As Emily Reeves has mentioned, a peer-reviewed paper in BIO-Complexity tackles the question of the origin of how certain fish species live in extremely cold Antarctic waters. Titled “The Cardiovascular System of Antarctic Icefish Appears to Have Been Designed to Utilize Hemoglobinless Blood,” and authored by medical researcher Gregory Sloop, the paper argues that, “The circulatory system of Antarctic icefish may have been designed to prevent high blood viscosity at low temperatures by taking advantage of the increased solubility of oxygen at low temperatures, allowing use of hemoglobin-free blood.” He argues that this complex system “could not have evolved via a series of gradual steps” because:

The hemoglobinless phenotype requires simultaneous customization of the heart, vasculature, and blood, including its viscosity. Simultaneous, coordinated acquisition of multiple unique features, as required by the absence of hemoglobin, is inconsistent with Darwinian evolution, which postulates that species develop by small, incremental changes over time.

More Cold, More Viscous

When liquids become cold they tend to become more viscous. Sloop observes, however, that “the viscosity of icefish blood at its native temperature, approximately 0°C, is very similar to that of human blood at 37°C.” Fish that lives in very cold waters, such as Antarctic icefish, must therefore solve a problem: How are they able to pump blood through their bodies at such low temperatures? 

One way Antarctic icefish solve the problem is by having no erythrocytes (red blood cells) in their blood. So how do they deliver oxygen throughout their bodies? Some believe it’s due to nitrous oxide (NO) dissolved in the bloodstream, which actually causes hypoxia in icefish tissues. But Sloop maintains that it is thanks to “a customized cardiovascular system [rather] than a conventional one that was pressed into service when a mutation caused the loss of hemoglobin expression.” Some of these “cardiovascular customizations” include:

• Solubility of O₂ increases with lower temperature, allowing the blood to carry more O₂ at such a low temperature. 
• Increased cardiac output. 
• A special “high-output, low resistance circulation” where “trunk skeletal muscle capillaries are two to three times greater in diameter than those of typical teleosts, reducing vascular resistance” and “Icefish retinas are more densely vascularized than those of red-blooded notothenioids, increasing O₂ delivery to this metabolically active tissue.”
• This larger cardiac output and special vasculature require “a blood volume two to four times greater than that of red-blooded fish” which “in turn requires a customized heart that is heavier and has a larger stroke volume than that of red-blooded notothenioids.” As a result “stroke volume of the icefish heart is 6 to 15 times greater than in other teleosts.”
• To sustain this larger heart, special heart contractile cells called “cardiomyocytes” are in the icefish “relatively large and contain a relatively large number of mitochondria.” Indeed, in one icefish species the percent of cardiomyocytes devoted to mitochondria are “the highest in any teleost and higher than in any vertebrate except for the Etruscan shrew.”
• Special kidneys to accommodate the low-pressure blood circulation. 
• Special “corpuscle” blood cells unique to icefish which convert carbon dioxide and water into carbonic acid. 

An “Example of Teleology in Biology”

The effect of all of these special features is to allow Antarctic icefish to have lower hematocrits (the percent volume used by red cells in the blood), which lowers blood viscosity, making it easier for the fish’s heart to pump blood under such cold conditions. Sloop concludes:

The customized icefish heart, vasculature, and blood form a system with mutually dependent parts. … The hemoglobinless phenotype requires simultaneous, coordinated acquisition of multiple unique features. This is difficult to explain with Darwinian evolution, which postulates that species develop by small, incremental changes over long periods. … Multiple customized components are necessary to utilize hemoglobinless blood. Actualizing the design for the icefish cardiovascular system requires each customized component to be in place simultaneously. This is more innovation than can be accomplished by random mutation as postulated in Darwinian evolution.

Sloop offers this potent observation: “Proponents of intelligent design see customizations to decrease blood viscosity as examples of teleology in biology.”