The ocean sunfish, (Mola mola)is classified as the heaviest bony fish in the world! This large and quite unusual looking animal is a common visitor to New England waters in the summer and fall. When in the waters of the Gulf of Maine, the ocean sunfish is often seen swimming on its side at the water's surface. This is why this species received its common name, the ocean "sunfish." The scientific name, Mola mola, refers to the large size of the animal and its somewhat rounded shape. Mola comes from the Latin word Molidae which means "millstone."
- Kingdom - Animalia
- Phylum - Chordata
- Class - Actinopterygii
- Order - Tetraodontiformes
- Family - Molidae
- Genus - Mola
- Species - mola
Ocean sunfish that are seen in our New England waters are typically 4 to 8 feet in length, weighing 300 - 1,000 lbs. Preliminary studies conducted by the New England Coastal Wildlife Alliance (NECWA) has conducted in conjunction with NOAA's Aging and Growth Laboratory in Woods Hole, MA, indicate that the majority of these fish are juveniles who have not spawned. The large adults have been reported to be up to 10 feet in length and weigh over 2,000 lbs.
The ocean sunfish has a very unusual appearance, almost as if this animal is all head and no body! It swims using its very tall and very powerful dorsal fin and anal fin. Even more bizarre is that the stubby tail of the ocean sunfish is not really a tail or caudal fin at all. This structure is called the clavus and is formed from extensions of both the dorsal fin and anal fin. Although small in size, the clavus acts as a rudder helping the fish to swim. Despite the lack of a true tail fin, ocean sunfish are powerful swimmers that dive to depths of over 2,000 feet and can jump or breach out of the water. Ocean sunfish have a small pectoral fin on each side of the body, just behind the operculum where water exits the body.
The skin of the ocean sunfish is very thin, but rough like sandpaper. The skin's color ranges from dark gray to silvery-white, and this helps to create a unique pigmentation pattern on the body. The skin is covered by copious amounts of mucus that act as a defense against external parasites. Some researchers feel that the mucus may help to keep external parasites from attaching and reduce skin irritations. Ocean sunfish are said to have the largest infestation of parasites, both external and internal, of any marine fish.
Ocean sunfish are said to eat gelatinous organisms like jellyfish, ctenophores, and salps. Some tuna fisherman who fish in Cape Cod Bay and Massachusetts Bay have commented that they see ocean sunfish nibbling on their bait bags that are filled with cut-up fish.
Although found in all oceans of the world, little is known about the biology and ecology of the ocean sunfish. What is known about ocean sunfish in the western North Atlantic is that this species moves into the cold northern waters of the Gulf of Maine in the later spring, spending much of its time in offshore waters. Later in the summer, some individuals within the population migrate into shallower, more coastal waters and that is when fishermen, boaters, and beach walkers start seeing them on a more regular basis. Ocean sunfish stay in these coastal areas until the early fall and then they migrate south to spend their winter in more tropical waters.
As they migrate south for the winter, a small number of ocean sunfish become trapped in Cape Cod Bay and can't figure their way out. These unfortunate individuals will eventually strand on our New England shores. Most strandings occur from late August through December and fish stranding from October through December are often cold-stunned. Typically 15 to 20 ocean sunfish carcasses wash up on the northern shores of Cape Cod each season. To learn more about ocean sunfish strandings in New England, visit our Stranding page for this species by clicking HERE.
Photo-Identification of Ocean Sunfish
Photographs taken of ocean sunfish can be used to identify individuals within a population. Ocean sunfish have unique features on their body which can be used to identify one individual from another. The unique features for photo-identification include:
- pigmentation pattern on the body
- size and shape of the dorsal fin and anal fin
- cuts and notches on the dorsal fin, pectoral fin, and clavus
- cuts and scratches on the body, including cuts from boats and fishing gear.
The diagram below shows the various parts of the ocean sunfish. Notice that clauvus has rounded projections (called ossicles) that create a variable margin. Although the dorsal fin and anal fin are large in size, the pectoral fin is small and more broadly shaped. Just forward of the pectoral fin is the operculum which allows water that has entered through the mouth and over the gills to move out of the body. Ocean sunfish have nostrils on both sides of the head which are small and difficult to find. Click HERE to view a PDF file of this same image.
When in New England waters, ocean sunfish spend a large amount of time near the water's surface. This animal presents a low profile to boats in the area and is difficult to see, especially in rough waters, they often are hit by fast moving boats, large and small. Boat and propeller scars can be very useful when photo-identifying ocean sunfish. Also, some ocean sunfish show signs of previous entanglements in fishing gear. All of the features mentioned above can be used to identify individual ocean sunfish which can then be tracked from one location to another.
Tips when Photographing Ocean Sunfish for Photo-ID
- Set your camera to the highest resolution and fastest shutter speed.
- Ensure that the sun is not in the background of the picture.
- Take photos of the dorsal fin from both sides, if possible.
- Take photos of the tail and snout of the shark.
- Take photos of the animal's mouth to see if it is open (feeding).
Basking Shark or Ocean Sunfish?
In our New England waters, the two large fish that are typically seen at the water's surface are the basking shark and the ocean sunfish (Mola mola). For both species, what attracts your attention is often a large dorsal fin visible above the surface of the water. It is helpful to get a good look at the size, shape, and movement of this fin to help you determine whether you are looking at a basking shark or an ocean sunfish. Don't forget that basking sharks have two dorsal fins and the first dorsal fin is quite large and triangular in shape. Ocean sunfish only have one dorsal fin that is more pointed and elongated.
The first dorsal fin on a basking shark is located in the middle of the body, past the insertion of the pectoral fins. This fin is typically erect and for adults, can be 1 meter tall. The second dorsal fin is much smaller in size and is positioned behind the first dorsal fin and forward of the caudal or tail fin. For some reason that is not understood, a small number of individuals have a first dorsal fin is not erect, but instead, flops over onto one side of the animal's body.
Ocean sunfish have a tall dorsal fin, but the shape is not as triangular as that of the basking shark. When swimming at the surface, an ocean sunfish will often swim on its side as it repeatedly lifts the dorsal fin in and out of the water. This behavior is called skulling and is quite unique to this species. By examining the movement of the dorsal fin and the overall swimming pattern of the animal, you can distinguish between these two species. If you see the dorsal fin cutting smoothly through the water in a forward motion, then you are looking at a basking shark. If on the other hand, you see that the dorsal fin is being skulled above the water's surface, then you are looking at an ocean sunfish.
The video below illustrates the different swimming patterns between a basking shark and an ocean sunfish swimming at the water's surface..
Ocean sunfish populations off the United States are not well documented, but it is assumed that they are healthy and robust since this species is not targeted by the commercial fishing industry. They are not listed as threatened, protected, or endangered in any waters of the United States. However, this species is directly targeted in fisheries in other parts of the world, in Japan, Korea, and Taiwan.