Do you want to live forever?

By Kali Horn

THIS IS A GREENLAND SHARK

Ever thought immortality would be neat? Maybe like a vampire or a mythical being? Well the closest you can get on this planet requires living in the depths of the freezing cold ocean and eating dead things. The Greenland Shark, Somniosus microcephalus, is one of the longest living vertebrates on this planet, living up to 345 years old!!!!  345 years ago was 1672. I’m no historian, but I do know the USA was not even in existence yet!

Unlike early American history, and vampires, which we all know lots about…. we know only a few of the puzzle pieces that make up Greenland sharks lifestyle and their resulting environmental impact. They are often found as by catch, meaning the un-intended catch by fisherman, and that is where most of our information on their biology comes from. For example, we think female sharks take at least 130 years to reach sexual maturity, but are unsure as to how we affecting their populations. We also know they get large, and they obviously need to eat to get large, but we can’t really say how dominant their role in the arctic food web is. Scientists are just beginning to scrape the iceberg that is Greenland sharks. (Nielsen 2016)

Let’s go through what we do know about Greenland sharks and how they physiologically manage to live such crazy long lives.

Step one: Live somewhere deep and cold. Greenland sharks live around Greenland (surprise!), the North Atlantic Ocean near Iceland, Norway, and Canada.

Figure 1: Grey shaded region is the known range of the Greenland shark (MacNeil et al. 2012)

Their preferred temperature range is from -0.6-12 C^o and in the arctic (not surprisingly) there is a lot of ocean that fits that temperature range. Greenland sharks have been observed close to shore in less than 1 meter of water, but also at abyssal depths greater than 1200 meters! Also, during those really deep dives they experience hypoxic conditions (aka low oxygen) (MacNeil 21012). Low levels of oxygen in living organisms leads to oxidative stress, which relates to aging and cell death via the production of free radicals. Free radicals are uncharged super reactive molecules that ultimately mess up things in the cell, like proteins, lipids, etc. = not good for you or me or the sharks. This oxidative stress theory has taught scientists a lot, but has a few holes, one being the Greenland shark. Through this theory, it is posed that long-lived species are more resistant to oxidative damage and have lower antioxidant levels (substances that prevent oxidation or remove nasty by products) since they produce less free radicals. Yet, Greenland sharks were found to still have relatively high levels of antioxidants likely due to their adaptation to deep dives that take them into these hypoxic zones (Constantini 2017). Species that deal with repeated episodes of hypoxia (like the Greenland sharks diving for meals) have higher levels of antioxidant defenses to protect themselves from free radical production during re-oxygenation. So yes, drink your POM juice and eat your blueberries for those days when you feel like diving a mile down in the ocean.   

Figure 2: Swimming behavior of a Greenland shark.
These graphs show a Greenland shark’s tail beat frequency(Hz) depth (m)
swimming speed (m·s⁻¹) and pitch (degrees) (aka angle)
over 24 hours in June 2009. They are slow pokes.(Wantanabe 2012)

Step two: Be super low key.  Even though Greenland Sharks are about the same size as the charismatic great white shark, at an average of 23ft long, they are way more laid back. Greenland sharks are part of the shark family Somniosidae, or the sleeper sharks, named for their apparently slow swimming speeds, low activity, and non-aggressive nature. Basically they are super chill and that helps them live super long. They have a tail-beat frequency of 0.15 Hz and mean max speed of 0.74  m·s⁻¹ and acceleration during burst swimming (0.008 m·s⁻²). These rates are some of the LOWEST for a fish of their size (Watanabe 2012). Mola molas and Atlantic Cod are faster. Scientists have only tagged a handful of these sharks and most of the times don’t even see them hit their max speeds. In Figure 2, they were able to show one tagged Greenland shark’s behavior and only really saw them speed up when they were bombing down to some deep water. Because of their low levels of activity and large size, scientists assume they have low metabolic rates (Watanabe 2012)… but they don’t have any actual numbers to prove it.

Figure 3:  Gut contents from sharks over three geographic areas in Greenland. 

You can see it is mainly seals and fish. YUM (Neilsen 2014)

Step three: Don’t be a picky eater. They mainly live off of fish (although they are rarely observed hunting being so slow) and any dead remains they happen upon, from seals to polar bears, and even moose and horses have been found in their gut (gross)! The smell of rotting meat underwater actually attracts these sharks.  Since these beings live in such cold dark waters, they rely heavily on their sense of smell.  

Figure 4: This is what the Greenland shark olfactory organ looks like 

when removed from a dead shark and cut up (Ferrando 2016)

Their olfactory organ, or olfactory rosettes, consists of a number of lamellae (think internal skin folds) that increase the surface area on the olfactory epithelium. Although this is common in shark species – Greenland sharks have a much higher surface area than most. The higher surface area allows for them to detect more subtle chemical cues similar to scents in the water(Ferrando 2016). Additionally, since they are slow, but like to eat seals, they have to work smarter not harder (because they are too slow anyways). They use their keen olfactory rosettes to sniff out arctic seals that sometime sleep in the water to avoid predation by polar bears. If the seals were awake they would be much too fast for the Greenland shark to catch. (Watanabe 2012).

So you still want to live forever? Hope you like to live in the deep cold depths of the arctic, move slow, and smell/eat dead things with the Greenland sharks! That is….. unless you would rather be a bivalve -  the ocean quahog (a clam) can be over 500 years old! Wonder how they manage that physiologically…. NEXT TIME!

If you want to learn more about the Greenland shark here are some neat vids 

References:

Constantini, D., Smith, S., Killen, S., Nielsen, J., Steffensen, J. ( 2017) The Greenland shark: A new challenge for the oxidative stress theory of ageing? Comparative Biochemistry and Physiology. 203:227-232.

Ferrando, S., Gallus, L., Ghigliotti, L., Vacchi, M., Niesen, J., Christiansen, J.S., Pisano, E. (2016) gross morphology and histology of the olfactory organ of the greenland shark Somniosus microcephalus. Polar Biology 39:1399-1409.

MacNeil, M.A., McMeans, B., Hussey, N., Vecsei, P., Svavarsson, J. , Kovacs, K., Lydersen, C., Treble, M., Skomal, B., Ramsey, M.,  Fisk, A. (2012) Biology of the Greenland shark Somniosus microcephalus. Journal of Fish Biology. 80:991–1018.

Nielsen, J., Hedeholm, R., Simon, M., Steffensen, J. (2014).Distribution and feeding ecology of the Greenland shark (Somniosus microcephalus) in Greenland waters. Polar Biology. 37:37-46.

Russo R, Giordano D, Paredi G, Marchesani F, Milazzo L, Altomonte G, et al. (2017) The Greenland shark Somniosus microcephalus— Hemoglobins and ligand-binding properties. PLoS ONE 12(10): e0186181.

Watanabe, Y., Lydersen, C., Fisk, A., Kovacs, K. (2012) The slowest fish: Swim speed and tail-beat frequency of Greenland sharks. Journal of Experimental Marine Biology and Ecology. 426:5-11.

Photos: 

http://blog.wwf.ca/blog/2014/01/09/tagging-greenland-sharks-jaws-north/