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The wonderful net: why mackerel sharks are not coldblooded killers

By Dorien Schröder, Dutch Shark Society

In our Instagram profile on the Porbeagle shark, we briefly mentioned their amazing countercurrent exchange system that allows them to be warm-blooded, the retia mirabilia. This system occurs in all sharks of the Lamnidae family, the mackerel sharks. The family consists of the Porbeagle, white shark, longfin and shortfin mako and the salmon shark. The common and bigeye thresher are also warm-blooded.

A salmon shark is a warmblooded animal / picture by Andy Murch, Big Fish Expeditions

A salmon shark is a warmblooded animal / picture by Andy Murch, Big Fish Expeditions

The body temperatures of most species fluctuates with the temperature of their environment. This is known as cold-bloodedness or ectothermy. However, birds and mammals are able to regulate their body temperature, known as warm-bloodedness or endothermy. Water conducts heat faster than air, draining body heat more efficiently, which is why we use wetsuits to stay warm when diving or surfing to prevent hypothermia. Because of this, many marine animals, including most sharks are ectothermic and therefore more abundant in warm waters. However, the sharks mentioned above have the ability to elevate and maintain body temperatures above the ambient water temperature. Their flanks and organs are warm, while the heart and gills are at environmental temperatures. The mackerel sharks have a special organ in their circulatory systems, allowing them to retain much of the heat generated by swimming and muscular activity. In cold-bodied sharks, this heat is radiated from the body surface and lost to the environment. Warm-bodied sharks are able to greatly reduce this heat loss by redirecting blood from enlarged arteries along the flanks inward through a dense bundle of small arteries and veins called the rete mirable (or wonderful net). The arteries in each rete carry cold, oxygenated blood from the gills, while the veins carry metabolically-warmed, deoxygenated blood. These small arteries and veins pass very close to one another, carrying blood in opposite directions. Because of this intimate countercurrent blood flow, most of the heat is transferred from the veins to the arteries and cycled back to the muscles and organs that produced it originally. The warm-bodied mackerel sharks have three sets of retia: one in the swimming muscles, another in the organs, and a third surrounding the brain. Mackerel sharks with this modified circulatory system are able to maintain body temperatures well above that of the water in which they swim.    The advantages of being warm-bodied are amazing. A 10 °C rise in body temperature can result in a three-fold increase in the speed and strength of muscle contraction, providing more power from a given muscle mass. For mackerel sharks, this may afford an increase in sustained swimming speeds. Being warm-bodied might also allow mackerel sharks to make excursions into colder or deeper waters. To remain alert in the mind-numbing chill, mackerel sharks have a rete ‘brain heater’ to help stave off the grogginess brought on by their cold-water environment. Elevated organ temperatures may increase the rate of digestion and absorption of food, making available more caloric energy per day. The visceral retia also warm the uteri. By holding their young in warm uteri, mackerel sharks may somehow enhance the development of their embryos. Perhaps this shortens gestation, increasing the number of pregnancies a warm-bodied shark may have over her lifetime.

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A woman views a plastinated shark showing the structure of the blood vessels in the ‘Animal Inside Out’ exhibition at the Natural History Museum on April 3, 2012 in London, England.

But the advantages of endothermy are costly. To maintain a warm body in cold water, a mackerel shark may need more than ten times as much food as a cold-bodied shark of the same size. The thin biomass of tropical seas cannot readily support such high-energy carnivores. Since large fishes and large, energy-rich marine mammals tend to live inshore in cold temperate waters, mackerel sharks are most abundant in these areas. The distribution of warm-bodied sharks is thus dictated largely by availability of prey. For many people, sharks are the epitome of the ‘cold-blooded killer’. The warm-bodied mackerel sharks are spectacular reminders that many of our stereotypes are inaccurate. Perhaps a greater appreciation of the elegant adaptations of these incredible animals will help us see their intrinsic value.   More information: http://www.elasmo-research.org http://www.sharksavers.org/en/education/biology/endothermy/

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