Biology of the Megamouth Shark (Megachasma pelagios) |
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 - including renowned American shark biologists Eugenie Clark and José Castro - 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 - lastly - 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 - like dead men - 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). Recognising 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 - 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 - if the shark were to remain alive long enough - 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 - from time of capture to time of release - 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 - 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) - as though confused and searching for bearings - 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 - representing two complete day-night cycles - 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 - though slightly faster - 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 - whether Megamouth changes depth due to light levels directly (by avoiding depths brighter than 0.4 lux) or indirectly (by following its food supply) - 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 - 11 days before Otto Elliot snagged Megamouth #6 in his swordfish nets - 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 - like so many others that day - 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 (23%) of the 13 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 - as the first known female Megamouth - postumously 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.
