84 Carol E. Johnston 



Hunter and Hastier (1965) found the nest to be the least important 

 stimulus in attracting spawning redfin shiners, Lythrurus umbratilis 

 (Girard), and Kramer and Smith (1960) found that golden shiners, 

 Notemigonus crysoleucas (Mitchill), would spawn over nests of large- 

 mouth bass, Micropterus salmoides (Lacepede), built in a variety of 

 substrates. These observations suggest that something other than the 

 substrate of the nest causes nest associates to spawn over the nests of 

 other species, possibly the parental care of the host or benefits from 

 other factors. 



A potential advantage to nest associates of spawning over the nest 

 of another species is increased egg survivorship with no costs of parental 

 care (nest building and defense). Increased egg survival can occur in 

 gravel-mound nests such as those built by Nocomis because of better 

 aeration and reduced siltation. Survival is also increased because male 

 Nocomis cover the eggs and defend the nest against predators. A 

 consequence of the nest-associate strategy, for both associate species 

 and nest builders, is hybridization. Hybrids between nest builders and 

 associate species have been described (Greely 1929, Raney 1940b, 

 Reighard 1943, Miller 1963). Hybridization can be viewed as a cost, 

 especially if hybrid progeny are inviable or sterile, or as a benefit if new 

 and adaptive gene combinations are introduced into the populations by 

 viable hybrid backcrosses (Futuyma 1986). 



The advantage to nest builders, such as Nocomis, of having other 

 species use their nests is difficult to understand, because associates do 

 not contribute to nest building and most associate species are too small 

 to drive egg predators away. Evidence suggests that the larger species of 

 associates aid in driving egg predators from the nest (Hankinson 1932, 

 Vives 1988). It is possible that nest builders benefit from a selfish-herd 

 effect (Hamilton 1971), in which the eggs of the nest associate are as 

 likely to be eaten as those of the nest builders, or more likely to be 

 eaten. The result would be increased egg survivorship for the host. 

 Because so many nest associates usually spawn over a given nest, they 

 may also benefit from the selfish-herd effect; spawning adults may 

 benefit directly from this effect as well as through the survival of eggs. 

 In that case the relationship would be mutually beneficial, and the nest 

 builder would be expected to tolerate, or even encourage, nest associates. 

 If the reproductive success of the nest builder is lowered as a result of 

 the intrusion of nest associates, the relationship would be parasitic, and 

 one would expect the nest builder to actively discourage nest associates 

 from spawning over his nest. It is possible, however, that nest builders 

 are energetically incapable of driving nest associates away. If that is the 

 case, one might expect to find adaptations to prevent associates from 

 finding nests, temporal shifts in spawning, or other mechanisms to 



