What is a megamouth shark? Is it still a scientific mystery? - Scientific American Scientific American The Sciences What is a megamouth shark? Is it still a scientific mystery? Filipino fisherman got more than they bargained for last week when they pulled up their nets: an extremely rare, humongous fish By Nikhil Swaminathan on April 9, 2009 Advertisement | Report Ad On the morning of March 30, fishermen casting their nets in the Burias Pass, a centrally located channel in the Philippine archipelago, got the catch—not to mention surprise—of their lives: a megamouth shark so rare that some people still consider it a " cryptid ", a creature that is seen so infrequently science can't confirm its existence. That's likely an overstatement when it comes to the megamouth, first spotted in 1976 in waters near the Hawaiian island of Oahu.

But, according to the Florida Museum of Natural History (FLMNH) in Gainesville, which dubbed last month's catch "megamouth 41" (this being only the 41st observation of one of these sharks)—this is the eighth specimen snagged in the Pacific Ocean near the Philippines. The docile leviathan, measuring 13 feet (four meters) in length and weighing in at about 1,100 pounds (500 kilograms), died after becoming entangled in the nets. The anglers brought it back to the municipality of Donsol where, over the objections of the local chapter of the World Wildlife Fund (WWF–Philippines), they butchered it for food. (Shark is considered a delicacy in the Philippines.) "Whale and megamouth sharks, manta rays, dolphins, and other charismatic giants indicate that the region’s ecosystem is still relatively healthy ," Elson Aca, a WWF–Philippines project manager who identified the carcass as that of a megamouth, said in a statement.

"By protecting megafauna, we help maintain the dynamic balance of our seas, and ensure the entire ecosystem's resilience and natural productivity." WWF-Philippines reports that megamouth 41's belly was full of shrimp larvae, which it caught and gobbled in its roughly 3.5-foot- (1-meter-) wide mouth. The shark is a sluggish swimmer and can end up being chum for other marine animals: Previous specimens have born scars of circular bites made by so-called cookie-cutter sharks, which are faster and more aggressive than these behemoths. Among the sightings catalogued by FLMNH : a megamouth shark being attacked by a sperm whale in 1998 off the coast of Indonesia. To find out more about the mysterious shark, ScientificAmerican.com spoke with George Burgess , director of FLMNH's Florida Program for Shark Research. [An edited transcript of the interview follows.] What is a megamouth shark? A megamouth shark is a member of the same order of sharks that includes white sharks, basking sharks and mako sharks.

It's in its own family and its own genus as a single species (Megachasma pelagios), and it's in the Megachasmidae family. It's one of a kind, or monotypic, as we say in the scientific world. It's the only member of its group, the only member of its family, and the only member of its genus. It was not discovered until 1976 in Hawaii. The specimen had gotten tangled up in what's called a sea anchor, which is basically a parachute that's put behind a boat on the open sea to keep the boat from drifting too fast. This is a species that basically eats small shrimp—the little krill in the water. It probably spends most of its time in at mid-depth but does come to the surface, mostly to chase the large masses of krill as they go up and down in the water column during their natural, daily migrations. It's pretty amazing that this animal, which gets reasonably large, was not discovered until the year that it was. Where it lives and the lack of sampling in that kind of an area probably have a lot to do with that.

There's been a number of specimens found and, in fact, every year we're finding more and more, suggesting that this animal actually has been around, in terms of contact with humans, for quite some time now. It's just that there was a lack of recognition by the humans that what they had was something new at the time or unusual. Because this critter is seen or encountered oftentimes in Third World countries or in areas where there's less scientific presence, it's become pretty clear that it just simply wasn't recognized in the catches. The most recent megamouth was caught in the Philippines. Is that the part of the world where they dwell? Now that it is being recognized, we're getting quite a number of reports of them in certain places: Most of them are coming [in the Pacific,] from southern Japan to the Philippines and down to New Guinea. We're also hearing from people on the ground in those areas when these things are caught, through interviews, that oftentimes the fishers in the areas say, "Oh yeah, we've seen these things several times." It is a worldwide critter. It has worldwide distribution—in all the major tropical areas of the world.

It does seem to be a warm-water critter, but the [capture] records in the Atlantic are few and far between. There's one in Brazil. There's one off the west coast of Africa—off of Dakar [in Senegal]. We've got a record off of South Africa, but obviously it's not as common in the Atlantic as in the Pacific, based on capture records. They seem to be relatively common in the eastern part of the Indian Ocean. Part of this could be a function of how much fishing effort is being done in some of these areas and the opportunities to capture some of these critters. How are these sharks typically captured? If you're going to catch a critter like this, one of the ways that we've seen them being caught often is in nets that are used for tuna fishing, which are called purse seines. A net is swung around a school of tuna and then the bottom is tightened so that you end up having a bowl of netting that is gradually bought to the boat with the school of fish in the middle of the bowl. So, that [kind of rig] could capture a large animal if it happens to be in the area. Megamouths also could be seen as wash ups.

A couple of specimens have washed ashore. Whether they were the result of a capture event by a fisherman and then a release and the animal didn't make it or whether it's just the occasional sick, dying animal that washes ashore as we've seen in many other species of fishes, we can't say for sure. These are exciting animals in the sense that they are one of the rarest sharks. We still actually record them by number. Obviously, with the exception of a few deepwater sharks—all [those shark] species that are found on the bottom, in very deep water—this is one of the rarer sharks. Because of their large size, when they are found, they are a cause célèbre. With fewer than 50 sightings, how much do we actually know about megamouths? There is only one [megamouth] shark that has been captured that any science has been done on at all. Many of those 41 that turned up are in areas where they were eaten or discarded before any scientist could do anything with them. The one that was tagged was caught in October of 1990 in the Pacific off of Dana Point, Calif.

This is really the only specimen that offers us a great deal of information, because a [locator] tag was put on it to follow its movement. It was a male, about five meters [16 feet] in length, and for two days scientists were able to track it. Observations of its movement, as recorded by the tag, showed that it was a vertical migrator: It moved up and down in the water column, spending its days in deeper waters and then moving into mid-depth waters at night. Undoubtedly, its vertical migration was in response to the things that it was eating: krill, which are little shrimplike items and apparently the preferred food of the megamouth, are known to migrate to deeper waters during the day and all the way to the surface at night. So, that's our little tidbit of information based on the one megamouth that's ever been tagged. If one looked at the animal itself, just its morphology, you can get a pretty good idea of where it's spending its time and what it's up to: Like many other deepwater sharks, it has a poorly calcified skeleton.

Of course, all sharks lack bone, but there are levels of calcification of their vertebral centra—what would be backbone on a fish. The megamouth has really very poorly calcified skeleton, which is a characteristic of all deepwater animals, be they sharks or bony fishes. There's less of a need for a solid skeleton because the pressure is very strong and it sort of pushes you together. The skin tissue tends to get very flabby in deepwater animals, including the megamouth, because pressure there doesn't reward the increased turgidity of tissue. So, all of the morphology then tells us that this is an animal that lives in deep water most of the time and, of course, the tagging definitely underscored that. Given that it's called "megamouth," how does the shark feed? Is it a filter fish? The precise details of the feeding process are unknown because we haven't seen them feed, we don't have any underwater video of them feeding. What we can infer from the morphology of the mouth is that it is huge and the big fish has the ability to protrude its jaws upward and outward. Hence, of course, the name, "megamouth".

What undoubtedly it does is it protrudes its jaws and expands the buccal cavity (the mouth) and inhales the krill and then closes the mouth, forcing those krill down the tube to the stomach. It's probably more of an active feeding than what we see in, for instance, the whale shark or the basking shark, which are filter-feeding fish; they simply swim with their mouths wide open and the plankton, which they feed on, then becomes stuck on gill rakers and periodically is swallowed. What is it about the biology of the shark that vaulted it to one of a kind, complete with its own family and genus? All sharks have commonalities that give us clues to their evolutionary relationship. It has a lot of commonality with Lamniformes—the order of sharks that includes the families of the white shark, the mako, the megamouth, sand tigers and so forth. It shares a lot of the internal skeletal characteristics with those species. What makes it unique is this very large mouth with its protruding jaws. That is highly unique among all other sharks. You wouldn't confuse it with any other species just by looking at the head.

And at that size, it's the largest of the deepwater animals—that certainly separates it from other sharks, as well. It's the only shark that does what it does, which is to be a vertical migrator that feeds on these small shrimps, so it's in a unique habitat. The habitat itself sort of defines what it is, as well. How much bigger is a megamouth than an average shark? Well, the maximum size is about 17 feet [five meters] or so. As such, the only sharks that get bigger than it are the whale shark, basking shark and a very large white shark, [all of] which [can] grow to be just under 20 feet [six meters] in length. There's another species that lives on the bottom that gets to the same size, one of the deepwater, bottom-dwelling sharks [the Greenland shark], but [the megamouth] is really the fourth-largest shark. Given the lack of sightings, some people still consider the fish a cryptid? Is that a fair assessment? It's at a stage right now scientifically that each and every specimen that is caught is worthy of scientific attention.

The problem is we don't always get to the critter quickly enough. Because these are taken in fisheries and in developing countries, sometimes by the time the word comes out it's already on a plate somewhere. This is particularly the case in the Philippines, where there's been a fair number of them taken, as well as in Taiwan and Japan. Those three areas account for most of the specimens that have been seen. In all three cases, they were taken in fisheries. The reports of them sometimes are obscure. We're getting better reporting because of the presence in some of these areas of conservation NGO representatives who happen to be around in the area usually studying some other critter: whale sharks and other charismatic megafauna, such as porpoises and whales. Some of the observations have come from people who are there to look after some of these other things and by chance come across the megamouth when it's being brought into port. By and large, those animals are immediately butchered and put into circulation like any other piece of protein in that area. So, unfortunately, the most we get for some of these things are their length and sex, if the person there is cognizant of [the specimen's] value.

Once in awhile they'll get dissected or a biologist will be there as it's being butchered to add on some other observations. Little by little we get a bit piece here, but it's been rare that specimens are at the avail of scientists to do full-scale work-ups. Megamouths only pop up on our radar from time to time. Is there any way to determine if they're endangered or if we need a concerted conservation effort? No. We don't have an understanding of its population dynamics, what the natural population size would be, and whether the fishing pressure (that has resulted in the capture of these sharks in the fisheries) are a threat to the population. It's very difficult to say. But, it is a large species, and whenever you have a large species—whether it's shark or fish or mammal or whatever—they tend to get into trouble first because of overharvesting by humans. Thankfully, at this point, there is no targeted fishing for the megamouth, so it's probably in pretty safe hands. But, still, there's a lot of fishery effort in the western Pacific Ocean, between Japan and Indonesia, and, not surprisingly, this is where most of the captures have occurred.

As fishery managers and conservationists, we need to keep an open eye because that is an area where overfishing could occur. But, we certainly don't have enough scientific information to know one way or another. Megachasma pelagios :: Florida Museum of Natural History Megamouth shark. Image courtesy Compagno (1990) NOAA Tech. Rep. NMFS 90 Distinctive Features Body stout, tapering posteriorly. Head bulbous, wide and long; snout very short and broadly rounded. Gill slits moderate long but not reaching dorsal surface of head. Mouth broad, terminal, corner extending behind eyes. Caudal peduncle without keels or ridges. First dorsal fin origin closer to pectoral fin bases than to pelvic fin bases; dorsal fins relatively low; second dorsal fin less than half size of first dorsal fin. Pectoral fins shorter than head length in adults. Caudal fin asymmetrical, with pronounced ventral lobe. Megamouth shark. Photo © Tim M. Berra Coloration Dorsal surface of body, pectoral and pelvic fins, dorsal fins, center of anal fin and caudal fin are blackish brown.

Ventral surface of body, below level of pectoral end pelvic fins, tips and posterior margins of pectoral and pelvic fins, abruptly white, as the posterior margins of dorsal and anal fins and postventral caudal margins. Dorsal side of head is blackish brown, except areas between nostril and eye and between eye and spiracle, which are paler than the rest of dorsal area of head. Lateral side of head behind mouth corner is darker. Lower jaw dark with silver tint and many small dark blotches. Ventral side of head behind lower jaw dirty white. Megamouth displaying distinctive white band on anterior surface of snout. Photo © José I. Castro Mouth roof on dorsal and lateral parts, and oral membranes, silvery. Tongue purplish brown with slight silvery tint on both sides, dorsally and ventrally. Both sexes seem to have a white band on the anterior surface of the snout. This white band could be considered a feeding behavioral characteristic, because it is so contrasted by the dark coloration of the snout and upper jaw, and becomes very prominent when the upper jaw is protruded. Probably under low light this white band may be more visible.

This band might also be related to recognition of other individuals of the same species. Dentition The megamouth has approximately fifty rows of very small and relatively numerous teeth on each jaw, but only three rows are functional. Females seem to present fewer teeth rows than males. Upper and lower jaws have a symphyseal (where the two halves of the jaw meet) toothless space, but it is larger in upper jaw. A difference between the upper and lower teeth was recognized on a female specimen. The first five upper teeth are smaller than the first five lower teeth; the more distal upper teeth are smaller than the lower teeth; the cusps of the lower teeth are more acute and longer than those of the upper teeth. Megamouth tooth in A, lingual; B lingual; C lateral; and D, basal views. Image courtesy Compagno (1990) NOAA Tech. Rep. NMFS 90 Mucous and Dermal Denticles Megamouth sharks have very small mucous (on the tongue) and dermal (on the skin) denticles that differ in shape and size at each region of the body.

There are many pigment cells on the dorsal side, but none on the ventral side. Size The sizes of all reported megamouth sharks are listed in a table ( see Table ). Maximum size is at least 550cm (17ft). Males mature by 400cm (13ft) and female by 500cm (16ft). Behavior Only two observations of megamouth provide information about this species behavior. The 6th specimen from Dana Point, California (21 October 1990) offered the most important insights into the behavior of this species. The male specimen, with 494cm in total length, was tagged and tracked for two days. One of the conclusions of these observations is that megamouth is probably a vertical migrator on a diel cycle spending the daytime in deep waters and ascending to midwater depths at night. This vertical migration may be a response to the movements of the small animals on which it feeds. The krill that make up part of megamouth's diet are known to migrate from deep waters to the surface.

Megamouth, in contrast to many other deep-water sharks, shows a decrease in specific gravity in the form of a soft, and poorly calcified cartilaginous skeleton; very soft, loose skin; and loose connective tissue and muscles. Others epibenthic (live in the water just above the bottom) and epipelagic sharks often have an enlargement of their abdominal cavity and increased liver volume. The huge liver allows for greater production of liver oil in order to reduce specific gravity and increase hydrostatic support. Megamouth eating. Images courtesy Compagno (1990) NOAA Tech. Rep. NMFS 90 The 13th sighted also offered important megamouth behavioral observations. This sighting documented sperm whales attacking megamouth shark. Observers reported that the megamouth was swimming slowly and apparently confused at the surface. The shark showed signs of the whales' attack, on the base of its dorsal fin and gills. Food Habits The megamouth shark, which reaches over than 500cm in length, is one of the three giant filter-feeding sharks in the sea.

The other two are the basking shark (Cetorhinus maximus) and the whale shark (Rhincodon typus). Precise details of feeding behavior are unknown due to the lack of observations on a live, feeding specimen. However, some inferences can be made from morphological observations on the captured specimens. The megamouth shark, as its vernacular name indicates, has a huge mouth that is terminal and extends behind the eyes. Scientists believe that this shark swims slowly through aggregations of euphasiid shrimp ("krill") and other small prey with its mouth open (A). It feeds by protruding its jaws and expanding its buccal cavity in order to sucks the prey inside (B). Then the mouth is closed and the jaws retracted. This action decreases the pharyngeal volume and makes possible the expelling of water through the gill openings (C). Stomach Contents All analyzed specimens presented euphausiid shrimp in the stomach, indicating a filter-feeding habit. The stomach of the first megamouth captured contained only one type of shrimp, Thysanopoda pectinata.

The second megamouth's stomach contents included fragments of euphasiids, copepods (animals that make up plankton) and the jellyfish, Atolla vanhoeffeni. In general, euphasiids are the main alimentary item of megamouth diet. The megamouth feeds on large quantities of krill. Photo courtesy NOAA Origin of Filter-Feeding There are two conflicting hypotheses about the origin of filter feeding in megamouth exist. The first theory suggests a single origin for this habit between megamouth and basking sharks. In contrast, the other suggests that adaptations for filter feeding evolved independently in the two lineages from different ancestral origins. Results of recent studies argue strongly in favor of the latter hypothesis. This makes sense when considering morphological and behavioral differences between the two sharks. The second hypothesis also suggests that megamouth may have evolved its distinctive feeding apparatus from an ancestral sand tiger-like shark (family Odontaspidae) through jaw size exaggeration, acquisition of papillose (bud-like) gill rakers, and modification of jaw protrusion for suction feeding. Reproduction The megamouths II and VI were both mature males, and both showed evidence of recent mating.

The claspers of megamouth II were oozing spermatophores and those of the sixth specimen were abraded and bleeding, a common occurrence in sharks that have just mated. The 6th megamouth also had a fresh wound on the lower jaw, a feature found in other sharks that grasp one another's mouth during mating. Megamouth II was taken in late November and megamouth VI in late October, these factors make scientists believe that Southern California might be a mating area for megamouth sharks in the fall. The claspers of the first captured megamouth were fully described. Its claspers were relatively slender, with tip elongated, forming a very narrow, slender process. The 7th megamouth observed, a female with 471cm in total length was heavily studied and one of the conclusions is that this specimen is immature. This judgement was based on uteri that were enlarged only posteriorly, a poorly developed ovary and ostium and the small size of the oocytes. The ovary of the megamouth is similar to other mackerel sharks and this suggests that megamouth embryos are oophagous (the first well-developed embryo eats the other eggs in the uterus).

The 12th megamouth captured is the only known mature female. The total length of this specimen was 5.44m and the expanded uteri measured 260mm. The right ovary possessed a large number of whitish yellow eggs. Predators The only confirmed register of a megamouth predator is an isolated event of sperm whales (Physeter macrocephalus) attacking this shark. This occurred in Manado, North Sulawesi, Indonesia (30th August 1998) near midday, while some researchers were observing the whales. The base of the dorsal fin and the gills of the shark showed signs of the whales' attack. Sperm whales are usually considered squid feeders but there are a few notes about small deep-sea sharks in their diet. This behavioral observation significantly alters our views on the relationship between whales and sharks ( see notes ). Parasites Almost all megamouth reported presented scars in different parts of body. These scars are considered bite marks of the cookiecutter shark (Isistius brasiliensis).

Cookiecutter sharks are small dogfish sharks that attach to prey with the help of suctorial (aiding in suction) lips, and a modified pharynx. The cookiecutter removes a conical plug of flesh from the sides of the prey, leaving a crater-like wound. This parasitic shark is presumed to be a vertical migrator on a diel cycle, spending the daytime in deep waters and ascending to midwater depths and to the surface at night. The similar suggested pattern of megamouth combined with its slow swimming speed would make it easy prey for the active cookiecutter shark. Some specimens, males and females, of a parasitic copepod (Dinemoleus indeprensus), also have been found on the megamouth body surface. A few species of parasites are found internally in megamouth sharks, including specimens of a cestode worm species (Corrugatocephalum ouei) and specimens of a poorly known trypanorhynch (Mixodigma leptaleum) recovered from the intestine contents of the 7th megamouth. A microscopic parasite (Chloromyxum) was also found in this megamouth from its gall bladder.

Taxonomy When the first megamouth was captured in 1976, a new shark family, genus and species had to be erected. There are conflicting phylogenetic hypotheses regarding the evolutionary relationships between the Megachasmidae and other shark families. One theory suggests that the Megachasmidae is evolutionary derived and form a monophyletic (they have one single common evolutionary ancester) family with basking shark, Cetorhinidae. Others disagree with this idea and suggest that the Megachasmidae is relatively derived and forms a sister group to the Cetorhinidae, Lamnidae (mako, white and porbeagle sharks) and Alopiidae (thresher sharks). Recent studies suggest that Megachasma pelagios is the most primitive living species within the order Lamniformes, which contains all the aforementioned families, and has independently evolved the filter feeding mode, shared with the basking shark , Cetorhinus maximus. The currently valid genus Megachasma is derived from the Greek "megas, megalos" = great and "chasma" = cave, while the species name pelagios is also Greek, meaning of the sea.

Prepared by: Carol Martins & Craig Knickle Last Revised: February 20, 2013 Biology of the Megamouth Shark Biology of the Megamouth Shark (Megachasma pelagios) Field Marks Body very large, blubbery; head large and relatively long; snout extremely short, broadly rounded; mouth huge and terminal; gill slits lined with papillose (finger-like) gill rakers; caudal fin strongly asymmetrical, with very long upper lobe (but less than 1.5 times length of lower lobe); lacking a pattern of light bars and spots. Color dark brownish-grey to bluish-black above; pectoral and pelvic fin apices and rear margin of most fins white; mouth lining in life silvery (black in preservative) Size No data on size of pups at birth; most specimens have been 13-16 ft (4-5 m) in length, a 16-ft (4.8-m) individual weighed 1 800 lb (800 kg); maximum length to at least 17 ft (5.2 m).

Range Known from only 15 specimens to date ï¿½ of which 5 (33%) were from Japanese waters Habitat Mesopelagic; apparently a vertical migrator (a specimen tracked off southern California spent the day at depths of 395-545 ft [120-166 m], and at night moved to depths of 40-82 ft [12-25 m], returning to the depths before dawn); vertical movements apparently correlated with light levels, probably inhabits DSL, moving upward and downward with this community.

Occasionally observed at the surface during daylight hours; in October 1990, observers saw what was almost certainly a large Megamouth rise slowly and swim briefly along the surface off southern California; in August 1998, a roughly 16-ft (5-m) individual (Megamouth #13) was seen at the surface off Manado, Indonesia, apparently being attacked by a pod of 3 Sperm Whales (Physeter macrocephalus); known depth range is 0-545 ft (0-166 m), but possibly to a depth of 3 280 ft (1 000 m) or more. Feeding Known prey consists entirely of planktonic organisms, including euphausiid shrimps, copepods, and pancake jellyfish (Atolla vanhoeffeni); prey probably sucked in by expansion of buccal cavity; hypothesized to use bioluminescence to lure prey, but subsequent investigation failed to reveal light-generating tissues or symbiotic bacteria. Reproduction Almost certainly ovoviviparous, probably featuring oophagy; number of pups and gestation period unknown.

Two male specimens captured off southern California in the late fall displayed evidence of recent or impending mating activity; the claspers of Megamouth #2 were oozing sperm and the clasper tips of Megamouth #6 were extensively abraded and bleeding (possibly indicating recent copulation). Lower jaw of Megamouth #6 also showed abrasion, possibly due to the female grasping it during mating. Taken together, such evidence suggests that mating in this species off California may occur during October-November. Age & Growth Males mature at a length of about 13 ft (4 m); no data on length at maturity for females, age at maturity or longevity for either sex. Danger to Humans Minimal. Utilization Not fished or utilized commercially at present, although one of the specimens caught accidentally (#11, from the Philippines) was eaten. Remarks Probably widely distributed in deep-waters around the globe; it may only be a matter of time before a commercial fishery is established. At about 9:30 in the morning on 29 November 1994, a 21-year-old student out bird watching on the northern coast of Honshu, Japan, happened upon a remarkable discovery.

Kazuhisha Ohue was going about his ornithological business on the northeast coast of Hakata Bay, near Fukuoka City, when he spotted a large object on the tidal flats about 300 feet (100 metres) from the water's edge. At first, based on its size and rounded head, he thought it was a baby whale that had become stranded. But, noting the multiple gill slits and sandpapery skin, he deduced that it had to be some kind of shark. Ohue quickly phoned the nearby public aquarium, Marine World Umino-Nakamichi. Within 15 minutes, aquarium staff arrived and were startled to discover that the freshly-dead animal was a rare Megamouth Shark. The shark measured 15.7 feet (4.8 metres) long and 1 760 pounds (800 kilograms) in weight. It was only the seventh confirmed Megamouth specimen and the first female individual since the species was first discovered off Oahu, Hawaii, on 15 November 1976. By noon, the find was announced on CNN worldwide satellite television and the next day a photograph of the shark was in every major newspaper in Japan.

Unimo-Nakamuchi, expanding its already famous aquarium and planning a grand opening of new facilities on 16th April 1995, seized the opportunity to obtain the fresh carcass of this impressive and poorly-known shark in order to prepare it for display in the main lobby at their forthcoming opening. Thanks to foresight and careful planning, Aquarium director Seiichiro Wakisaka, curator Kouji Takada, and their staff quick-froze the carcass and set up a 'Megamouth Office' at Unimo-Nakamuchi. Japanese shark scientists and a few foreign scientists ï¿½ including renowned American shark biologists Eugenie Clark and Josï¿½ Castro ï¿½ were invited to study the specimen before it was prepared for permanent display. Leading Japanese shark biologist, Kazuhiro Nakaya was placed in charge of the dissection, choreographing the complex task so that both scientific and curatorial interests could be served. Nakaya supervised the careful cutting of the specimen so that the Aquarium's veterinarian, Satoki Kudo, could sew it back together again for a life-like display.

Over 100 people were in attendance, including some 40 scientists and their assistants, reporters, photographers, and television cinematographers. Nakaya arranged the dissection in five stages: external measurements, mouth and gill examination, cloacal examination (revealing that the shark was a virgin, dashing all hopes that she might contain embryos), heart and stomach cavity examination and taking samples of the various internal organs, and ï¿½ lastly ï¿½ the taking of a sample of the vertebral column (for age and growth studies). The dissection continued far into the night. After the scientists had taken their measurements and tissue samples, the curatorial team descended on the well-worn but still valuable specimen. Veterinarian Kudo's team hosed out the shark's body cavity, stuffed it with five big, grey blankets, and stitched up all the incisions. These delicate tasks took almost until daybreak. The shark was then transferred to a huge, water-filled acrylic tank that had been specially constructed at a cost of 7 000 000 Yen (over 70 000 US dollars) to contain the Aquarium's priceless centerpiece.

The huge shark, with its wing-like pectoral fins spread, was positioned in the tank by scuba divers and then formalin was injected into its flabby muscles. The next day, aquarium director Wakisaka, had a small piece of Megamouth epaxial (upper back) muscle prepared fried, poached with a French sauce, and tempura style, as a celebratory treat to honor visiting scientists Clark and Castro (who, in a 1995 editorial, report that the tempura was best). In March 1997, 18 scientific papers spawned by the dissection of Megamouth #7 were published in an important symposium volume entitled, Biology of the Megamouth Shark . This volume was edited by Kazunari Yano, John F. Morrissey, Yoshitaka Yabumoto, and Kazuhiro Nakaya, and summarizes much of what is known about the spectacular and elusive Megamouth. Little is known about the ecology, life history, or behavior of the Megamouth Shark. Most of what is known or suspected about the biology of this animal has been deduced from dead specimens.

The largest known Megamouth specimen was a female from Nanto, Japan, measuring 17 feet (5.2 metres) in length, so we know that the species grows at least that large. It has a soft, flabby body with a poorly-calcified skeleton, huge, rubbery lips, and terminal bathtub-sized jaws filled with a large, mobile basihyal ('tongue') and lined rows of tiny teeth. These features suggest that Megamouth is a filter-feeder, employing its highly protrusible jaws and mobile basihyal to create a powerful pharyngeal suction. Stomach contents have revealed that Megamouth is a deep-water planktivore, straining tiny, weak-swimming creatures such as euphausiid shrimps, copepods, and pancake jellyfishes (Atolla vanhoeffeni) from the water using its finger-like gill rakers. Only two other species of large, filter-feeding sharks are known, the Whale (Rhincodon typus) and Basking Shark (Cetorhinus maximus), both of which feed near the surface.

Plankton is generally more abundant near the surface, and how Megamouth manages to find enough to eat at depth presented an intriguing mystery. The discovery of two sheets of guanine-silvered tissue hanging from its upper jaw led to speculation that Megamouth may lure its prey with bioluminescence. If this scenario is correct, it would make Megamouth by far world's largest bioluminescent organism. Unfortunately for this neat theory, histological examinations failed to reveal any light-producing tissues in its mouth. But there is a definite limit to how much can be learned about Megamouth from examination of carcasses. Dead fish ï¿½ like dead men ï¿½ tell few tales. Early in the morning of 21 October 1990, rare scientific opportunity swam into a drift gillnet some 7.5 miles (12 kilometres) west of Dana Point, California. At about 2:30 a.m., on board the Moonshiner, fisherman Otto Elliot and his son, Rob, were hauling in the net when Otto noticed a large creature tangled in the webbing about 75 feet (23 metres) down.

As the unusual catch was hauled near the surface, it became clear that it was not a Swordfish (Xiphias gladius) or any of the pelagic sharks he was expecting. Otto had been a commercial fisherman for some 16 years and had seen a lot of strange things at sea, but he had never seen anything like this creature. It had shark-like features, yet it looked like a baby Killer Whale (Orcinus orca). Recognizing that the weakly struggling catch was unusual and potentially valuable, Otto secured a tailrope around the shark's caudal peduncle (tail stalk) and slowly towed it back to Dana Point Harbor ï¿½ a 5.5-hour journey. At the dock, the strange shark was left in the water, tied to the boat with a relatively short tether. Elliot then hurried off to report his unusual catch. When ichthyologist Robert J. Lavenberg arrived shortly after 12:30 p.m., the creature's identification and condition were quickly confirmed: they had a living Megamouth on their hands.

It was the sixth specimen ever recorded, and only the second from California waters (the first, Megamouth #2, was collected dead in a gillnet off Santa Catalina Island, only 33 miles away). Lavenberg readily convinced Elliot to keep the shark alive until underwater observations, measurements, photographic and video records could be made. It was further decided that ï¿½ if the shark were to remain alive long enough ï¿½ it would be released and telemetric tracking would be attempted. To everyone's surprise, and with increasing spectator and media attention, the shark did survive its day-and-a-half of restraint in the shallow, murky harbor. On the afternoon of 22 October, the shark was towed gingerly from the harbor to open water for underwater filming, attachment of sonic tags, and release. The total time of restraint for Megamouth #6 ï¿½ from time of capture to time of release ï¿½ was at least 39 hours. To Lavenberg, the 16-foot (4.9-metre) shark appeared to be in relatively good condition upon release.

Although its vigor had deteriorated noticeably during its long period of being tethered in the harbor, Megamouth #6 seemed to revive somewhat during the slow tow back to sea and continued to convalesce when it was tethered on a long tail rope while being filmed by divers. The stunning images captured by underwater photographer Mark Dell'Aquila at this time (which have since been widely reproduced) revealed at long last what a Megamouth Shark looks like alive and in its natural environment. While the constellation of divers swam around and photographed it, the shark actively pumped water over its gills, maintained an upright orientation in the water, and even continued sculling its tail in what seemed a 'normal' swimming motion ï¿½ as if unconcerned about the frenetic human activity, whirring cameras, and flashing strobes. Just prior to its release, two acoustic transmitters (one signaling position and depth, the other a simple 'pinger' to serve as a back-up tracking device should the other unit fail) were dart-attached to the shark's mid-dorsal region by a diver.

When the tailrope was slipped off at about 5:30 p.m., Megamouth #6 accelerated downward, bumbled about the bottom at a depth of about 130 feet (40 metres) ï¿½ as though confused and searching for bearings ï¿½ for about 10 minutes, then slowly headed offshore and away from all the bubbling, flashing, and poking primates. Megamouth #6 was tracked continuously for 50.5 hours ï¿½ representing two complete day-night cycles ï¿½ providing intriguing insights into this species' activity patterns. The tracking was done from the R/V Discovery, the personal research vessel of shark behaviorist Donald R. Nelson, and the results of this study were published in a 1997 paper written by Nelson, Lavenberg, and five other co-workers. During the period it was tracked, the shark moved slowly southward at an average rate of 0.7 miles (1.15 kilometres) per hour, but this speed was probably against a steady current at depth of 4 to 10 inches (10 to 25 centimetres) per second.

Thus, adjusting for the opposing current, this individual Megamouth's actual speed through the water was about 0.9 to 1.3 miles (1.5 to 2.1 kilometres) per hour, which is less than half the average cruising speed recorded for large predatory sharks such as the White (Carcharodon carcharias) and the Tiger (Galeocerdo cuvier) and is far below the theoretical optimum swimming speed for a fish of its size. Since similar ï¿½ though slightly faster ï¿½ swimming speeds have been recorded for the planktivorous Basking and Whale Sharks, its filter-feeding habits may have dictated the glacial pace of Megamouth #6. In any case, no significant differences in its swimming speed were noted between day and night. Perhaps most significantly, Megamouth #6 exhibited distinct vertical migrations at dawn and dusk transition periods. During daylight hours, the shark remained at depth but well above the bottom, which along its route occurred at depths of 2 295 to 2 790 feet (700 to 850 metres).

During the day, the swimming depth of Megamouth #6 ranged from 395 to 544 feet (120 to 166 metres) and averaged about 490 feet (149 metres). At night, the shark moved into upper layers of the ocean, its depth ranging from 40 to 82 feet (12 to 25 metres) and averaging about 56 feet (17 metres). In the tracking data, the four twilight depth-change events were very distinct and always spanned the times of sunrise or sunset. The shark's descent and ascent profiles reported by the acoustic transmitter closely match a light level at depth of about 0.4 lux (a measure of luminous intensity of visible light from a point source evenly distributed over a square metre), calculated from water transparency measurements. Further, the steepest parts of the shark's depth changes corresponded closely to the times of maximum rate of change in ambient light. To Nelson, Lavenberg, and their co-authors, these findings suggest that the Megamouth #6's changes of depth were largely in response to changes in light levels.

But why would a shark move up and down in the water column in response to light? The answer has to do with a mysterious oceanic phenomenon called the Deep Scattering Layers (or DSL's, referring to the layers' ability to scatter echoes from depth soundings). Ever since oceanographers began using sound pulses to measure the depths of the sea in the 1940's, they noted a curious 'false bottom echo' that changed depth depending upon when the measurements were taken: during daylight hours, the DSL's occurred at depths of 3 000 feet (1 000 metres) or more, while at night they occurred at 300 feet (100 metres) or less. It was not until the mid-1960's that it was demonstrated that these sound-reflecting layers were due to gas-filled floatation organs in vertically migrating creatures.

We now know that much of the upper Deep Scattering Layer is comprised of enormous scintillating constellations of lanternfishes (family Myctophidae, most species of which have a gas-filled swim-bladder) and much of the lower comprised of a gelatinous soup of siphonophores (a cnidarian order characterized by free-floating colonies of several types of specialized polyps, some of which are gas-filled and serve as buoyancy regulators for the colony as a whole). In between these layers are additional strata of crustaceans, bioluminescent sergestid shrimps below the lanternfishes, krill and other non-bioluminescent euphausiids below them but above the siphonophores. Flowing among these stratified animal populations is a veritable bestiary of wondrous and bizarre creatures: dagger-toothed viper and hatchetfishes, black, globular anglerfishes, bioluminescent deep-sea dogfishes (Etmopteridae, Dalatiidae), pulsating jellyfishes and ctenophores, jerkily rowing copepods, and jet propelled deep-sea squids.

Most of the animals of the DSL's occur in stunning profusion and the vast majority are quite tiny. But not all of them: for this realm of permanent twilight is also home to the occasional titan, such as the elusive Giant Squid (Architeuthis spp.) and the mysterious Megamouth Shark. Therefore, DSL's represent astonishingly diverse and vibrant assemblages of deep-sea organisms. Many of these creatures are tiny, free-floating packets of proteins and lipids that can provide a rich source of food for Megamouth and other predators of the DSL's. Since the planktonic creatures that comprise the DSL's migrate surfaceward at night and return to the depths by day, the Megamouth Shark does also. Thus ï¿½ whether Megamouth changes depth due to light levels directly (by avoiding depths brighter than 0.4 lux) or indirectly (by following its food supply) ï¿½ it is a crepuscular commuter, part of an enormous, vertically migrating community. Occasionally, however, Megamouth Sharks are seen at the surface during daylight hours. Why they might do so is something of a mystery.

On 10 October 1990 ï¿½ 11 days before Otto Elliot snagged Megamouth #6 in his swordfish nets ï¿½ two credible observers saw what was very probably a Megamouth Shark. At about 10:30 in the morning on that day, harbormaster Wendy Costello and her colleague, Jim Lund, spotted a very large, strange-looking animal rising to the surface just 2 miles (3.2 kilometres) off Dana Point. At first, they thought it was some kind of whale, but could not see any trace of a tell-tale spout that would confirm it. The animal rose slowly in the water and swam along the surface for a short period before descending. It swam like a large fish, snaking its body gently from side-to-side rather than displaying the arch-backed posture of a marine mammal. Costello and Lund saw a single dorsal fin and another whitish fin (possibly a pectoral). Thrice the brown-grey back of the animal came to the surface, then slowly and quietly disappeared. It's tail never broke the surface. When Wendy and Jim returned to port, they described their sighting to colleagues. All were mystified.

On Sunday morning, 21 October, they heard about Otto Elliot's unusual capture and ï¿½ like so many others that day ï¿½ couldn't resist dropping by to get a peek at the mystery creature. As the shark was brought to the surface, Wendy exclaimed, "That's it!" As the sex of the shark observed by Costello and Lund was never determined, it is impossible to say whether or not they saw Megamouth #6 before it became entangled in Elliot's nets. Since both Megamouth #2 and #6 showed evidence of sexual activity and were both caught during late fall in the same general area, perhaps the shark observed by Costello and Lund was engaged in some manner of mating-related activity. Or perhaps it was simply ill. At least three (20%) of the 15 known Megamouth specimens have been found dead or moribund on various beaches. The first of these, Megamouth #3, was found near-death on the beach at Mandurah, near Perth, Western Australia. Attempts to 're-float' the animal proved futile; after it died, the carcass was preserved and deposited in the permanent ichthyological collections of the Western Australian Museum.

The next known Megamouth, specimen #4, was also found dead on a beach, this time at Hammamatsu, Japan. It was washed back out to sea, so the only evidence we have to confirm its identity is photographs (Thank Neptune for the Japanese penchant for taking pictures!). And lastly, two days before birdwatching Japanese student Kazuhisa Ohue discovered Megamouth #7 dead on the beach at Hakata Bay, a very large shark was reported swimming in the Bay. There is no way to confirm whether this was the same shark that later beached itself and ï¿½ as the first known female Megamouth ï¿½ posthumously stimulated much new and valuable research. In its deepest areas, Hakata Bay is only 16 feet (5 metres) deep. It seems likely that Megamouth #7 entered Hakata Bay at night with the incoming tide and was stranded before it could get back out to deep water. Just why Megamouth #7 entered such a shallow, enclosed bay in the first place is just one of the many mysteries that surround this fascinating shark.

But it is perhaps worth bearing in mind that much of what we think we know about Megamouth may, in fact, be based on individuals that are sick, injured, or otherwise abnormal. Megamouth shark videos, photos and facts - Megachasma pelagios | ARKive Top Megamouth shark biology Since its discovery in 1976, only 17 sightings have been recorded of this elusive fish (3) and the majority of these have been of dead specimens; either accidentally caught or stranded individuals (5) . Consequently, very little is known of the megamouth shark's natural ecology and behavioural observations have only been possible from the tagging of one individual for a brief two-day period in 1990 (3) . This shark was observed to undergo vertical migrations, spending the day in deep water and ascending to midwater at night; it is likely that this migration is undertaken in response to the movements of prey species such as krill (3) . Megamouth sharks are thought to feed by swimming through groups of small prey items with their mouths open; however, no direct observations have yet been achieved (3) .

Rare megamouth shark caught off Japan | Fox News Rare megamouth shark caught off Japan Published May 08, 2014 Facebook 0 Twitter 0 livefyre Email Print FILE: Fishermen off the coast of japan pulled in a rare megamouth shark, similar to the one pictured, marking only the 58th time one was encountered by humans, Japanese news outlets reported. (Reuters) Fishermen off the coast of Japan hauled in a rare megamouth shark recently, marking the 58th time in history one of its kind were seen or caught by man, Japanese news outlets reported. The Japan Daily Press reported Thursday that scientists performed an autopsy on the 1,500-pound female shark in front of onlookers at the Marine Science Museum in Shizuoka City. The shark was reportedly caught from a depth of about 2,600 feet. It's unclear precisely when it was nabbed, according to the report. The first megamouth was discovered in Hawaii in 1976, prompting scientists to create an entirely new family and genus of sharks. The megamouths are docile filter-feeders with wide, blubbery mouths.

RARE GOBLIN SHARK CAUGHT OFF KEY WEST Others megamouths — considered one of the rarest fish in the world — have been encountered in California, Japan, Taiwan, Indonesia, Brazil, Ecuador, Senegal, South Africa, Mexico and Australia. It's known to inhabit the Indian, Pacific and Atlantic oceans, according to the Florida Museum of Natural History. "As with the two other filter-feeding sharks, the basking and whale sharks, this species is wide-ranging," according to a profile of the animal on the museum's website. "However, the megamouth is considered to be less active and a poorer swimmer than the basking or whale sharks." The megamouth primarily feeds on large quantities of krill and its maximum size is at least 17 feet long. The sperm whale is its only known predator, researchers say. In 2009, fishermen in the Philippines accidentally caught and later ate a megamouth shark. The 1,100-pound, 13-foot megamouth died while struggling in the fishermen's net off Burias island in the central Philippines. It was taken to nearby Donsol in Sorsogon province, where it was butchered and eaten.

The Associated Press contributed to this report. Megamouth Shark - Megachasma pelagios - Details - Encyclopedia of Life Description[ edit ] Mouth of preserved specimen The appearance of the megamouth is distinctive, but little else is known about it. It has a brownish-black colour on top, is white underneath, and has an asymmetrical tail with a long upper lobe, similar to that of the thresher shark . The interior of its gill slits are lined with finger-like gill rakers that capture its food. A relatively poor swimmer, the megamouth has a soft, flabby body and lacks caudal keels . Megamouths are large sharks, able to grow to 5.5 metres (18 ft) in length. Males mature by 4 m (13 ft) and females by 5 m (16 ft). Weights of up to 1,215 kg (2,679 lb) have been reported. As their name implies, megamouths have a large mouth with small teeth, and a broad, rounded snout, causing observers to occasionally mistake megamouth for a young orca .

The mouth is surrounded by luminous photophores , which may act as a lure for plankton or small fish. Their mouths can reach up to 1.3 m (4 ft 3 in) wide.[ citation needed ] Behaviour[ edit ] In 1990, a 4.9-m (16-foot) male megamouth shark was caught near the surface off Dana Point, California . This individual was eventually released with a small radio tag attached to its soft body. The tag relayed depth and time information over a two-day period. During the day, the shark swam at a depth of around 120–160 m (400–525 ft), but as the sun set, it would ascend and spend the night at depths of between 12 and 25 m (39–80 ft). Both day and night, its progress was very slow at around 1.5–2.1 km/h (1–1.3 mph). This pattern of vertical migration is seen in many marine animals as they track the movement of plankton in the water column.

[5] The shark captured in March 2009 was reportedly netted at a depth of 200 m (660 ft). Reproduction[ edit ] Reproduction is ovoviviparous , meaning that the young sharks develop in eggs that remain within the mother's body until they hatch. Discovery[ edit ] The first megamouth was captured on November 15, 1976 about 25 miles off the coast of Kāneʻohe, Hawaiʻi , when it became entangled in the sea anchor of United States Navy ship AFB-14. [6] Examination of the 4.5-m (14.7-ft), 750-kg (1,650-lb) specimen by Leighton Taylor showed it to be an entirely unknown type of shark, making it – along with the coelacanth – one of the more sensational discoveries in 20th-century ichthyology .

[7] Main article: List of megamouth shark specimens and sightings Worldwide sightings of the megamouth shark As of January 2015 [update] , only 60 megamouth specimens have been caught or sighted, the most recent being one found in the Philippines. They have been found in the Pacific , Atlantic and Indian oceans. Japan , the Philippines , and Taiwan have each yielded at least 10 specimens, the most of any single area, amounting to more than half the worldwide total. Specimens have also been sighted in or come out of the waters near Hawaii , California , Mexico , Indonesia , Australia , Brazil , Senegal , South Africa and Ecuador . On March 30, 2009 off Burias island in the Philippines , an 880- to 1,100-pound (400- to 500-kilogram) 4-metre (13-foot) megamouth shark died while struggling in a fisherman's net and was subsequently taken to nearby Donsol in Sorsogon province, where it was examined by scientists, before being butchered and sold.

[8] [9] [10] On 12 June 2011, a 3-m (10-ft) dead juvenile male was found by fishermen near the western Baja California peninsula coast, in Sebastián Vizcaíno Bay . It was picked up by the same fishing vessel that in 2006 captured another megamouth specimen in Sebastián Vizcaíno Bay, which has led Mexican scientists to believe that the megamouth could be a seasonal visitor to the Baja California peninsula. The new specimen was taken to Ensenada , where it was photographed and sliced in order for Scripps Institution of Oceanography and Mexican researchers to study the structure of its muscles and gills. [11] On May 7, 2014 a 4 m (13 ft) 680 kg (1,500 lb) female shark was captured at a depth of 800 metres (2,600 ft) off the coast of Shizuoka, Japan . The body was dissected in front of the public, by staff at the Marine Science Museum in Shizuoka City, Japan.

[12] On June 30, 2014 a 500 kg (1,100 lb) female shark was captured in the shallow waters of Cagayan de Oro in the Philippines. Samples were sent to the Bureau of Fisheries and Aquatic Resources in northern Mindanao (BFAR-10) and the outer skin is to be stuffed for a local museum. [13] On January 28, 2015 a 4.5 m (15 ft) deceased megamouth shark was found by residents of Barangay Marigondon, in Pioduran town, Albay, Philippines. It is now on display at the Albay Parks and Wildlife, opened to the public since March 3, 2015 which coincided with Albay's celebration of the World Wildlife Day. The megamouth shark was preserved through taxidermy, and is now considered one of the more precious collections of Albay, a leading province in environment protection.

[14] Hawaiʻi Sharks | Discovering Megamouth Discovering Megamouth Home » Hawaiʻi Sharks » Discovering Megamouth Discovering Megamouth In the 1970’s it was believed there were about 300 species of sharks worldwide. The number is now estimated at around 380. Finding a new species of shark, or any other creature for that matter, is one of the most exciting events a biologist can experience. In 1976 a particularly remarkable discovery was made in the waters of Hawai‘i. On November 15, 1976, a U.S. Navy research ship was operating in deep waters about 25 miles off Kahuku Point, the northern-most spot on O‘ahu. The crew had deployed two large parachute-like sea anchors to a depth of 500 feet, in order to hold the ship in position. When the sea anchors were hauled up, one of them had entangled a 14-1/2 foot shark, the likes of which nobody had ever before seen. The shark did not survive.

It took a lot of hard work, but the 1,500 pound shark was finally brought aboard the ship and transported to a Navy lab at Kane‘ohe Bay. Local marine biologists were summoned to the site, and calls were made to shark specialists around the country. None had ever seen or heard of such a shark. One of the first marine biologists on the scene was Waikiki Aquarium director Leighton Taylor, who eventually had the honor of describing and naming the new shark. The press immediately picked up on his nickname “megamouth,” chosen because its mouth was about 3 feet wide. It would take another seven years before the fish was formally described scientifically. It belonged to a new family (and therefore a new genus and species), and Taylor named it Megachasma pelagios, Greek for “giant yawner of the open sea.” Megamouth was studied extensively, particularly its mouth, teeth, and stomach contents. Tooth and tissue samples were made available to researchers worldwide. It became clear early on that megamouth was a plankton feeder, hence its large mouth.

It had fed mostly on deepwater euphausiid shrimp, and was only the third plankton-eating shark known, joining the whale and basking sharks in that category. It was placed in the order Lamniformes, which includes the basking shark, but also the white shark, threshers, makos, and sand tigers. It would take eight years before another megamouth was found, this one off California’s Catalina Island. Megamouth seems to be a deepwater species, possibly following the deep scattering layer of vertically migrating plankton which come closer to the surface at night. As of the end of 2003, 18 megamouths had been found, and there was an unconfirmed report of a May 26, 2003 sighting near Dana Point, CA. Megamouth I (the holotype, or first one described) is still held in the Bishop Museum icthyology collection in Honolulu. Megamouth remains one of the world’s most rarely seen sharks, and it’s amazing a fish that size could remain undiscovered for such a long time. You can learn more about Megamouth from the Florida Museum of Natural History’s Megamouth page.

Deep Sea: Megamouth Shark Deep Sea: the Twilight Zone and Beyond Megamouth Shark First captured off Oahu, Hawaii, in November 1976 and now known from only 14 specimens from scattered locations around the globe, the Megamouth Shark (Megachasma pelagios) is one of the most spectacular ichthyological discoveries of the 20th Century.

Just the Facts: Birth: unknown; smallest free-swimming specimen was 6 ft (1.8 m) long Maturity: males 14 ft (4 m); females no data Maximum: 17 ft (5.2 m) Reproduction: Maturity: no data for either sex Mode: ovoviviparity Adult: euphausiids, copepods, sea jellies Habitat: Open Ocean, Deep Sea Depth: 40-545 ft (12-166 m), possibly to 3,300 ft (1,000 m) Distribution: Central Pacific, Tropical Eastern Pacific, Amazonian, West African, South East Asian, Western Australian, Japanese Named for its huge, bathtub-sized jaws and rubbery lips, Megamouth is highly unusual among deep-sea sharks in that it is apparently a filter-feeder. Along the inner lining of its gills are rows of cartilage-cored, finger-like gill rakers, which the animal almost certainly uses to strain food from the surrounding seawater.

Known prey of the Megamouth Shark consists entirely of planktonic animals, including euphausiid shrimps, copepods, and the Pancake Jellyfish (Atolla vanhoeffeni). Yet most plankton is found near the surface, so it is something of a mystery how Megamouth manages to find enough to eat. Megamouth gill rakers Because the Megamouth Sharkï¿½s upper jaw features great sheets of silvery, reflective tissue, it has been speculated that its lower jaw might be bioluminescent and the creature may use living light to draw planktonic prey to itself. Had this been true, Megamouth would have been by far the largest known bioluminescent organism. Unfortunately, histological examination of its lower jaw tissues revealed no light-producing organs ï¿½ another beautiful theory slain by an ugly fact. Examination of Megamouthï¿½s anatomy has revealed that its jaws are highly protrusible and its basihyal (ï¿½tongueï¿½) is large and unusually mobile.

In theory, coordinated jaw protrusion and basihyal flexure could significantly expand the volume inside Megamouthï¿½s throat, creating a partial vacuum to help suck in planktonic prey. While this makes good intuitive sense, confirmation or refutation of this scenario will have to await documentation via film footage of a Megamouth feeding in the wild. As well established as is the field of comparative anatomy, there is only so much one can learn from a lifeless specimen. In October 1990, scientists had a rare opportunity to track the daily movements of a Megamouth (specimen #6) off Dana Point, southern California. A 16-foot (5-metre) male specimen, it spent the daylight hours at depths of 400 to 550 feet (120 to 166 metres). At night, it rose to depths of 40 to 80 feet (12 to 25 metres) and returned to the depths before dawn.

This individual Megamouth was tracked continuously for only 50.5 hours ï¿½ two complete day-night cycles ï¿½ but that was long enough to determine that its vertical movements were strongly correlated with ambient light levels. From the sum total of data collected at the time, the researchers concluded Megamouth is a vertical migrator that probably moves up and down as part of a complex community of mesopelagic animals known collectively as the Deep Scattering Layer (for its ability to scatter depth sounding echoes). The tracking data also revealed that Megamouth swims very slowly. The telemetered individual moved southward at an average rate of 0.7 miles (1.2 kilometres) per hour, but this speed was probably against a steady current of 4 to 10 inches (10 to 25 centimetres) per second. Adjusting for the opposing current, the sharkï¿½s actual speed relative to the water was about 0.9 to 1.3 miles (1.5 to 2.1 kilometres) per hour.

This is less than half the average cruising speed recorded for large predatory sharks, such as the White Shark (Carcharodon carcharias) and Tiger Shark (Galeocerdo cuvier), but only slightly slower than swimming speeds recorded for other large planktivorous sharks, such as the Basking Shark (Cetorhinus maximus) and Whale Shark (Rhincodon typus). But, at present, we cannot be sure whether Megamouthï¿½s slow cruising pace is due to its filter-feeding mode of life, a strategy for saving energy in the nutrient-poor mesopelagic, or simply an artifact of the physiological sluggishness of muscles in the cold water characteristic of the deep-sea. Little is known about the reproductive biology of Megamouth. Two male specimens captured off southern California in the late fall displayed evidence of recent or impending mating. The claspers of Megamouth #2 were oozing sperm and the clasper tips of Megamouth #6 were extensively abraded and bleeding, possibly indicating it had mated recently. The lower jaw of Megamouth #6 also showed abrasion, possibly due to a female grasping it during mating.

Taken together, such circumstantial evidence suggests that Megamouth mating in the tropical eastern Pacific may occur during October and November. The Megamouth Shark is occasionally observed at the surface during daylight hours. In October 1990 a pair of sea kayakers off Dana Point, California saw what was very probably a large Megamouth rise slowly and swim briefly along the surface. In August 1998, a Megamouth (#13) ï¿½ estimated to be 16 feet (5 metres) long ï¿½ was observed and photographed at the surface off Manado, Indonesia, apparently being attacked by a pod of three Sperm Whales (Physeter macrocephalus). Whether the whales were attempting to feed on the hapless shark or simply playing with it, as a cat does with a mouse, is not clear from the reports and photos. Vulnerability to predators may partially explain why Megamouth is so rarely seen at the surface during daylight hours. And it may also explain why such a large and otherwise conspicuous species was not ï¿½discoveredï¿½ until so very recently.

Most of our accumulated knowledge about Megamouth as a living creature has been deduced from dead or moribund specimens. At least three Megamouth specimens have been found stranded on shallow, sandy beaches, one in Western Australia (#3) and two in Japan (#4 and #5). It is hard to imagine a healthy deep-sea shark entering such shallow waters and stranding. Even the 16-foot (5-metre) male tracked off southern California (#6) was caught in a gillnet and kept overnight tethered on a short leash in a shallow, silty bay before it was outfitted with a telemetric device and released. Thus, a great deal of what we think we know about the Megamouth may be based on aberrant, ill, or moribund individuals. But, as our exploration of the deep-sea continues into the 21st Century, opportunities to observe, film, and study the Megamouth Shark will undoubtedly increase, revealing new secrets and raising new questions about its mysterious life.

Question: What type of fish is a Megamouth ?
Answer: SHARK