Something took a giant bite out of the backside of this sunfish. Luckily, the bite missed its vital organs—so the sunfish survived!
Sunfish (Mola mola) are the largest bony fish in the world. Adults can weigh up to 3 tons. That’s about the weight of a pickup truck. Marianne Nyegaard is a marine biologist who studies the anatomy of sunfish at the Auckland War Memorial Museum in New Zealand. To study the fishes’ body structure, she dissects specimens that wash up on the beach, cutting them open to carefully examine their body parts. “By the end, you’re covered in fish juice,” jokes Nyegaard.
Unlike most fish, sunfish don’t have fully formed tail fins. Instead they have a dense mass of collagen on their backside. This tough, stringy protein provides structure and support for their bodies. Because a sunfish’s rear end is just thick tissue, says Nyegaard, “the back is the best place to lose a bit.”
Nyegaard’s recent research hints that sunfish may rely on their rumps to survive attacks from predators. She studied videos of sunfish being hunted by orcas. These large ocean predators are also known as killer whales. Nyegaard observed sunfish quickly flipping upside down as an orca came near. “It looked as if they were trying to keep their back ends toward the orca so it wouldn’t bite where their organs are,” says Nyegaard. Sticking out its rear might be an adaptation to help the sunfish survive getting chomped.