106 FRESHWATER ANIMALS 



the unwary investigator is great, because each population may 

 exhibit considerable variation. Furthermore, in the late spring 

 and early summer, when the Daphnia are likely to be most 

 numerous, their shapes tend to be least distinctive (cf. Figs. 3, 



4). 



There is, however, a further possible complication to species 

 recognition which may be the result of the continued coexistence 

 of several species. This is the oft-noted tendency for two popula- 

 tions of different species inhabiting a lake to show some resem- 

 blance to each other (see, for example, some of Woltereck's pa- 

 pers; Kiser, 1950). Thus, each of the two common species in 

 Moosehead Lake, Maine, Daphnia dubia and Daphnia galeata 

 mendotae, tend to differ slightly in body shape from their char- 

 acteristic forms (cf. Fig. 8) — the helmet of galeata may be more 

 slender, that of dubia broader usual. Other structures may show 

 some intermediacy, yet the specific identity of each is not in 

 doubt. This phenomenon has been interpreted by Woltereck as 

 being a direct effect of the peculiar features of the water of the 

 lake in question on the species living in it. A more reasonable 

 hypothesis is that the genes of the two species are somehow ad- 

 mixed in the individuals of such populations. One possibility is 

 that these populations have developed from a chance hybrid 

 between the two species, at one of the infrequent periods of 

 sexual reproduction. A hybrid individual genetically very like 

 one of its parents might be able to compete successfully with its 

 parents. Of course, only one viable female hybrid is necessary 

 to start a clonal population. If its genotype were superior in the 

 peculiar environment of that lake, especially at some critical time, 

 it could conceivably replace the parental species most like itself. 



Hybridization. Tims we are introduced to the third aspect of 

 the species problem in Daphnia. This is the occurrence of popu- 

 lations partaking of the characteristics of two species. The follow- 

 ing facts about these intermediate populations are noteworthy. 



1. They occur more commonly in pond-dwelling species than 

 in lake species. This is consistent with the fact that periods of 

 sexual reproduction are more frequent in the pond species and 



