several days to a few weeks. Species living in tem- 

 perate climates have either one or two generations 

 per year. Some females drop their eggs while in flight 

 but others crawl under the water to deposit them. 

 The eggs are laid in masses in either a single-layered, 

 cement-like encrusting form or in a jelly or gelatinous 

 matrix that swells in water. Eggs are sometimes de- 

 posited on objects above water. Usually, 10 to 24 

 days are required for their hatching (Balduf 1939). 



The common hellgrammite of North America ap- 

 pears to require three years to complete its life-cycle, 

 of which it spends two years and eleven months as 

 an aquatic larva. When ready to pupate, the larva 

 crawls out of the water and underneath some loose 

 stone or piece of wood. The adults do not eat and 

 live only a few days. The female lays her eggs in 

 masses attached to supports situated near water or 

 to the upper surface of leaves. Upon hatching, the 

 larvae make their way back into the water (Balduf 

 1939). 



In the crayfish Orconectes propinquus copulation 

 occurs in cool climates from July to November. Fur- 

 ther southward, copulation is delayed until Septem- 

 ber, continues until cold winter weather, and is re- 

 newed again during March and April. Eggs are laid 

 beginning in late March or early April and are car- 

 ried around by the female, attached to her pleopods, 

 or swimmerets. The eggs hatch in 4 to 6 weeks, and 

 the young are carried for another week or two before 

 they become free-swimming. The majority of the 

 young become sexually mature at the end of the first 

 growing season in early October (Van Deventer 

 1937). 



The female adult black fly deposits her eggs in a 

 mass or string on a stone or other object at water 

 level during late afternoon, usually with only the tip 

 of the abdomen submerged. If the eggs become ex- 

 posed to the air they do not hatch ; normally, the lar- 

 vae appear in four or five days at medium water 

 temperature of 20°-22°C. The larval stage persists 

 13 to 17 days, the pupal period a little more than 4 

 days, and the adult stage a little over a week when the 

 adults feed, or only 5 or 6 days when they do not 

 (Wu 1931). 



Stream snails attach their eggs in a jelly mass to 

 the sides of stones during late spring and summer, 

 and development leads directly to the adult. Clams 

 of the family Unionidae, however, have a peculiar 

 mode of reproduction. The sexes are separate, and 

 fertilization of the eggs takes place in the supra- 

 branchial chambers of the female. Development takes 

 place through several weeks in these marsupial gills, 

 and each egg grows into a minute glochidium. These 

 larvae are later shed into the water, where further 

 development requires that the glochidia become at- 

 tached to the gills, skin, or fins of fish. The larvae 

 may be parasitic, feeding on nutrients absorbed from 



the fish ; this stage may last from 9 to 24 days. Later, 

 the cyst formed by the fish around the glochidium 

 weakens, and the young animal escapes to take up a 

 free-living existence. Breeding occurs from May to 

 August in Quadrula and Unio, while in some spe- 

 cies {Anodonta, Lampsilis) breeding does not occur 

 until late in the summer and the glochidia are re- 

 tained in the female over winter. 



The life history of clams is of special significance 

 in showing that dispersal depends, to a large extent, 

 on the movements of the fish to which the clams are 

 attached. There is evidence that some species of 

 clams cling to particular species of fish only, so that 

 distribution of the two forms in the stream is closely 

 correlated. The fingernail clams Sphaeriidae, on the 

 other hand, are hermaphroditic and lack the glo- 

 chidial stage. The fingernail clams are annuals ; the 

 larger unionid clams may live 10 to 15 years (Coker 

 et al. 1922, Boycott 1936, Matteson 1948). 



Some sponges, and perhaps also bryozoans, are 

 perennial, although they may become fragmented as 

 a result of floods or freezing during the winter ; they 

 may die during times of low water. Both kinds of 

 animals have vegetative buds, gemmules in sponges 

 and statoblasts in bryozoans, that become free of the 

 parent body. The buds are adapted to withstand un- 

 favorable drought or winter periods, and to germinate 

 and form new colonies when favorable conditions re- 

 turn. 



The nesting habits of some stream fishes have 

 already been mentioned. Some of the darters and 

 dace defend their nests, or small territories around 

 their nests, against intruders ; other species appear 

 to not do so. Individuals of territorial species do little 

 wandering, and it is possible that a darter may persist 

 through several generations in the same riffles. There 

 is increasing evidence that some larger species of 

 stream and pond fishes have definite home areas, and 

 that the fish population of a small stream with riffle- 

 pool development may be considered as a series of 

 discrete, natural units. This has been demonstrated 

 with tagged individuals for species of bass, sunfish, 

 suckers, and bullheads (Gerking 1953). Homing 

 tendencies, however, are developed to varying de- 

 grees, and some species appear to move around in a 

 quite random manner (Thompson 1933). 



FOOD COACTIONS 



The basic food substances for stream ani- 

 mals are detritus, diatoms, and filamentous algae. 

 Detritus consists of dead fragments of plants ; par- 

 tially decomposed, finely divided, plant material ; and 

 a certain amount of dead animal matter. Plankton, 

 either plant or animal, is not normally a common 

 source of food, except in outlets from the lakes and 



54 Habitats, communities, succession 



