Curiosity 42: Ocean sunfish

I have known and loved ocean sunfish (Mola mola) ever since I started volunteering at the Monterey Bay Aquarium (MBA) in 1990. They are one of the largest bony fishes in the world, growing to sizes of up to 2,000 pounds (and more) and ten feet tall and long. For all that, they can barely maneuver, relying on two long fins, dorsal and anal, to move their rigid, disk-shaped body around in a sculling motion.

Its common English name refers to its habit of sunbathing at the surface of the ocean. Many languages call it the equivalent of "moonfish," in reference to its moonlike appearance. In German it is known as the Schwimmender Kopf, or "swimming head." Its Chinese name translates to "toppled wheel fish." Which reflects its Latin name as well, Mola meaning millstone. It is one of three Mola species, all in the family Molidae, in the same order as pufferfish, porcupinefish, and filefish, Tetraodontiformes, referring to their fused beak of a mouth with four apparent "teeth." They lack swim bladders and even spines, and their skin is covered by denticles and mucus, rather than scales.

Because they have no real jaw, sunfish suck food into their wide-open mouths, consuming jellies and salps, small fish and fish larvae, squid, and crustaceans. They can further process this food with pharyngeal teeth, located in their throats. They grow very quickly: one specimen at the MBA increased from 58 pounds to 880 in a mere 15 months. Moving it out of its tank and back to the ocean required a bit of engineering.

Ocean sunfish are found in temperate and tropical waters throughout the world. I was surprised in Oslo some time ago to see a restaurant with a sign that featured a sunfish. The picture above is from the Nordsøen Oceanarium in Denmark. 

While they are fairly safe from predation as adults, orcas, killer whales, and sealions do eat them. In Monterey Bay, these animals like to fling small molas around like frisbees. I am not making that up.

I chose this topic today because of a graphic I saw (where else?) on Facebook, of the other sunfish. I had no idea there were other sunfish! One of them is Mola alexandrini, found in the southern hemisphere, the heaviest bony fish in the world at a record 6,049 pounds. The rarely encountered Masturus lanceolatus is distinguished from the Mola species by having a sharp projection on its clavus (pseudo-tail), hence its common name, sharptail sunfish. And then there is the (relatively) dimunitive slender sunfish (Ranzania laevis), only 3 feet in length. It swims faster than its cousins, and is apparently an able predator on squid. (If you are interested in watching a short video about larval R. laevis, go here. The larvae are covered in spikes, very medieval.)

How marvelous that these bizarre creatures live in our seas!

P.S. And I must have been channeling big goings-on at the Monterey Bay Aquarium as well, because on 12/9 they posted the news that they have a new Mola mola in their Outer Bay Tank!