Have I Ever Told You How Cool Crocodile Hearts Are?
When I first met my partner, I would constantly babble about whatever cool thing I had read about in my papers that day. After years of putting up with me talking about penguin knees, grass screaming, the anomalies of statistics and much much more, he suggested I find an outlet “If you find it interesting, why do you think no one else will?”. So here I am giving it a go; the first “random blurb I blurted out to him – “Have I ever told you how cool crocodile hearts are?”
Evolution constantly presents us with amazing variation and adaptions to life on our planet. Take for example the heart, simply a pump to transport, distribute and dissipate what an organism needs around its body. Over 500 million years ago the first heart appeared (Ma et.al., 2014) as a self-contracting single layered tube. Now, we have a plethora of hearts across the animal kingdom – none more interesting to me than that of the Crocodilians (which includes Crocodiles and Alligators).
What most people think of as a heart is the bird and mammal heart consisting of four valved chambers, two atria and two ventricles, but there is much greater variety to behold. Octopi have three hearts, a systemic heart (true heart) and two gill hearts (Wells, 2013) and Earthworms have five hearts (Edwards and Bohlen, 1996). Reptilian hearts have only three chambers, two atria and a single, incompletely partitioned ventricle (Koshiba-Takeuchi et.al., 2009). The partitioning of the heart into chambers means that oxygenated (from the lungs) and deoxygenated (to the lungs) are kept separate. Furthermore, valves ensure that blood does not travel in the wrong direction (Simões-Costa et.al., 2005).
Crocodilians, however, have completely separated left and right ventricles, as seen in birds and mammals. Crocodilians also possess an adaption as yet unseen in other reptiles, a structure called the foramen of Panizza (Stephenson, Adams and Vaccarezza, 2017). This structure allows Crocodilians to alter their circulatory system for when they are above or under water. During surface circulation, when the croc is breathing normally, blood enters from the body into the right aorta, travels to the right ventricle where the blood is then sent to the lungs to pick up oxygen. Oxygenated blood then returns to the heart by the left aorta where it flows into the left ventricle to be sent back around the body (Katano et.al., 2019). Conversely, when diving, the foramen of Panizza directs blood away from the lungs and straight to the left aorta, therefore bypassing the reoxygenation step, this is accompanied by the heart rate slowing (Kelly et.al., 2006).
The remaining mystery is why the organism does this. It was originally believed that this was to allow for longer periods submerged under the water as the need to replenish oxygen is decreased. More recently, it has been found that the foramen of Panizza may play a role in digestion, as increased carbon dioxide concentrations in the Crocodilian body increases secretion rate of gastric acid alongside aiding in the function of the liver, small intestine and pancreas (Farmer et.al., 2008). This could be especially beneficial in the consumption of larger prey and in dissolving bone. Farmer et.al. (2008) showed that dissolution of bone in crocodilians incapable of enabling their foramen of Panizza was significantly slower than those still capable. Farmer also goes on to suggest that it may have a function in enriching blood destined for digestive muscles with Oxygen. Others attest that it may be involved in thermoregulation.
The real answer to the function of this unique heart? Well, we do not know. Perhaps my favourite answer science can give, and the best reason to keep sharing my little random facts. We just do not know.
Sources
Edwards, C.A. and Bohlen, P.J., 1996. Biology and ecology of earthworms (Vol. 3). Springer Science & Business Media.
Farmer, C.G., Uriona, T.J., Olsen, D.B., Steenblik, M. and Sanders, K., 2008. The right-to-left shunt of crocodilians serves digestion. Physiological and Biochemical Zoology, 81(2), pp.125-137.
Katano, W., Moriyama, Y., Takeuchi, J.K. and Koshiba‐Takeuchi, K., 2019. Cardiac septation in heart development and evolution. Development, Growth & Differentiation, 61(1), pp.114-123.
Kelly, L., 2006. Evolution's Greatest Survivor Crocodile. Allen & Unwin.
Koshiba-Takeuchi, K., Mori, A.D., Kaynak, B.L., Cebra-Thomas, J., Sukonnik, T., Georges, R.O., Latham, S., Beck, L., Henkelman, R.M., Black, B.L. and Olson, E.N., 2009. Reptilian heart development and the molecular basis of cardiac chamber evolution. Nature, 461(7260), pp.95-98
Ma, X., Cong, P., Hou, X., Edgecombe, G.D. and Strausfeld, N.J., 2014. An exceptionally preserved arthropod cardiovascular system from the early Cambrian. Nature Communications, 5(1), pp.1-7.
Simões-Costa, M.S., Vasconcelos, M., Sampaio, A.C., Cravo, R.M., Linhares, V.L., Hochgreb, T., Yan, C.Y., Davidson, B. and Xavier-Neto, J., 2005. The evolutionary origin of cardiac chambers. Developmental biology, 277(1), pp.1-15.
Stephenson, A., Adams, J.W. and Vaccarezza, M., 2017. The vertebrate heart: an evolutionary perspective. Journal of Anatomy, 231(6), pp.787-797.
Wells, M.J., 2013. Octopus: physiology and behaviour of an advanced invertebrate. Springer Science & Business Media.
Written by Libby Bowles
I’m a PhD Researcher at Prifysgol Aberystwyth, studying the perenniality syndrome in Grasses. I’m passionate about public engagement in science and the sharing of knowledge regarding the world we live in, especially the weird and wonderful.