- 

 rent, while a very large percentage of pool animals 

 are swept away by the current or are forced to with- 

 draw into their shells. Caddisfly larvae, free of their 

 cases, are not very able to withstand a strong cur- 

 rent, although within their cases they readily main- 

 tain position. 



Blackfly larvae can tolerate water currents as 

 swift as 180 cm/sec, and studies indicate that their 

 clinging to the substratum is a response to current 



rather than to any associated factor, such as food or 

 o.xygen requirement (Wu 1931). 



When tested experimentally, 80 per cent of the 

 stream crayfish Orconectes propinquus were able to 

 maintain position in currents of 50 cm/sec, but only 

 about 20 per cent of the pond crayfish O. jodiens 

 were able to do so (Bovbjerg 1952). 



Fishes generally respond to current by showing 

 nearly 100 per cent positive response, regardless of 

 whether they be taken from streams or ponds. Since 

 the response involves a tendency to swim upstream, 

 other factors must be involved for the fish to main- 

 tain a constant location in the stream ; otherwise they 

 would all move to its headwaters. 



Some stream fishes, such as the blacknose dace 

 and the common shiner, can be shown experimentally 

 to respond visually to landmarks on stream bank and 

 bottom to maintain their location. Some pool fishes, 

 such as the sunfish and topminnow, likewise respond 

 visually, but much more sluggishly, and irregularly. 

 Darters are entirely unresponsive to visual stimuli, 

 depending on the tactile stimulus of contact with the 

 bottom for maintaining position (Lyon 1905, Clausen 

 1931). Smell may be important to some fish for 

 orientation. The backswimmer Notonecta (Schulz 

 1931 ) and whirl-i-gig beetle Dineutus (Brown and 

 Hatch 1929) have also been shown to use visual ori- 

 entation in running water. 



RESPONSES TO BOTTOM 



The segregation of stream animals be- 

 tween riffles, and sand- and mud-bottom pools may 

 be, in part, a response to type of bottom. With no 

 current flowing, the species listed in Table 5-3 were, 

 in another experiment, given a choice between a hard 

 bottom and a sand bottom. Eighty-five per cent of 

 the riffles animals selected the hard bottom, but only 

 10 per cent of the pool animals did so. Of the pool 

 animals, all species made 100 per cent response to 

 sand, except the damselfly naiad, Calopteryx macu- 

 lata, which divided equally between the two types of 

 bottom. When the riffles animals were given a choice 

 between loose stones and a bare bottom, nearly all 

 individuals selected the stones, and they distributed 

 themselves among the stones or on top or underneath 

 in the manner one would expect of them under nat- 

 ural conditions (Shelford 1914). Stream crayfish, 

 when given a choice between mud and cinders, ori- 

 ented 88 per cent to the cinders, while the pond cray- 

 fish responded 40 per cent to cinders and 60 per cent 

 to mud (Bovbjerg 1952). 



Type of bottom is important to invertebrates for 

 support and locomotion. Sand bottoms are note- 

 worthy as unstable and shifting. Insect larvae and 

 naiads find footing very uncertain ; planarians, 



50 Habitats, communities, succession 



