Notes on Two Darters of the Genus Boleosoma

Abstract

TWO currently recognized subspecies of darters, Boleosoma nigrum nigrum (Storer) and B. nigrum olmstedi (Rafinesque) have geographical ranges that overlap along the south shores of Lake Ontario (Hubbs and Lagler, 1941) and both forms are reported in the Allegheny-Chemung drainage (Greeley, 1938). The University of Rochester Museum of Natural History collections include specimens identified as the western form, B. nigrum nigrum, from as far east as Mudge Creek, Wayne County, and others identified as the eastern form, B. nigrum olmstedi, from as far west as Otter Creek, Orleans County. That the two groups should continue to be recognized as subspecies of a single species is somewhat dubious. Their ranges are compatible with their present status, but clear evidence of intergradation in their zone of overlap is lacking. The diagnostic key characters listed by Hubbs and Lagler (1941) for B. nigrum nigrum are: dorsal soft rays usually 11 to 13, and nape, breast, and cheeks scaleless; while for B. nigrum olmstedi they are: dorsal soft rays usually 13 to 15, and cheeks scaly. By these characters, (1) specimens taken within the zone of range overlap can be rather readily assigned without ambiguity to one or the other subspecies, and (2) collections from a single stream or even a single sample include both forms. The latter situation exists in Irondequoit Creek, Buttonwood Creek, Salmon Creek, and North Creek, Monroe County, New York (see Table I). In considering these facts it was felt that the intergradation to be expected of subspecies from the zone of range overlap might appear if more characters were studied. Furthermore, the separation of two such forms is somewhat subjective, in part because they have one number of soft dorsal fin-rays in common, namely 13. This is roughly the number one would expect in intergrades. But a collector might understandably assign any given 13-ray specimen to one or the other subspecies, on the basis of scutellation. This procedure would ignore the fact that intergrades might well resemble one subspecies in one character and be intermediate between the two subspecies in other characters. It seemed desirable, therefore, to re-investigate these two forms inside and outside their zone of range overlap with respect to characters in addition to those given by Hubbs and Lagler (1941). It has been customary, where a character complex has been studied, to establish the taxonomy of closely related forms by treating each character difference separately. Ginsburg (1938) and Hubbs and Perlmutter (1942) have demonstrated pictorial and statistical representation of such differences. Wherever a taxonomic judgment is to be based on a complex of characters, the problem immediately arises as to how the evidence from separate character differences can best be combined into one conclusive judgment. There have been three general approaches: (1) the worker examines the chosen characters of each specimen and arrives at a largely subjective decision concerning the material; (2) the worker examines a series of meristic characters,