Ocean Sunfish | Mola Mola | seen off the coast of Nantucket
Island Science

Basking in the Sun, Ocean Sunfish

by Dr. Sarah D. Oktay, Director, University of Massachusetts Nantucket Field Station

We have had three visitors this week, a Mola Mola otherwise known as an ocean sunfish, a beautiful humpback whale out by Great Point and an errant washashore/rode-ashore raccoon, and I thought I would write about one of them. We will save the exciting raccoon on the loose story for next week when I’ll write about terrestrial mammals. One of my favorite animals is the Mola mola or sunfish. The first time I saw one, for some reason it reminded me of a garbage can lid flipped sideways, with a long fin sticking out of the water. This is a strange creature in the league of platypuses and armadillos and certainly worthy of a column. These fish are often seen in Nantucket Sound and south and east of the island and also get swept into the harbor when dead. There is nothing quite as odd as seeing one flopping around in the ocean.

Ocean Sunfish | Mola Mola | seen off the coast of NantucketI covered Mola Mola’s in a story back in 2010 (http://yesterdaysisland.com/2010/science/20.php); one was photographed and reported on Facebook near Brant Point by Rob Ranney last week which inspired people to speculate whether it was a shark or not. Back in 2012 I wrote about various fin shapes and sizes to help in identification of marine creatures (“The Boy who Cried Shark; Fin Sighting 101” in http://yesterdaysisland.com/the-boy-who-cried-shark-fin-sighting-101/. Check that out so that you’ll know if that fin belongs to a great white or a sunfish. If it is flopping around looking like I usually look when I try to swim then it likely belongs to the ocean sunfish, Mola Mola, or common mola, which is the heaviest known bony fish in the world. Common mola’s have an average adult weight of 1,000 kg (2,200 pounds over a ton). According the 1995 edition of the Guinness Book of World’s Records, the largest recorded one weighed in (actually a guesstimate) at two tons four hundredweight, which converts to 4,927 pounds. The fish measured 3.1 meters (10 feet) in horizontal length, 4.26 meters (14 feet) in vertical length. Since that record was set even heavier ones have been found.

This portly (big boned) species is native to tropical and temperate waters around the globe. It resembles a fish head with a tail, and its main body is flattened laterally (like a pancake). Sunfish are typically taller than they are long from front to back when their dorsal and ventral fins are extended. The common name “sunfish” is used to describe the marine family, Molidae, as well as the freshwater family, Centrarchidae. The common names “ocean sunfish” and “mola” refer only to the family Molidae and can be applied all three Molidae species. Presently, three distinct species are recognized within the family Molidae including: the common mola, Mola mola (Linnaeus 1758); the sharp-tailed mola, Masturus lanceolatus (Lienard 1840); and the slender (relatively!) mola, Ranzania laevis (Pennant 1776).

The German name for this fish is “Schwimmender Kopf” meaning swimming head, which gives you some idea of what a Mola mola looks like. Mola molas are often seen near the surface, swimming either upright, dorsal fin flailing from side to side as it seems to waddle underwater, or flipped over swimming on its side, basking on its sides soaking up the rays of the sun and hoping for some free grooming by birds (http://www.dive-the-world.com/creatures-sunfish.php ; accessed June 12, 2015). In fact, this tendency to bask at the surface led to Mola mola’s common name of Sunfish.

The word “mola” comes from Latin and means millstone—in reference to these fishes’ roundish shape. The common name “ocean sunfish” comes from the Mola mola’s habit of lying atop the surface of the ocean appearing to sunbathe. Fossil jaw parts of the genus Eomola suggest this family descended from coral reef fishes sometime in the middle Eocene, roughly 40 million years ago. Since their first appearance, sunfish have spread into every tropical and temperate ocean. Their unusual appearance gives one the idea that Mola Molas were an evolutionary mistake. While the Molidae may appear primitive, they are in fact relative latecomers to the fish world. Fishes first emerged over 500 million years ago and the evolutionary radiation (species rapid expansion) leading to most modern fishes occurred about 100 million years ago. It took another 50 million years for Molidae to appear. In fact, molas are thought to be one of the most recently derived fish groups in the sea.

Ocean sunfish are member of the order Tetraodontiformes, which also includes pufferfish, porcupinefish, and filefish, the sunfish shares many traits common to members of this order. The name Tetraodontiformes refers to the four fused teeth that comprise their characteristic beak. While their inflatable relatives can defend themselves by sucking in water and puffing up, molas suck and spit water primarily as a means of manipulating prey items into manageable pieces. Long claw-like teeth in their throat help this process.

Mola Molas are grayish white, sometimes light brown and mottled in appearance. Molas are capable of color changes particularly when stressed or under attack from a sea lion or other predator and can turn from light to dark within a matter of moments. Their skin is very thick, covered with denticles (“little teeth”) and feels like sandpaper and, to top it off, is slathered with mucus. Apparently in spite of the mucus, parasites love M. Molas and up to 40 different species of parasites call molas home. Molas use a variety of ways to rid themselves of these freeloaders, like hanging with reef wrasses and other cleaner fish, basking sideways on the surface to encourage birds to “de-flea” them and breaching up to ten feet above the sea surface to knock them off, which has to be a pretty amazing site. One parasite frequently found on M. Molas is a larval shark tapeworm, which then enters shark guts when the mola is consumed by a shark, completing their optimistic (for the tapeworm) lifecycle. Although classified as a bony fish, molas do have large amount of cartilage substituting for bones in their bodies which allows them to attain their incredible size.

This site, http://www.oceansunfish.org/ features an excellent map showing recent sightings of Mola molas around the world so you can get an idea of their distribution. I’ll be reporting the most recent one seen at Brant Point. For a fish that does not have the most aerodynamic shape and swims pretty much like I do (poorly), they get around. This site also has lots of information for classrooms and children and gives visitors a chance to “adopt a sunfish.” I also recommend this TED talk by marine biologist Dr. Tierney Thys, who tags and researches these amazingly odd creatures (http://www.ted.com/speakers/tierney_thys accessed June 12 2015). The research done in Monterey Bay by Dr. Thys and colleagues at the Monterey Bay Aquarium Research Institute (MBARI) indicates that recent die-off events of young Mola molas appear to be caused by sea lions. Sea lions don’t have sharp enough teeth to penetrate the thick mola skin and actually consume them, so they just tear off the fins and then the fish sink to the sea floor to endure a slow ironic coup de grâce by starfish.

Sunfish live on a diet that consists mainly of jellyfish, but because this diet is nutritionally poor, they consume large amounts in order to develop and maintain their great bulk. Mola mola eat a variety of foods, including Portuguese man-o-war, ctenophores, and salps. Squid, sponges, eel grass, crustaceans, small fishes and deepwater eel larvae have also been found in M. mola guts indicating that they forage both at the surface, among floating weeds, on the seafloor and into deep water. They are found in Nantucket Sound because there are plenty of jellyfish squid and salps here to keep them fat and happy. Sometimes they will feed deeper in the water column along a thermally developed density layer that traps their squishy prey.

Females of the species can produce more eggs than any other known vertebrate; a median female M. mola (4.5 feet) was found to have 300 million eggs in one ovary (Carwadine, 1995)! Sunfish fry resemble miniature pufferfish, with large pectoral fins, a tail fin and body spines uncharacteristic of adult sunfish. They look like little pincushions and they slowly lose some of their youthful spikiness as they get older, like we all do.

In 2005 and 2006, Inga Potter (Department of Biological Sciences, University of New Hampshire) conducted a microwave telemetry program as part of the Large Pelagics Research Lab program to tag these large fish and see where they were going. She tagged several fish off Nantucket and observed them to make the long trip all the way down south to the Gulf of Mexico through the Florida Straits in about 130 days, often swimming at depth instead of at the surface as many people believed the species favored. Nantucketer Josh Eldridge (captain of the F/V Monomoy) helped her with her research (http://oceansunfish.org/Potter%20and%20Howell%202010.pdf) . Very little is known about these gentle giants and information from tagging programs can help us determine if bycatch (incidental non targeted species caught by non-selective fishing methods), ship strikes, finning, and the gaining popularity of the fish on Asian markets is effecting their populations. Mola Mola makes up a large portion of bycatch in Pacific and Mediterranean bluefin longline fishery and Mediterranean swordfish driftnet fishery, comprising between 70-90% of the total catch between 1992-1994. In the Pacific, ocean sunfish are the most common bycatch in swordfish driftnet fisheries, making up approximately 25%, the largest of any species recorded. Drift netting is now banned under a UN moratorium and in the U.S. Finning, or removing the fins of a creature and then releasing it back into the water to die, is sometimes seen in washed up species and is the result of fishermen annoyed by the creatures or convinced they are stealing bait. I can’t imagine anyone being annoyed or intimated by one of these things, seeing them swim is one of the more surreal and comical sea scenes I’ve seen. According to Inga’s paper linked above, “M. mola is a common resident of the offshore waters of the Northwest Atlantic during the spring and summer months, with an estimated summer population of 18,000”.

This web site has a lot of great pictures of Mola molas alongside divers so one can see how huge they can get http://www.dive-the-world.com/creatures-sunfish.php, accessed June 12th, 2015. Remember they do seem to flail about a lot and they are much slower swimmers than sharks. The speed of movement is key to determining whether you are looking at a shark or dolphin or a slower species. And Molas molas have a very straight dark dorsal and anal fins which is usually very tall; their caudal (butt) fin is very short and rounded giving it the swimming head look. To learn more, go to the Monterey Bay Aquarium website http://www.montereybayaquarium.org/conservation-and-science/our-programs/research/husbandry/ocean-sunfish (accessed June 12, 2015); they have been tagging, tracking, and raising Mola molas for years. And last but not least, here is an excellent article and funny article about these awkward gentle giants http://www.realmonstrosities.com/2012/05/ocean-sunfish.html

More references: http://oceansunfish.org/references.php

Image courtesy of Plymouth Whale Watching http://plymouthwhalewatching.com/october-14th-2013-1100-am-whale-watch-tiffany/