Sea water freezes at -1.9C which presents a problem for any creature that wants to live in Arctic or Antarctic seas. Some species of fish have antifreeze proteins (AFP’s) that allow them to live in such environments. Such proteins bind to ice crystals and inhibit their growth. Consequently the freezing point of blood and body fluids in such fish is about 2 degrees below the freezing temperature of seawater. The best studied species of fish with AFP’s are the notothenioids which live in the Antarctic. Several of which are pictured below.
AFP’s evolved from pancreatic trypsinogen some 5-14 million years ago. It is an excellent example of a gene being co-opted for a new function. It used to thought that such proteins were present at hatching, but new research, published in the Journal of Experimental Biology has cast some doubt on this assumption. Researchers examined freezing resistence in three species of fish (Gymnodraco acuticeps, Pagothenia borchgrevinki and Pleuragramma antarcticum) and discovered only one of them (P. borchgrevinki) had AFP levels high enough to offer protection. So how do the larva of the other two species survive without AFP’s? Here is the problem:
The average freezing point of the larval fish fluids was about -1.3 degrees Celsius, according to testing with a nanoliter osmometer. Yet the fish hatch into water at almost -2 degrees Celsius. “With all this ice around, there is no way they can prevent freezing,” Cheng said. “At -2 degrees Celsius, internal fluids would freeze instantly and the baby fish would die.”
“This 0.7 of a degree Celsius is small but very significant,” Cziko said. “In adults, we find ice in their bodies but these small crystals don’t grow because of antifreeze proteins. Finding that larval fish don’t have enough antifreeze really threw off how we understand survival in fish.”
So how do they survive? According to the Science Daily article (I don’t have access to the Journal of Experimental Biology) it is because of:
Looking more closely, the researchers discovered that the gills of all three species were undeveloped at hatching, minimizing the risk of ice passing through them to get inside.
The delicately thin skin of the larval fish may offer additional protection, because their skin hasn’t yet been exposed to environmental damages, Cheng said. [I’m a little skeptical of this part, but Science Daily doesn’t give enough info for me to really say one way or the other – afarensis] The skin and undeveloped gills, Cziko said, may combine to allow time for antifreeze levels to rise.
In these two species AFP levels start increasing at around 87 days after hatching and reach adult levels at around 147 days.
If I were snarky I would point out that the intelligent designer doesn’t much like fish larva, much prefering to protect adult fish rather than their offspring…
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