The Insane Biology of: The Sunfish

Summary

The ocean sunfish, or Mola mola, is one of the most bizarre and fascinating creatures in the ocean. Despite its awkward appearance and slow-moving nature, this fish has mastered survival in ways that defy expectations. With oversized fins, a lack of a swim bladder, and an extraordinary method of thermoregulation, the sunfish thrives in both shallow and deep ocean waters. Known for its enormous size and its ability to lay up to a billion eggs, the sunfish’s unique traits reveal that sometimes being weird is the best strategy for survival.

Takeaways

• 😀 The ocean sunfish, despite its awkward appearance, is the heaviest bony fish in the world, weighing up to 6050 pounds and reaching lengths of over 3 meters.
• 😀 Sunfish are evolutionary newcomers, having appeared only 50 million years ago, making them one of the most recent fish families to evolve.
• 😀 Sunfish have a bizarre body structure with a truncated spine, no pelvic fins, and back fins that fold in on themselves, giving them their characteristic lumpy shape.
• 😀 Unlike most fish, sunfish lack axial muscles, relying instead on their massive dorsal and anal fins for propulsion, similar to the wings of flying animals.
• 😀 Sunfish have evolved a unique vertical fin structure, which sets them apart from other fish and even animals like manta rays or penguins, using them for efficient lift-based thrust.
• 😀 Sunfish have a stiff, gelatinous exoskeleton made of collagen and elastin, which helps reduce drag, making them one of the most efficient swimmers despite their slow speed.
• 😀 While sunfish are slow swimmers (about 3 km/h), their large fins allow them to generate enough thrust without needing to swim quickly.
• 😀 Sunfish are cold-blooded and can dive to depths of over 1000 meters into the coldest waters of the ocean, where most fish would struggle due to their swim bladder.
• 😀 Instead of a swim bladder, sunfish maintain neutral buoyancy due to their dense bodies and stiff exoskeleton, allowing them to move up and down the water column efficiently.
• 😀 The sunfish uses behavioral thermoregulation by basking at the surface to warm up between dives, and the biggest sunfish can dive deeper and for longer due to their ability to retain heat.
• 😀 Sunfish lay an extraordinary number of eggs, with a single female capable of producing up to 300 million eggs at a time, making them one of the most prolific egg layers in the animal kingdom.

Q & A

What is the ocean sunfish and why is it often misunderstood?

The ocean sunfish (*Mola mola*) is the heaviest bony fish in the world, known for its strange appearance with a truncated spine, missing pelvic fins, and a permanently open mouth. Its awkward and slow movement often leads people to misinterpret it as a deformed or half-eaten fish, though it has unique evolutionary adaptations that allow it to survive and thrive.

How does the ocean sunfish move if it doesn't have typical axial muscles?

Instead of using axial muscles like most fish, the ocean sunfish propels itself using its large dorsal and anal fins, which it flaps simultaneously. These fins are adapted for vertical movement, creating lift-based thrust, allowing the fish to move forward with minimal body movement.

What is unique about the sunfish's fins compared to other fish?

The ocean sunfish has oversized dorsal and anal fins that are vertically oriented, unlike most other fish whose fins are asymmetrical. This unique configuration of fins allows the sunfish to move efficiently through the water, resembling the winged propulsion seen in manta rays and penguins.

How does the ocean sunfish maintain buoyancy without a swim bladder?

Instead of a swim bladder, the ocean sunfish has a dense body structure with a thick, rubbery hypodermis that provides neutral buoyancy. This tissue's density is similar to seawater, allowing the sunfish to remain buoyant without the need for a swim bladder.

Why do ocean sunfish sometimes dive to extreme depths?

Ocean sunfish dive to depths of over 1,000 meters to forage for gelatinous plankton, particularly deep-water siphonophores. These dives help them access prey that isn't available in shallower waters. Despite the cold temperatures at these depths, the sunfish can regulate its buoyancy and perform repeated dives.

What is 'behavioral thermoregulation' in ocean sunfish?

Behavioral thermoregulation refers to the ocean sunfish basking at the water's surface to warm its body. By absorbing heat from the sun, the fish can increase its body temperature, which helps it maintain energy and prepare for its next deep dive.

What is the significance of the ocean sunfish's large size when it comes to reproduction?

The ocean sunfish's enormous size enables it to produce an astounding number of eggs—up to 1 billion in some cases. This massive egg production increases the likelihood that at least some of the offspring will survive, making it one of the most prolific reproductive strategies in the animal kingdom.

How do ocean sunfish adapt to the deep-water environment despite being cold-blooded?

Despite being cold-blooded, the ocean sunfish can dive deep into the ocean where temperatures can be as low as 4°C. It compensates for its inability to regulate its internal temperature by using behavioral thermoregulation, spending time at the surface to warm up before diving again.

What is the ocean sunfish's primary food source, and how does it find it?

The ocean sunfish primarily feeds on gelatinous plankton, including jellyfish and siphonophores. It hunts for these prey by diving to great depths during the day, taking advantage of the bioluminescence of its prey when sunlight is absent.

How does the ocean sunfish's appearance and behavior contribute to its survival strategy?

The ocean sunfish's seemingly 'weird' appearance, with its large, awkward fins and slow movement, is an evolutionary advantage. It allows the fish to avoid predators and access food in deep waters while minimizing energy expenditure. Additionally, its unusual body structure helps it survive in a variety of ecological niches.