FRESH-WATER MUSSELS. II9 



ANIMAL ASSOCIATES. 



In the previous discussion of the Naiades in relation to the physical environment, 

 there has been shown to be an adaptation by certain species to particular physiographic 

 situations, as to pond, lake, river, swift or quiet water, hard or soft bottom, etc. In 

 any habitat each mussel is in association \vith other mussels of the same or other species 

 and with animals and plants of various classes, all more or less adapted to the same 

 en\"ironment. Such an association of organisms forms a community, the members of 

 which interact more or less upon one another and upon their environment. The con- 

 sideration of these communities with reference to th^ir members and to the environment 

 often reveals important relations. Because of the mutual relations existing, a dis- 

 turbance or destruction of any one element, by affecting a balanced condition, may 

 cause a marked disturbance of the whole community. (See Shelford, 1913, p. 17.) 

 Some of the relations between mussels and their associates may be described as com- 

 petition, symbiosis and commensalism, parasitism, and preying. A description of a 

 typical habitat with its inhabitants will illustrate the variety of life associated with 

 mussels. For Oneida Lake, N. Y., Baker (1916, p. 94) gives an account of a particular 

 sort of habitat which he designates the bulrush-waterwillow type, where there is not 

 great exposure to waves, where the bottom is more or less covered with stones and 

 bowlders, but with sandy spots here and there, where the depth varies from i to 4 

 feet, and where the vegetation consists of bulrushes, waterwillows, and pickerelweed. 



The principal differences between this habitat and the bowlder type are the less exposed situation, 

 the density of the vegetation, the deeper water, and the sandier bottom. Such a habitat is particularly 

 favorable for black bass, sunfish, rock bass, and others, because of the hiding and breeding places 

 provided by the thick vegetation, the attachment for eggs by the roots and stems of plants, and the 

 excellent feeding ground, by the abundance of animal life, insect, crustacean, and molluscan. The 

 largest number of molluscan species, 39, occur in this type of habitat, including upwards of 15 which 

 are known to be eaten by bottom-feeding fish. [The following numbers of species are listed: Mussels, 

 II, including several species of Anodonta and Lampsilis; univalves, 16; crustaceans, i (crayfish); 

 Sphseriids, 10; leeches, 5; insects, 4.] 



A typical association of mussels and other species in Andalusia Chute, Mississippi 

 River, near Fairport, Iowa, is as follows (Howard, unpublished notes) : 



Bottom — gravel, rock, and sand. 



Water — depth -sX to 7.'2 feet. Current at surface estimated 2 miles. 



Haul — 250 feet in length, with cro\vfoot drag 10 feet wide and with dredge 18 inches wide. 



Distance from edge of water — 20 feet. 



Mussels — Lampsilis gracilis, 3; Plagiola elegans, i; P. donaciformis, 3; Quadrula ebenus, 1; Q. 

 metanevra, 3; Q. pustulosa, i; Q. utuiata, 2; Strophilus edentulus, i; and Unio gibbosus, 3. Total, 18. 



Bivalve — Musculium transversum Say, i. 



Bryozoa — Plumatella polymorpha Kraepelin, i colony. 



Snail — Vivipara subpurpurea Say, 36; Pleurocera elevaitttn, Say i. 



Flatworm — Planarian . 



Leech — Placohdella parasitica Say. 



Insects — Stonefly, Perla sp. (larvae); ma^-fly, Chirotenetes. i (larva); Heptagenia, 14 (larvse); 

 Polymitarcys, 2; dragonfly, Gomphtts extermis, 5, Argia, 3 (larvse), Neurocordulia, i; caddisfly, Hydro- 

 psyche, 70 (larvae); beetle, Pamids, 2 (adult). 



Crustacea — Crayfish, Cambarus. 



In communities of animals and plants, as the individuals increase in numbers there 

 may develop the keen competition for food which has been designated as the struggle 



