Fig. 5. Hematocrit is related to vascular density index (VDI) and intervessel distance (IVD). (A) Among four red-blooded notothenioids with a >2.3-fold range of Hct, VDI in retinal tissue is inversely correlated with Hct (r²=0.934). (B) Intervessel distance in retinal tissue, in the same four species, is positively correlated with Hct (r²=0.898). Values are means ± s.e.m.; N=5 for C. aceratus and P. charcoti; N=2 for C. gunnari; N=3 for G. acuticeps and T. hansoni; N=4 for N. coriiceps. Channichthyids were not included in the regression analysis relating Hct to anatomical vascular characteristics because blood flow in icefishes is many times that of +Hb species. These high blood flows permit adequate oxygenation at lower vascular densities and greater spacing between vessels than would be expected based upon the relationship between Hct and these parameters (broken red lines).