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.