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26%. The eye tissue Why The Greenland Shark Often Goes Blind And Survives

For decades scientists have assumed that Greenland sharks were essentially blind because they live in near-total darkness at great depths & would not need vision, similar to many cave-dwelling species like Mexican blind cavefish, cave salamanders, cave crayfish & cave amphipods, all of which lost their eyes because they are useless—there’s no light to detect. This is known as regressive evolution—if a trait is costly to maintain & provides no benefit. Greenland sharks, one would think, would follow the same example, but it turns out, according to a new study published in Nature Communications, this is not the case. By mapping the genome & analyzing the eye tissue, researchers have discovered that not only can these sharks see, but their eyes are marvels of biomechanical engineering that break all the rules of what govern eyesight at those depths, in addition to the parasites that were long supposed to remove eyesight from them.

Those are not the only misconceptions about their eyes. They thought that the extreme depths (around 3,000 meters (9,840 ft)), enough to implode submarines, would likewise implode their eyeballs. No. That is countered by highly flexible photoreceptors; their adapted cartilaginous skull adapted to deep-sea compression, stable proteins & membranes, & their eyes evolved for millions of years for very low light. Instead of the cones required for color, their retina is full of long rods and incredibly powerful DNA repair pathways that guard against age-related macular degeneration. They can live up to 400 or 500 years, making them the longest-living vertebrate animals on the planet! They maintain very high levels of rhodopsin, the rod photoreceptor pigment sensitive to blue-green wavelengths, which dominate in the Arctic Ocean.

Because of the extreme cold (0 to 4°C [32 to 39°F]), one would think that the eyes of any very long-lived animal would become stiff, brittle, & dysfunctional after centuries in near-freezing water, but they aren’t. Why? Because their eyes are loaded with a fish oil, an omega-3 fatty acid known as DHA (docosohexanoic acid), which lubricates their eyes, acting as a sort of antifreeze & keeping them from getting stiff and useless. To compare, a Greenland shark's eyes contain 41% DHA vs. a cow, which has just 26%. Now as for those parasites that permanently attach to the cornea of their eyes, they are called Ommatokoita elongata & measure about 1.2 inches (30 mm). The parasites attach themselves with their mouthparts to the shark’s eyes & feed on corneal cells & mucus. This does cause scarring on the surface of the eye, producing a slight cataract but not blindness. Crucially, it does not invade or damage the retina, optic nerve, or photoreceptors. It’s often just one parasite dangling from one spot. The rest of the cornea may be unobstructed & the other eye may not have a parasite. Also, in the deep sea, vision is not about detail or sharp edges. It’s about detecting movement, not clarity. Their vision is more than sufficient for hunting, navigating & mating. Exactly what a 400-year-old shark needs.

#science #biology #marine biology #evolutionary physiology #ophthalmology #parisitology #biochemistry #oceanography

Most biologists regard a complete digestive system as an evolutionary improvement over an incomplete digestive system because it permits functional specialization -- different parts of the system may be specially adapted for various functions of food digestion, nutrient absorption, and waste excretion.

Digestive and Excretory Systems