Something very unexpected landed near the Port Hardy seaplane base on October 20th – a dead Mola mola. This is the largest of the world’s ocean sunfishes and looks like a cartoon character rather than a relatively fast-moving, deep-diving fish whose design has been perfected by millions of years of evolution.
This fish species is aptly named since Mola means “millstone” in Latin and indeed this fish looks like a huge, flat, gray circle and has rough skin. It appears to have no body, only a giant, round, flat head with a small beak-like mouth. It is propelled by two pointy fins (dorsal and anal) and is steered by a wide, rounded, rudder-like tail.
Mola molas are found in all temperate and tropical seas and are relatively common in the open ocean off our coast; often getting misidentified as sharks. They were believed to be passive drifters who travelled only at the surface, wherever the current took them. However, satellite tracking studies have revealed that they dive deeper than 600 m and travel an average of 10 to 20 km per day, the same distance traveled by open-ocean shark species.
They are certainly a rarity on the inside of Vancouver Island however and I greatly appreciate that Matthew Drake let me know about this find and that he undertook a necropsy of the giant together with Louisa Clarke and Natasha Dickinson. (I only recall there being a similarly sized one on the beach in Port Hardy in 2005).
This Mola mola measured 2.00 m wide, from beak to tail fin, and 2.06 m long, from the tip of one pointy fin to the other. It may have weighed more than 200 kg. Remarkably, this is small for its kind. Mola molas hold the record for being the largest bony fish on earth with an average mass of 1 tonne. The largest Mola molaever recorded was 2,235 kg and 3.10 m by 4.26 m (it was struck by a boat near Australia in the early 1900s). Note that the whale shark can be more than 9 times bigger than this but, it is not a bony fish.
Matt and the team concluded that the Port Hardy Mola mola was female which meant that she could have up to 300 million eggs in her one ovary. This is another record for the species: having more eggs than any other animal with a backbone. Another astounding fact is that the larvae could grow to be 60 million times their weight at hatching.
The investigation also revealed partially digested jellyfish in her gut, which is the typical prey of Mola molas. Their diet also includes small fish, eelgrass and crustaceans and they are able to spit out and pull in water and food with their unique mouthparts. As with all species that feed on jellies, a conservation concern is that they mistake plastic bags for their food. However, there was no evidence for this being the cause of death for this particular Mola mola.
Maybe parasites were a factor in her death? The team found lots of skin and intestinal parasites! Some of the round worms in the guts were even still alive. Parasites are common for Mola molas. In fact, it is now believed that the behaviour of “sunning” at the surface (hence, ocean “sunfish”) might be so that birds can feed on the skin parasites and that jumping more than 3 m out of the water might help dislodge some parasites too. Mola molas are also found associated with drifting kelp patches, where small fish can clean away the pests.
HOLY MOLA you never know what you are going to find in our amazing marine backyard.
Beware! Fabulous Mola mola parasite pictures below!
Advance only if you are NOT about to eat lunch and/or if you a biology-type like me who can view these kinds of photos in rapturous fascination anytime!
- ARKive images. Click here to see an image of gulls cleaning a Mola mola. Click here for video.
- oceansunfish.org (Where sightings should be reported in order to aid science)
- Smithsonian.com; May 6, 2015; Goofy Looking Ocean Sunfish Are Actually Active Swimmers and Predators; The idea that these giant fish are lazy is just wrong
- The Biogeography of Ocean Sunfish (Mola mola) by Lisa Wilkinson
- Sims DW, Queiroz N, Humphries NE, Lima FP, Hays GC, 2009 Long-Term GPS Tracking of Ocean Sunfish Mola mola Offers a New Direction in Fish Monitoring. PLoS ONE 4(10): e7351. doi:10.1371/journal.pone.0007351
- H. Dewar, T. Thys, S.L.H. Teo, C. Farwell, J. O’Sullivan, T. Tobayama,1, M. Soichi, T. Nakatsubo,Y. Kondo, Y. Okada, D.J. Lindsay, G.C. Hays, A. Walli h, K. Weng, J.T. Streelman, S.A. Karl. 2010 Satellite tracking the world’s largest jelly predator, the ocean sunfish, Mola mola, in the Western Pacific Journal of Experimental Marine Biology and Ecology. JEMBE-4916
- Six fish parasites you don’t want to miss . . . or catch. Deep Type Flow (blog)
- And thank you Erika Grebeldinger for the clever “Holy Mola” play on words