This one just popped into my head this morning when I was trying to think of something to blog about. It’s been a while since I wrote about a fish, so why not blog about a strange-looking, and let’s face it, pretty ugly fellow. It’s ocean sunfish time!

Ocean sunfish are bizarre creatures. They are the largest bony fish in the world, growing up to 3.1 meters in length, 4.2 meters in height, and weighing over 2000 kilograms. That is a pretty large fish. Not only are they super huge, but they also very oddly shaped. Unlike most fish, which have the standard dorsal, ventral, pectoral and tail fins, ocean sunfish seem to have done away with the tried and tested model other fish have used for thousands of years of evolution. Instead, they have very tiny (and pretty useless) pectoral fins, elongated dorsal and ventral fins, and a non-existent tail fin. The tail fin is modified into a clavus, which is used for steering and not propulsion. This is what this strange fish looks like:

An ocean sunfish with a diver for scale. Image By Edgard Dias Magalhães – Own work, CC BY-SA 3.0

How then, does this ridiculous thing swim? Why, it waves its dorsal and ventral fins back and forth. Because that’s a sensible way to get around. But apparently it works quite well for sunfish, as they can reach speeds of 3.2 km/h (I know, that’s crazy fast). They can also propel themselves out of the water, sometimes leaping into boats. Can you imagine relaxing on your yacht only to have a two tonne fish land on it? I don’t think it would be very much fun.

Ocean sunfish’s teeth are fused into a beak, and they use this to eat plankton and jellyfish. As you can imagine, jellyfish aren’t the most nutritious of meals, so sunfish have to eat a massive amount of them to maintain their size. In order to get to their food supply, sunfish will migrate vertically during the day, often venturing into colder waters to do so. Unfortunately for the ocean sunfish, sustained temperatures of under 12 degrees celsius causes disorientation and eventually death. So after sunfish dive into deep waters they swim to the surface and turn sideways, soaking up the sun. Often times boats will run into these bathing sunfish, causing damage to the boat and undoubtedly discomfort to the relaxed sunfish.

A sunfish basking at the water’s surface. This may also be a method of parasite removal, as seabirds will come feed on external parasites on the sunfish. Public Domain,

Unfortunately little is known about sunfish reproduction, but we do know a bit about their development. Female ocean sunfish can lay more eggs than any other vertebrate animal, depositing up to 3 million in one go. When sunfish hatch they don’t look anything like their adult counterparts, and spend the first part of their lives looking like a tiny pufferfish. As they grow, they shed their spines and the tail is absorbed and turns into the clavus. It’s possible that young sunfish are even stranger looking than adults. Here’s a picture, I’ll let you decide:

Image By G. David Johnson – An image featured on the website of the Australian Museum, CC BY-SA 3.0

Though we don’t know a whole lot about them, from what we do know it’s fairly obvious that sunfish are weird animals. While I think I’d be terrified to meet one (even though they’re very docile), I appreciate their strangeness. At least it made a good blog post.

Cover image By U.S. National Oceanic and Atmospheric Administration – U.S. National Oceanic and Atmospheric Administration, Public Domain