Megamouth Sharks: Characteristics, Size, Behavior, Mating

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Megamouth sharks (Scientific name:Megachasma pelagios) are very large plankton-feeding sharks that reside in relatively shallow water. One of the least known of all sharks, they were only discovered in 1976, when one accidentaly got caught near Oahu after it became entangled in an anchor of US navy research vessel. As of 2006, only about 21 had been found in the Atlantic, Indian and Pacific Oceans. As of 2023, there had been just 273 sightings of megamouths, most involving sharks caught in fishing gear. Only five have been spotted swimming freely in the wild. As of 2019, all that was known about megamouth sharks was based on 124 records. [Source: Live Science, Animal Diversity Web (ADW)]

Megamouths can reach a length of 5.5 meters (18 feet) and weigh 790 kilograms. Not only are they a unique species and genus they represents a new family of sharks. Their gills and fins configurations are like that of other sharks but their head and teeth are very different. Somewhat related to mackerel sharks, megamouth sharks have round heads that grows large at the mouth end. They have small teeth between six and seven millimeters in length, ideal of collecting plankton. Their body is very soft.

Megamouth sharks are one of three plankton-eating shark species, along with whale sharks and basking sharks. They feed like these sharks by using a filtration system. Based on information gleaned from transmitters implanted on individuals, they swim during the day at depths of 120 to 170 meters (393 to 558 feet) and climb at night to swim at depths of only 10 to 20 meters (3 to 66 feet). They are harmless to humans and may be preyed upon by sperm whales.

The lifespan and longevity of megamouth sharks is unknown due to the lack of specimens and sightings. Some researchers believe basking sharks should be placed in the same family as megamouth sharks as they have similar morphology as megamouth sharks. Basking sharks have a lifespan of about 50 years and would not be a strech to assume megamouth sharks have a similar lifespan. [Source: Acalia Carter-Martin, Animal Diversity Web (ADW)]

Websites and Resources: Shark Foundation ; International Shark Attack Files, Florida Museum of Natural History, University of Florida ; Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase ; Encyclopedia of Life ; Smithsonian Oceans Portal

Megamouth Habitat and Where They Are Found

Megamouth sharks are typically found in warm, shallow coastal and deep, open-water pelagic waters. at depths of five to 1500 meters (16 to 4921 feet) between latitudes of 40°N and 40°S. These sharks inhabit the Atlantic, Indian, and Pacific oceans. Confirmed sightings or catches include coastal regions of the United States (Hawaii and San Diego, Catalina Island, San Clemente, California), Australia, Brazil, Indonesia, Japan, the Philippines, Taiwan, Senegal, Vietnam, Sri Lanka, South Africa, Peru, China, and Ecuador.[Source: Acalia Carter-Martin, Animal Diversity Web (ADW), Live Science]

Megamouth range

Megamouth sharks have been caught at depths of around 600 meters. They move vertically through the water according to the migration of their prey, which follows the light cycle. They swim deeper during the day and closer to the surface at night. Nocturnally, megamouths are found at depths between 150 meters and 500 meters. During the day, they swim at depths between 350 meters and 600 meters.

As of 2006, seven of the 21 confirmed megamouth catches took place off of Japan. A male caught in April 1997 off Owase, Mie Prefecture, weighed more than a ton and was 5.4 meters long. Others have been caught off Hamamatsi in Shizuoka Prefecture, in Hakata Bay in Fukuoka and in Tokyo Bay off Ichihara. A 4.2-meter, 460-kilogram specimen has been stuffed and is now displayed at a museum in Japan . Another four are preserved in formaldehyde in Japan, Los Angeles, Hawaii and Australia.

Megamouth Physical Characteristics

Megamouth adults range in length from 4.25 to 5.5 meters (14 to 18 feet) and reach the weight of 1,215 kilograms (2,679 pounds). Females are larger than males. Adult males are typically 4.25 to 5.15 meters (14 to 17 feet) long. Females are usually over five meters (16.4 feet) long. The largest recorded megamouth — a female — was 5.44 meters (17.85 feet) long and weighed 1,040 kilograms (2291 pounds). Another female was reported as “larger than 5.5 meters (18 feet)" by Nakaya (2001). A juvenile male megamouth caught off the coast of Brazil was 1.9 meters (6.2 feet) in length and weighed 24.4 kilograms (54 pounds). [Source: Acalia Carter-Martin, Animal Diversity Web (ADW) /=]

Megamouth sharks are gray to bluish-black in color and have a white underside, large tapering flipper-like pectoral fins, small pointed dorsal fins, relatively small pelvic fin, a small, pointed anal fin and a large upper tail lobe. Distinctive coloring in and around the shark’s mouth are believed to assist the sharks in feeding. There are dark spots under their lower jaws. A luminescent strip on the upper jaw and the silvery color of the roof of the mouth, visible when the fish swims with its mouth open, are thought to play a role in attracting prey. /=\

Megamouth sharks get their name from to their massive, terminal mouth which has distinct short, round snout. They have a bulbous head. Their jaws extend past their eyes. The sharks have small hooked teeth along the top and bottom of their jaws. Like other sharks, megamouth sharks have eyes that include key features found in most vertebrates including the retina, a cornea, and an iris, they also have a tapetum lucidum, which allows them to see in darker waters.

23-Million-Year-Old Megamouth

In 2013, scientists announced they had identified a new species of megamouth shark that lived about 23 million years ago, based on teeth first discovered in the 1960s and then forgotten. The prehistoric sharks likely roamed both deep and shallow waters, using its massive mouth to filter its food — plankton and fish. Unlike modern the megamouth sharks, the ancient creature had slightly longer, pointier teeth. "It was a species that was known to be a new species for a long time," said study co-author Kenshu Shimada, a paleobiologist at DePaul University in Chicago. "But no one had taken a serious look at it," said Shimada, who described the new species published his findings in the Journal of Vertebrate Paleontology. [Source: Tia Ghose, Live Science, November 4, 2013]

When scientists first found shark teeth from the species in the 1960s, they didn't know what to make of them as there were no similar living creatures and modern megamouths hadn’t been discovered yet. Live Science reported: Over time, researchers turned up hundreds of similar teeth along the coast of California and Oregon. All the specimens were tossed in a drawer and forgotten in the collections of the Los Angeles County Museum and a few other California museums. Scientists discovered the modern megamouth shark in 1976.

The ancient megamouth’s lightly longer, pointier teeth. “suggests that they probably had a wider food selection," Shimada told LiveScience. "They could have probably eaten plankton, but they were also probably feeding on fish." The team determined the ancient creature would've sported a slightly longer, less-wide snout than the modern megamouth shark. The extinct creature also likely grew to an average of 20 feet (6 meters), but the biggest megamouth individuals might have been nearly 27 feet (8 m) long, not much different from their modern relatives. Because the teeth were found in both deep-ocean and near-shore marine sediments, the extinct monster probably had already begun to migrate between the deep and shallow oceans in search of food. It's still not clear what caused the sharks to evolve to have wider mouths and adopt an exclusive filter feeding strategy, Shimada said.

Megamouth Behavior, Perception and Communication

Megamouth shark head
Megamouth sharks are generally solitary, crepuscular (active at dawn and dusk), motile (move around as opposed to being stationary) and migratory (make seasonal movements between regions, such as between breeding and wintering grounds). They have no known home range or territory. [Source: Acalia Carter-Martin, Animal Diversity Web (ADW) /=]

According to Animal Diversity Web: Megamouth sharks move vertically through the water, being caught at deeper depths during the day, and more shallow depths at night, and are slow, swimmers. This behavior likely occurs as megamouth sharks are following the movement of emphasiid shrimps and copepods. One radiotracked shark traveled vertically multiple times during a two-day period of time. At dusk, it was at depths of just 12 to 25 meters (39 to 82 feet) below the surface, retreating to 120 to 166 meters (390 to 544 feet) at sunrise. During daylight hours, it moved to 700-850 meters (2230- 2,790 feet) below the surface. Speeds of travel were measured at 1.5 to 2.1 kilometers per hour (.9 to 1.3 miles per hour) regardless of time of day. Other than some information gleaned from strandings and by-catches, not much else has been reported about the behavior of these sharks. /=\

Megamouth shark communicate with vision, touch, electric signals and chemicals usually detected by smelling and sense using vision, touch, electric signals and chemicals usually detected with smelling or smelling-like senses. Not much is known about how megamouth sharks specifically communicate with each other. As with all sharks, megamouth sharks have both a lateral line and ampullae of Lorenzini. The lateral line aids sharks in recognizing the presence of both predators and prey. The ampulla of Lorenzini uses electrical fields to detect prey. /=\

Megamouth Food, Eating Behavior and Predators

Megamouth sharks are filter feeders that sift food from the water captured in their enormous mouths. They feed mostly on krill and shrimp-like plankton, particularly shrimp from the family Euphausiidae and copepods. The white band on the shark’s top jaw, Kazuhiro Nakaya of Hokkaido University, believes serves two purposes — 1) to lure for plankton in low-light environments, and 2) as a way for individual megamouth sharks to recognize one another as they appear to be unique to megamouth sharks. [Source: Live Science]

Megamouth sharks use their mammoth mouths to engulf plankton-filled water, forcing the water through gills equipped with a filtering apparatus called gill rakers, which direct plankton into the digestive track. Occasionally, they consume jellyfish, including the species Atolla vanhoeffeni. Stomach contents from a shark in Hawaiian waters revealed primarily a euphausiid shrimp, Thysanopoda pectinata. In Japan, the shrimp species consumed there was identified as Euphausia nana. Megamouth sharks deeper-in-the-day and closer-to-the-surface-at-night movements follow the movements of their prey. [Source: Acalia Carter-Martin, Animal Diversity Web (ADW) /=]

The only recorded predator of megamouth sharks is cookiecutter sharks, which latch on to the larger shark and scoop out ice-cream-size chunks of flesh and swim away. In 1998, three sperm whales were observed attacking a five-meter (16.4-foot) -long megamouth in Indonesian waters. Humans catch megamouth sharks in occasional bycatch incidents.

Megamouth Mating, Reproduction, Development and Offspring

Megamouth sharks are ovoviviparous {eggs are hatched within the body of the parent) and iteroparous (offspring are produced in groups). They engage in internal reproduction in which sperm from the male fertilizes the egg within the female, with males using claspers for fertilization, and likely breed year round The extent of parental involvement in megamouth sharks is not known but is figured to be minimal as it is wit all sharks. Being ovoviviparous, the eggs remain in the female while they develop and are thus protected at that time.[Source: Acalia Carter-Martin, Animal Diversity Web (ADW) /=]

Male megamouth sharks have been caught with sperm coming from their claspers. This indicates internal fertilization by the male. A female was caught off Japan in 1994 had extensive scarring, which is believed to have been caused by a male during sex. Otherwise little is known about megamouth mating. They are believed to be polygynandrous (promiscuous), with both males and females having multiple partners. /=\

Megamouth sharks show signs of oophagy (eating their own eggs). The number of offspring, mass at birth and gestation period are unknown but it is believed they give birth to large offspring in small numbers, with offspring being less than 1.8 meters (5.8 feet) at birth. A juvenile caught off the coast of Brazil weighed 24.4 kilograms (54 pounds) and was 1.9 meters (6.2 feet) in length. It is thought that megamouth sharks mate year-round and give birth near the equator based on the locations where few juveniles have been caught. /=\

According to Animal Diversity Web: The stages of development are not entirely known. No female megamouth sharks with embryos or newborns have been caught. What is known is available is due to by-catches by fishermen and strandings. Female megamouth sharks are not fully mature until they reach about five meters (16.4 feet) in length. Males megamouth sharks fully mature at about four meters (13 feet) in length. More specifically, 50 percent of female megamouth sharks are mature at 5.17 meters (17 feet), and males at 4.26 meters (14 feet). Megamouth sharks are thought to exhibit indeterminate growth (they continue growing throughout their lives), as do all sharks./=\

Two Megamouth Sharks Caught on Video for First Time Ever — May Be Mating

In March 2023, video footage was posted of a pair of megamouth sharks swimming together off the coast of San Diego. Live Science reported: The video, captured by fishers in early September 2022, may show the deep-dwelling beasts in a courtship ritual and is one of just a handful of sightings of the creatures alive. Never before had two been seen swimming together.[Source: Joshua A. Krisch, Live Science, March 25, 2023]

A study analyzing the footage suggests that the two sharks were engaging in courtship or mating behaviors. "The curiosity of these fishermen benefited the field as a whole," study lead author Zachary Skelton, a graduate student at the University of California, San Diego, told Live Science. "The 10 minutes the fishermen had with the sharks contains the only knowledge we have on megamouth shark sociality."

"It's pretty darn rare to see one, let alone two at a time swimming at the surface during the day," Christopher G. Lowe, director of the Shark Lab at California State University Long Beach, who was not involved in the study, told Live Science. To better understand megamouth shark behavior, Skelton and colleagues analyzed the footage in light of whatever they could find in the literature on the social behaviors of other filter-feeder sharks, such as basking sharks and whale sharks. "Because the encounter was so brief, we had to heavily rely on other studies and species to try and make sense of why the sharks were at the surface, why they were together, and why at that specific place," Skelton said.

Visible male sex organs known as claspers suggested the smaller of the two sharks was male. And although the team could not confirm the sex of the other shark, they determined that it was probably female, based on a lack of obvious claspers and a series of scars on its back similar to the mating scars found on female sharks from other species. Given that the male was closely following the putative female shark and that neither shark was seen attempting to feed, the researchers concluded that the footage likely reflects a courtship display. The results were published March 13 in the journal Environmental Biology of Fishes. "This anecdotal observation has all the hallmarks of precopulatory mating behavior," Carl Meyer, an associate researcher at the Shark Research Lab at the Hawaii Institute of Marine Biology who was not involved in the study, told Live Science in an email. "We still know comparatively little about the biology and ecology of megamouth sharks so this observation is an interesting addition to our understanding of this species."

Neil Hammerschlag, director of the Shark Research & Conservation Program at the University of Miami, who was not involved in the study, was similarly impressed. "The paper does a good job of speculating what could be occurring," he told Live Science in an email. "The social behavior of [megamouth] sharks is still a bit of a black box to scientists, and observations like these are exciting, generating a bunch of questions and theories that can be further studied."

Megamouth Sharks, Humans and Conservation

Humans mainly utilize megamouth sharks for research, displays and education and, in rare cases, food. Specimens have been preserved and used for research. They have also been eaten in some countries where they have been caught including Brazil, Taiwan, and the Philippines. [Source: Acalia Carter-Martin, Animal Diversity Web (ADW) /=]

Megamouth sharks are considered to be a species of "Least Concern" on the International Union for Conservation of Nature (IUCN) Red List. The US Federal List and Convention on International Trade in Endangered Species (CITES), have no special status for megamouth sharks.

Megamouth sharks are typically caught by fishers in rare bycatch incidents, although they have also been found washed up on shores in Japan and other countries. Many specimens have been caught fishing for sharptail mola (Masturus lanceolatus), which are found at the same depths as megamouth sharks. From 2013-2015, 34 megamouth sharks were caught as bycatch. These sharks, by their rarity, would make overexploitation (intentional or not) a serious threat. /=\

In U.S. waters in the Pacific Ocean, the collection of megamouth sharks had been prohibited since 2004. As of 2015, it has been legal for fisheries to keep a megamouth specimens if they were caught accidentally — but it must be donated to a museum or for research facilities. In other countries such as Taiwan, megamouth sharks must be reported to the government if caught. Some scientists recommend that if megamouth shark catches continue to increase in number as bycatch of driftnet fisheries in Taiwan and other Asian fisheries, live release should be required. /=\

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated March 2023

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