Nature Notes
by Bob Thomas
Naturalists always maintain a mental list of "target species" that they really want to see. Some naturalists are fanatic in their quest, while others work at seeing their quarry, but are not bitterly disappointed when they do not. They just look forward to the next adventure. I am in the latter group. It is all about the adventure to me, but I can get very excited when I am successful.
On a Sunday in late September, a group of serious birders left Venice, Louisiana, on the Cougar, a 48-foot boat that normally takes fishers out to the rigs and open water of the northern Gulf of Mexico. That day's target was to leave Southwest Pass of the Mississippi River and travel another 35 miles to the Mississippi Canyon, an actual canyon that falls from about 500 ft at the surface to about 1500 feet at its bottom. This is where the government scientists recently found the giant squid. With all the fanfare of the discovering, no one mentioned that several years ago on another pelagic birding trip the group found a giant squid carcass floating on the surface.
Our real target species were pelagic birds, those that rarely if ever go to land, or at least are extraordinarily rare on our shores. But at sea, one never knows what might pop to the surface.
We were watching bridled and black terns along a rip when we noticed a fin protruding lazily from the water. A shark would be cruising about, and a bottlenose dolphin would normally be surfacing and diving. This one was simply staying stationary and flopping back and forth. It could be only one thing - an ocean sunfish, Mola mola, or one of its cousins. As we approached, we were rewarded with an excellent view of an ocean sunfish about four feet long.
The water was crystal clear and flat, and cameras were clicking, but I feared that glare would prevent nice images. I just stood and watched the fish move by us. It was my first Mola mola ever. What a joy!
These are interesting looking fish. They can be 13 feet long and weigh over 3000 pounds. They are laterally compressed and have one large dorsal and one large anal fin (no pelvic fins). The mucus-covered caudal fin consists of a series of lobes just behind the dorsal and anal fins, making them look like the fish has been chopped in half. They have no scales, a pectoral fin on each side, a very small opening to the gills nearby, and an eye on each side that is about equidistant between the gill opening and the tip of the snout. They have an obviously concave forehead. As in most pelagic animals, they are counter shaded, i.e. they are dark above and light below. Predators looking down on them see a dark back against the dark depths, and animals below see a silvery belly against a light sky.
When I first learned about this fish in the 1960s, I was told it is the largest plankton in the world. Plankton is a word used for critters that move in water at the whims of tides, currents, and the wind. Most planktonic species have slight abilities to maneuver in the water column, but they are not active swimmers like virtually all fish, turtles, etc.
Ocean sunfish can move efficiently through the water, and dive at will, to depths of 2000 feet or so. They are generally abundant, but experts tell us adults spend most of their time about 600 feet below the surface. The ones we see at the surface seem to prefer lollygagging about on the surface, often lying on their sides or waving their large dorsal fins. These are usually subadults.
Sunfish larvae and juveniles are gobbled up by many kinds of predators. Seals, sharks, and/or killer whales may attack small individuals. As they age, they get a bit gristly and become less palatable to most predators. Humans rarely eat them, but there is a sunfish festival in Hualien County, Taiwan, and the molid fishery there is thriving. Taiwanese call it mambo fish.
Sunfish have external fertilization. Females produce up to 300 million eggs (the most among vertebrates worldwide) and males squirt milt into the water to fertilize the eggs they find.
They feed primarily on jellyfish, ctenophores (comb jellies and the like), and relatives, although they will take smaller prey if abundant. Interestingly, another large jellyfish specialist, the leatherback sea turtle (Dermochelys coriacea), was also seen on the pelagic trip, not far from the sunfish sighting. It must have been a good day for jellies; we saw many moon jellies (Aurelia aurita) just below the surface in several places.
There are two other species of the family Molidae in our waters. One is the sharptail mola (Masturus oxyuropterus) that is smaller (up to 11 feet long and weighing several thousand pounds), has an extension of the middle caudal fin lobe that is rather pointed, a rounded forehead, and its eye nearer the tip of the snout than to the gill opening. The other is the slender sunfish, Ranzania laevis, that is much smaller, reaching only about three feet in length, and having silver or green bars in front of the gill openings, and its body being twice as long as high.
Wow! Another check on my life list of strange animals encountered in the wild.
Also published in Delta Journal, The Times Picayune, August 16, 1989.
The first indication of an ocean sunfish, As you get closer, the body becomes visible
Mola mola, is a sighting of a fin wobbling and you know it is a sunfish.
on the surface in the distance. Photo by Erik Johnson.
Photo by Bob Thomas.
This specimen has a body length of about At the surface, sunfish often stick parts of
four feet and is clearly identifiable as their bodies out of the water. This specimen
Mola mola by the shape of its caudal flap. is blowing bubbles through its mouth and
Photo by Amy Shutt. the top of the anterior region is exposed to
the air. Note the characteristic dip on the
forehead seen in this species. It is more
pronounced in juveniles.
Photo by Dan Purrington.
Underwater view of an ocean sunfish.
Photographer unknown.