| Abstrakt: |
Two distinct species of Hyalella Smith, with no intermediate forms, occur in the Arizona limnocrene known as Montezuma Well. One is referable to the inermis type of H. azteca (Sauss.). The other is a new and presumably endemic species, H. montezuma, characterized by elongate appendages, mucronation of tergites of pereonite 7 and pleonites 1 & 2, and remarkably enlarged mouthparts; the inner plate of maxilla 1 is broad and armed with up to 30 plumose setae, rather than the 2–3 apical setae that typify the genus. H. azteca occurs alone in a small stream emerging from the Well; both species coexist at the weedy margins of the Well; and H. montezuma swims alone in the open water, where it is a member of the plankton. A high level of free CO2 in the water serves as a barrier to fish, indirectly allowing Hyalella to colonize the limnetic zone where it represents, perhaps, one of four of the World's freshwater planktonic amphipods. Theoretically, the ancestors of H. montezuma arrived at the Well sometime more than 11,000 years BP, when it was a new steep-walled collapse basin lacking fish; H. azteca appeared later after a shallow littoral zone had developed. The two species are reproductively isolated at present. |
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