Bottom Fauna (continued)

Table 4 shows the snail fauna found in the bottom samples from Chillicothe to Peoria Lake. Even where there are surviving snails, Richardson finds them to be greatly reduced in number and weight in 1920 as compared to 1913-15.

Table 4: Snail Fauna of Bottom Muds, Chillicothe to Peoria Lake, 1913-15 to 1920 (Richardson 1920)

The Viviparidae, Pleurocidae, Valvatidae, Physidae and Ancylidae families are part of the Gastropod Class, or freshwater snails. The family Sphaeriidae is referred to as fingernail clams (Figure 20 & 21) because of their small size. They are no more than 3-20 cm (0.5 in) in length. Their habitat includes lakes and streams of all sizes in silt, mud, sand, or gravel. They vary widely in tolerance to pollution.

Figure 20: Fingernail Clam Shell
(Iowa State University)

Figure 21: Fingernail Clam
(Nordsieck)

Between 1912 and 1920, a number of species disappear from the river altogether: two formerly abundant species of Viviparidae, two species of Valvata, two Amnicolas, three or four species of Physidae and Planorbinae, one species of Ancylus, and at least three species of Sphaeriidae. In 1920, only three species of clams occur in the lake and river channel between Chillicothe and Peoria: Campeloma subsolidum, Pleurocera species, Musculium transversum, all of which are highly tolerant of polluted waters and substrate. Even the snails that continue to survive show great reduction in numbers and size. Table 5 shows the dramatic reduction in number of individuals of three categories of snail fauna, collected in three zones: the Upper Lake, Middle Lake, and Lower Lake

Table 5: Snail Fauna of Bottom Muds, Chillicothe to the Foot of Peoria Lake (Richardson 1920)

1. Average number per square yard

2. Percentage, by weight, of average total hauls contributed by snails