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Percent composition of pebble size classes from spawning and non-spawning substrates of nests were used to test the hypothesis that distribution of pebble sizes is random in nests of Exoglossum laurae, Exoglossum maxillingua, Nocomis leptocephalus, Nocomis micropogon, Nocomis platyrhynchus, and Nocomis raneyi in Virginia. In nests of the two species of Exoglossum, spawning areas (i.e., upstream bases of nests) contain significantly greater amounts of the 6.0 mm size class, and significantly smaller amounts of the 2.5 mm size class of stones than do non-spawning areas. Spawning areas (i.e., pits) in nests of N. leptocephalus contain significantly more 6.0 mm pebbles than non-spawning areas, whereas spawning areas (i.e., troughs) in nests of N. micropogon, N. platyrhynchus, and N. raneyi are composed of significantly more 6.0 and 11.3 mm pebbles than non-spawning areas. In all Nocomis species, there was significant selection against the largest (23.0 mm) size class of pebbles in spawning areas. Male Exoglossum and Nocomis expend significant amounts of time reorganizing substrate material in spawning areas of their nests before and during spawning. Reorganizing pebbles results in relatively uniform sizes of substrate material at spawning areas in nests of each species. We propose that selection of 6.0 and 11.3 mm size classes for spawning areas is related to spawning behaviors and enhancement of survival of eggs and larvae in nests. Sizes larger than 11.3 mm interfere with spawning behaviors, and sizes smaller than 6.0 mm form a compacted substrate, which can crush eggs and larvae during trough or pit reshaping, and impede water flow, and consequently, aeration of eggs and larvae.

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