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SUMMARYAntarctic icefishes possess several cardiovascular characteristics that enable them to deliver oxygen adequately in the absence of hemoglobin (Hb). To gain insight into mechanisms driving development of these cardiovascular characteristics of icefish, we chemically induced severe anemia in a red-blooded notothenioid, Notothenia coriiceps. After 10 days of treatment with phenylhydrazine HCl, the hematocrit and Hb concentration of N. coriiceps decreased by >90% and >70%, respectively. Anemic fish exhibited a significantly higher concentration of nitric oxide (NO) metabolites in their plasma compared with that of control animals, indicating that corporeal levels of NO are higher in anemic animals than in control fish. The activity of nitric oxide synthase (NOS) was measured in brain, retina, pectoral muscle and ventricle of control and anemic animals. With the exception of retina, no significant differences in NOS activities were observed, indicating that the increase in plasma NO metabolites is due to loss of Hb, which normally plays a major role in the degradation of NO, and not due to an overall increase in the capacity for NO production. To determine whether loss of Hb can stimulate remodeling of the cardiovascular system, we measured expression of HIF-1a, PHD2 and VEGF mRNA in retinae of control and anemic fish. Expression of all three genes was higher in anemic animals compared with control N. coriiceps, suggesting a causative relationship between loss of Hb and induction of angiogenesis that probably is mediated through nitric oxide signaling.

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The Colorado pikeminnow Ptychocheilus lucius is a federally endangered species restricted to the Colorado River. Dexter National Fish Hatchery and Technology Center, Dexter, New Mexico, maintains three separate captive stocks of the Colorado pikeminnow: 1974YC (Yampa River), 1981YC (Green–Colorado rivers), and 1991YC (Colorado River). We surveyed mitochondrial DNA diversity in these three stocks (879 base pairs [bp]; ND‐4L and ND‐4 genes; N = 30) and in museum specimens collected from 1890 to 1976 (450 bp; ND‐4; N = 11). All individuals had the same haplotype except for a 1‐bp variant detected in 2 of the 12 fish from 1981YC. The low variation is postulated to be the result of a post‐Pleistocene bottleneck in the wild and small sizes of the hatchery founder populations.