Life Lines: The surprising lives of sharks
By Elof Axel Carlson
I was reading an article on the Greenland shark, Somniosus microcephalus, and I thought of my only other encounter with a shark (other than a slab on my dinner plate). That was when I was getting my bachelor’s at NYU and taking comparative anatomy.
One organism we dissected was the dogfish shark, Squalus acanthias. The sharks have no bones. They have a skeleton made of cartilage. The difficult challenge for my classmates and me was dissecting the inner ear within the cartilaginous capsule encasing it. I learned to respect surgeons, especially those working on the ears (like correcting otosclerosis of its calcareous deposits without breaking the coated set of bones that normally help us hear).
I learned that most sharks give birth to live young (puppies) rather than depositing eggs. Sex for sharks is a bit of a contortion act since the male (usually smaller than the female) uses one of its modified tail fins in lieu of a penis to inseminate a female. I also learned that they are quite ancient in the evolutionary scale, dominating the seas in the mid-Devonian era (about 390 million years ago) before the bony fishes out did them in adaptability.
That brings us back to S. microcephalus, which translates from its Latin name to an insulting “sluggish shark with a tiny head.” As its common name implies, these fish are located mostly in the Arctic circle and are spared an endangered species status as they are toxic to humans (and other predators) because they accumulate trimethylamine oxide in their tissues.
Inuits and others who live in that frosty region have learned to treat and ferment the fish so it is not as toxic; but even as a delicacy for the adventurous, it is not a popular item for those who catch fish for a living.
The sharks grow very slowly (less than half an inch a year) and swim at a leisurely pace of about one foot per hour. In addition to accumulating the toxic trimethylamine oxide, they also accumulate large amounts of urea in their tissues, which also contributes to their unsavory reputation among gourmets.
To make matters worse, the Greenland sharks are pretty ugly because they have luminescent parasites (copepod Ommatokoita elongata) that attach to their eyelids and use this to attract prey to their mouths. Although an opportunistic predator with much of their diet being decayed meat from drowned tetrapods and dead fish — they can swallow the floating carcass of a caribou — the sharks have been known to ambush and eat sleeping seals.
So why would such a revolting creature be attractive to research biologists? The answer is surprising. Greenland sharks are the longest lived vertebrates, living to be about 392 (272-512) years from radioactive carbon dating of crystals that are deposited in lenses of their eyes, which are layered like onions. They become sexually mature at about age 150 and attain a full mature adult size of 18 to 21 feet in length.
There is an irony to some of life’s winners of desired traits. Want to live as long as a Greenland shark? OK, make yourself toxic and marinate in urea. Try visiting your relatives at a speedy swimming rate of one foot per hour. Want to be cancer free no matter how old you get? OK, be like a naked mole rat (if you like subterranean life and ant hill type living).
We admire diversity among the millions of species of living things; but in addition to the instructive lessons of life (“Go to the ant thou sluggard”), we can find irony and humor in the knowledge we gain.
Elof Axel Carlson is a distinguished teaching professor emeritus in the Department of Biochemistry and Cell Biology at Stony Brook University.