Megamouth Shark Sightings – Why This Large Shark Is Seldom Seen

With fewer than 300 sightings of the megamouth shark having been reported since the species’ discovery in the mid-seventies, the recent appearance of a pair of megamouth sharks just 30 miles off the coast of San Diego came as a surprise to the fishermen who spotted the rare fish.

Megamouth Shark
Photo: OpenCage, CC BY-SA 2.5, via Wikimedia Commons (cropped / resized by

What Is Known About The Megamouth Shark?

The megamouth shark was first discovered in 1976, and since then there have been fewer than 300 sightings of this reclusive fish. The majority of megamouth encounters have involved the shark being mistakenly caught as bycatch.

The megamouth grows to a length of up to 7 meters / 22.97 ft, and and has a large head and thick lips.

With a mouth that is up to 1.3 meters / 4.27 ft. wide, there are no prizes for guessing how the megamouth shark got its name!

Megamouth shark in museum
Photo: opencage, CC BY-SA 2.5, via Wikimedia Commons (cropped / resized by

Although its size may be intimidating, unlike the majority of sharks, the megamouth is not an active hunter. Instead, it is a filter feeder, swimming through the ocean with its mouth open, gathering food from the water as it goes. Structures known as gill-rakers within the shark’s mouth filter food from the seawater.

The megamouth is one of three sharks known to feed in this manner, the other two being the whale shark and the basking shark, which are the world’s largest and second-largest fish, respectively.

Food captured by the megamouth shark includes plankton, jellyfish, and small crustaceans. The shark’s large mouth, small teeth and rounded snout have sometimes led observers to mistake it for a young orca.

Bioluminescent Mouth?

It was once hypothesized that the shark’s white upper lip may possess some form of bioluminescence, acting as a light trap to lure prey into its mouth.

Studies have since disproved this, finding that this area of skin is actually made of the same substance that makes fish scales shine. However, the idea that this acts as a light trap is still plausible, even though it is not bioluminescent.

Where Is The Megamouth Shark Found?

The first megamouth shark was discovered in 1976, after having become entangled in a US Navy ship’s anchor off the coast of Hawaii.

It was another eight years before a second was found off the coast of California. Megamouth sharks have since been found in the Pacific, Atlantic, and Indian Oceans.

Over half of the world’s megamouth sightings have been made off the coast of Taiwan. It is not known whether this is because the shark’s population is largest in this region or because more megamouths are caught here due to the country’s active whaling industry.

Since 2013, Taiwanese fishing vessels have been required to both report the capture of any megamouth sharks and retain the body for samples. This has resulted in 136 reported megamouth shark captures.

Local environmental groups are campaigning for better protection for the megamouth shark. Although the fishing industry claims the sharks are being caught mistakenly as bycatch, it is alleged that the species is being intentionally targeted.

Why Is The Megamouth So Rare?

The megamouth is a deep-water shark and as a result is rarely seen by humans. The species can be found anywhere from the surface to depths of 15,000 feet (4600 meters).

Like many deep-sea creatures, the megamouth spends most of its life in the dark depths of the ocean, only coming to the surface at night.

How Fast Does The Megamouth Shark Swim?

Researchers attached a radio tag to an individual megamouth that was caught off the coast of California, before releasing it back into the ocean. The tracking data revealed that the shark traveled incredibly slowly, swimming at speeds of between 0.93 and 1.30 miles per hour. It moved up and down in the water column during the night and day respectively. The shark’s poor swimming ability is confirmed by its soft, flabby body, in contrast to more active filter-feeding sharks (source).

It’s likely that these sharks are naturally very rare, but scientists do not have enough data on the species to accurately determine its conservation status. They are considered vulnerable to overfishing because they grow so slowly and have very low rates of reproduction, so any population decline may be difficult to reverse.

An Unusual Sighting

Although the sighting of any megamouth shark is extraordinary, the fact that two were spotted together makes this even more interesting. So far sightings have been singular, and this is potentially the first time that two megamouth sharks have ever been seen together.

Scientists have speculated as to whether this may have been courtship behavior, potentially indicating that southern California could be an important breeding ground for this rare species.

How Much Do We Know About Megamouth Reproduction?

You can find out more about shark reproduction on the following pages:

Shark Life Cycle & Shark Reproduction: How Do Sharks Reproduce?

Do Sharks Lay Eggs? Facts On Shark Eggs & Mermaid’s Purses

There is little solid information about the reproduction of megamouth sharks, as no recovered specimens have carried embryos. However, observations from the ovaries of female sharks suggest that they are ovoviviparous, meaning that young sharks develop in eggs inside the mother’s body before they hatch as live young.

During the gestation period, the mother will feed her young with unfertilized eggs, rather than providing nutrients via a placenta, as other animals that give birth to live young do. When the young sharks are born, they are usually around 170cm in length and will immediately become filter feeders.

Research has provided very few answers about the megamouth’s courtship behavior, but some females have been caught with fresh mating scars on their bodies at many different times of the year, indicating that they mate all year round. It is also thought that the shark gives birth to young in lower latitude waters, as this is where most juvenile megamouths have been reported (source).

Typically, female megamouths are larger than males. The species, like other sharks, is thought to show indeterminate growth. This means that the shark never stops growing throughout its life, and will continue to increase in size for as long as its environment allows.

This means that there isn’t a specific age at which a megamouth shark reaches maturity, but a certain size. Half of all females studied reached maturity at 5.17 meters in length, whereas males reached maturity at 4.26 meters.

This trend is the same as in other shark species, where female sharks are larger because they need the extra girth to be able to carry their young (source).

More Questions Than Answers

Although our knowledge of the megamouth shark has increased over recent years, much of what we know is based on knowledge from other shark species, and there is little data directly relating to megamouths.

This means that there are still so many questions that scientists don’t have answers to.

For example, how large can megamouth sharks grow and how long do they live? Do they have any predators? How many pups do they have? Although mating scars have been reported, we still know so little about their courtship behavior and where they breed.

We don’t even know how widely distributed the sharks are, and without this detailed knowledge, it’s difficult to know how best to protect them. Each new megamouth sighting is crucial in providing scientists with fresh insights into these elusive creatures and their mysterious lives in the deep.

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