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.