Esteval phenology of an Acanthocyclops (Crustacea,Copepoda) in a Colorado tarn with remarks on the vernalis-robustus complex

Weekly mean lenghts of adult female Acanthocyclops robustus ranged between 1.14 and 1.41 mm in a Colorado alpine lake during the summer of 1966. The smallest animals occurred in late September and the largest in mid-August, corresponding to development in warm and cool water, respectively. The males also declined in mean length during the course of the summer and were about 70% of the length of the females on any date. About 75% of the females possessed a 3.4.4.4 spine formula; 8%, 2.3.3.3 and 17% an anomalous formula. Eight-one percent of the males had a 3.4.4.4 formula; 7%, 2.3.3.3 and 12% were anomalous. Morphological variation, and breeding experiments performed by Price (1958) suggest that A. robustus and A. vernalis as defined by Kiefer (1976) are likely species complexes, rather than just two species.