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medium ( Krogh and Leitch, 1919). The applicability of these generalizations to air-breathing fishes depends on the relative importance of air
breathing in their bimodal gas exchange. Among species emphasizing
water breathing, there is a tendency for higher 0, affinity to correlate
with the most 0,-deficient habitats. No clear-cut adaptive trends are
apparent from a comparison of the obligate air breathers and those
practicing occasional supplemental air breathing. In fishes like Electrophorus in which the 0,-rich blood is admixed with 0,-deficient blood
before perfusion of the arteries, a high 0, affinity would improve the
efficiency loss caused by the mixed condition of arterial blood.
Several authors have claimed evidence of a relationship between the
CO, sensitivity of the 0,-Hb dissociation and the habitat and air-breathing habits of fishes. Willmer (1934) offered convincing data to show that
tropical fishes from larger rivers and fast flowing creeks with wellI00
90
80
70
c
.s 60
e
3
50
8 40
8
30
20
10
0
c aJ
m
I
10
20
30 40
50 60 70 80
Po, (mm Hg)
Fig. 5. Oxygen dissociation curves showing a conspicuous Bohr effect in Myelus
living in well-aerated C02-free water. A slight Bohr effect is present in Electrophorus
living in stagnant oxygen-deficient, COa-rich water. Redrawn from Willmer ( 1934).