Evidence for Sympatric Species of Gasterosteus in Enos Lake, Vancouver Island, British Columbia

Abstract

The threespine stickleback, Gasterosteus aculeatus, is widely distributed along temperate coasts throughout the northern hemisphere. Throughout this vast geographic range the species occurs in both fresh and marine waters and is characterized by a bewildering array of ecological forms and morphologically distinctive geographic isolates. Ultimately, some of these forms and isolates will probably receive formal taxonomic recognition. At present, however, the taxonomy of Gasterosteus is confused (MILLER & HUBBS, 1969; HAGEN & MCPHAIL, 1970) and neither the geographical nor morphological limits of the nominate species (G. aculeatus L.) are clear. An example of this problem is the situation in Enos Lake on Vancouver Island, B.C. In this small (17 hectare) lake a limnetic and benthic form of Gasterosteus coexist (Fig. 1). Because of differences in male nuptial colour and courtship behaviour, the two forms are reproductively isolated (RIDGWAY & MCPHAIL, 1984). Morphologically, the limnetic form is slimmer, has a narrower mouth, and more and longer gill rakers than the benthic form. These differences are inherited and have remained stable for over a decade (MCPHAIL, 1984). The differences between the forms in trophic structures reflect differences in feeding behaviour. The limnetic form is a plankton-feeder; whereas the benthic form is a benthos-feeder. F1 hybrids are intermediate in feeding performance between the parental forms (BENTZEN & MCPHAIL, 1984). These benthics and limnetics also differ in allele frequencies at three loci (Mdh 3, Ck, and Pgm). At one of these loci (Mdh 3) the benthic form is fixed for the common allele; whereas in the limnetic form a variant allele is at 18% frequency (MCPHAIL, 1984). Such a difference is