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Why do basking sharks swim with their mouth open?

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The basking shark (Cetorhinus_maximus) isn't constantly yawning, it's eating, filtering food through its massive gills. Photo credit Greg Skomal.
The basking shark (Cetorhinus_maximus) isn’t constantly yawning, it’s eating, filtering food through its massive gills. Photo credit Greg Skomal.

By Blaise Jones

Many people consider it in poor taste to walk around with your mouth open all the time. However, for some animals it’s a matter of life and death. For example, the basking shark has a very important reason for why it’s always yawning.

Large and in Charge
Basking sharks are the second largest fish on the planet, surpassed only by their relative the whale shark. They reach sizes up to 40 feet (12.2m) and can weigh up to 5 tons. Members of the order Lamniformes, basking sharks are a bit of a black sheep when compared to their cousins the great white and mako sharks.

Slow-moving basking sharks spend most of their time at the surface, which led to their name. Scientists noted how these sharks seemed to be sun bathing, or basking, at the surface.

Unlike their more infamous cousins, basking sharks do not hunt fast-moving animals like seals and dolphins. Like most large marine animals, basking sharks are filter feeders.

Open Wide and Say, “Aaaaa!”
The basking shark feeds via ram ventilation. Opening its huge mouth, which can measure up to 3 feet (0.9m) in width, basking shark slowly swim forward and let the water flow passively into their mouths and out their gills. This is different from how other filter-feeding sharks, like the whale and megamouth sharks, which actively pump the water into their mouths via a process similar to buccal pumping.

But the mouth is only one tool the basking shark uses for feeding. Their gills are equally as important, and highly specialized. Proportionally, basking shark gills are huge, with the front gills actually circling around the bottom of the basking shark’s head and under the bottom jaw. These specialized gills are coated in “gill rakers,” projections that sprout out of the basking shark’s gill arches and serve as a filtration system.

As the shark swims forward with its mouth open, water rushes into the opening and out through its gills. Within the filtered water are billions of microorganisms called zooplankton. These tiny animals are caught in the gill rakers as they are carried through the shark’s gills, giving the basking shark a mouthful of high-energy microorganisms each time it closes its mouth.

Through this process an adult basking shark can filter up to 2,000 tons of water per hour.

Mighty Micro Meal
Why do the largest animals on the planet feed upon some of the smallest? The answer has to do with trophic levels. A trophic level is the place an organism occupies on the food chain. Near the top are the predators such as sharks, and near the bottom are animals such as small fish and zooplankton. Below zooplankton are the primary producers.

Primary producers are the closest link between organic life and the source of all energy on this planet: the sun. In most environments, primary producers are plants, and the ocean is no different. The primary producers in the ocean are seagrasses, algae, and phytoplankton. Phytoplankton are in turn eaten by zooplankton, meaning that zooplankton are only one trophic level away from the main source of energy. The closer a trophic level is to the main source of energy, the more of that energy the level contains. Therefore, predators that can efficiently prey upon these lower trophic levels have a source of high-energy food.

Combine this with how easy it is to feed on zooplankton, and how little energy filter feeders need to spend on feeding, and you can see just how efficient filter feeding is. When animals like basking sharks and manta rays filter hundreds of tons of zooplankton and other microorganisms an hour, they are able to eat tons of highly efficient energy sources.

It just goes to show that sometimes bigger isn’t always better.

The filter-feeding manta ray also eats with its mouth open. This manta ray was photographed in the Maldives.
The filter-feeding manta ray also eats with its mouth open. This manta ray was photographed in the Maldives.

Sources

  1. “Sharks: The Animal Answer Guide” by Steve Parker
  2. https://www.flmnh.ufl.edu/fish/discover/species-profiles/cetorhinus-maximus
  3. http://www.arkive.org/basking-shark/cetorhinus-maximus/
  4. http://www.sharks-world.com/basking_shark/
  5. http://australianmuseum.net.au/image/fish-dissection-gills-exposed
  6. http://www.elasmo-research.org/education/topics/lh_size_benefits.htm
  7. https://www.quora.com/How-do-Baleen-whales-grow-so-big-on-a-plankton-diet
  8. “Marine Biology” by Jeffery S. Levinton
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