To Chomp or Not to Chomp: Shark Cannibalism in question

Photo: QLD Primary Industries and Fisheries Dept

​Sharks are often mislabeled as the trash cans of the ocean, eating anything they come across.  Unlike trash cans, sharks are efficient, dynamic, ocean predators; each individual species has a preferred diet and adaptations to capture said diet. Picturing jawesome shark adaptations is less terrifying that picturing trash cans actively hunting on land. Some species feed on organisms as tiny as plankton, while others capture larger animals such as polar bears! In the ocean if you are a small fish, or small shark, chances are you’re someone’s prey.  Despite the fish-eat-fish, shark-eat-shark dynamic of the ocean, that doesn’t always mean cannibalism.  In this post we’ll dive a little deeper into the odd but not unusual circumstances of shark cannibalism.

To analyze shark diets we need to know what they are eating. Dissections allow scientists to see the immediate contents of an individual shark’s stomach, but we don’t want to dissect every living shark out there! Scientists can also observe external damages on sharks (and other prey animals) to see what larger predators try to convert them into nutrition and energy. Many species leave unique teeth marks, and even teeth, behind.  Tracking technology currently allows scientists to locate individual animals and study the food found in their habitat. With new technology, patience, and scientific study we can compile specific shark diets for individual species.
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A majority of shark cannibalism is intrauterine, occurring before newborns leave the uterus. Intrauterine cannibalism is broken into two categories, embryophagy and oophagy.  Embryophagy (also referred to as adelphophagy) was first documented in the sand tiger (or ragged tooth) shark. In this instance the first shark to develop will eat the other, no matter what stage of development the other individual is in. The large word itself translates to “eating ones brother” creating an intense sibling rivalry. This behavior can be beneficial to the individual because it supplies direct nutrients for growth, and limits immediate competition once out in the big blue. Oophagy (also referred to as oviphagy), the eating of unfertilized eggs in the womb, is found in multiple sharks species including thresher, shortfin mako, and porbeagle. Oophagy has been observed out of the ocean in various snake and insect species.  Eggs of any kind hold easy access nutrients that won’t fight back. Feeding on the other eggs enables the individual shark to grow and increase fitness before it leaves the womb. Both variations of intrauterine cannibalism can be beneficial since shark parents do not protect or raise their pups. Newborns must be efficient predators immediately; they do not crawl into life like humans do. 
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​​Observing a shark to know every detail of its diet is already challenging, and since we do not want to harm healthy populations through dissection, studying cannibalism in adults becomes even tougher. Mature individuals of certain species have been known to eat their own young, but this is not unique to sharks or even the ocean (check out grizzly bears, chickens, rabbits, etc)! Sharks are active hunters, and get food when accessible (healthy drive thru food isn’t available to them). While no scientific documentation of a mature shark species actively hunting another mature individual of that same species currently exists, science has made great strides in the study of shark behavior and diets, and we gain new knowledge every day! There is still work to be done, creating a great opportunity for new research and new scientists to make a splash!

click here for more information about intrauterine cannibalism.

GLOSSARY

• Intrauterine- occurring within the uterus
• Predator-an organism that eats other organism
• Plankton-microscopic (cannot be seen with the naked eye) plants and animals
• Prey- an organism that gets eaten by other organisms
• Cannibalism- the eating of the flesh of an animal of the same kind
• Dissection –to cut apart and examine
• Embryophagy/adelphophagy- when one embryo (unborn but developing animal) eats another for food in the uterus
• Oophagy/oviphagy – the eating of eggs within the uterus
• Adaptation- a characteristic or behavior that serves a helpful function in animal or plant

GUEST BLOGGER AND TFUI OFFICER CARISSA THIEL

Carissa Thiel graduated with a BS in marine biology and then spent a few years working in environmental education. She now works as a Guest Engagement Educator, spreading conservation messages to locals and visitors alike!

Carissa loves exploring, reading, talking to people, and awkward get to know you questions. If she could be any bird she would want to be an osprey, because they have wonderful adaptations to hunt fish and are always found by water. However, she would probably be an emu because they have long legs, make awkward faces, and only watch other birds fly.