Megamouth shark

The megamouth shark (Megachasma pelagios) is a species of deepwater shark. It is rarely seen by humans and is the smallest of the three extant filter-feeding sharks alongside the whale shark and basking shark. Since its discovery in 1976, few megamouth sharks have been seen, with fewer than 100 specimens being observed or caught. Like the other two planktivorous sharks, it swims with its enormous mouth wide open, filtering water for plankton and jellyfish. It is distinctive for its large head with rubbery lips. It is so unlike any other type of shark that it is usually considered to be the sole extant species in the distinct family Megachasmidae, though suggestion has been made that it may belong in the family Cetorhinidae, of which the basking shark is currently the sole extant member.

Description
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. The meg­amouth is con­sid­er­ably less ac­tive than the other fil­ter-feed­ing sharks, the bask­ing shark and the whale shark. The meg­amouth has a stout body and a long, wide bul­bous head.

Meg­amouths are large sharks, able to grow to 5.49 me­tres (18.0 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. Meg­amouth sharks can be found as far north­ward as north­ern Japan; south­ern Cal­i­for­nia (LACM 43745-1) and near Punta Eu­ge­nia, Baja Cal­i­for­nia; Hawaii. Oc­ca­sional off Cal­i­for­nia. Length to at least 5.75m (18.9 ft) TL. Depth: sur­face to 1,000m (3,280ft). Meg­amouth sharks have a color of Dark blue, brown­ish-black, or gray above, lighter below; white band along upper jaw; pos­te­rior mar­gin of fins white. Vert. 139-151.

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 pro­trud­ing in­side of the upper lip is a bril­liant sil­very-white, which is very vis­i­ble when the mouth is open. This lip was ini­tially thought to be pos­si­bly em­bed­ded by lu­mi­nous pho­tophores when the first shark were ex­am­ined in the early 1980s, which may act as a lure for plank­ton, while the team ex­am­in­ing the sec­ond shark in the mid-1980s in­stead pro­posed that the lower lip might glow with the white band used as a re­flec­tor of sorts, but nei­ther the­ory has been proven. This white band is pre­sent in both sexes and could be ei­ther a feed­ing mech­a­nism or pos­si­bly be used as a means of iden­ti­fy­ing other in­di­vid­u­als of meg­amouth sharks. Their mouths can reach up to 1.3 m (4 ft 3 in) wide. Meg­amouth sharks have up to 50 rows of teeth in their up­per-jaw and up to 75 rows of teeth in their lower-jaw.

Taxonomy and evolution
Re­searchers have pre­dicted the feed­ing pat­terns of meg­amouth sharks in re­la­tion to the other two plank­tiv­o­rous sharks; the three plankivourous sharks have ram feed­ing in com­mon, as it evolved from ram feed­ing swim­ming-type an­ces­tors that de­vel­oped their fil­ter­ing mech­a­nism to cap­ture small prey like plankton. In ad­di­tion to the liv­ing M. pela­gios, how­ever, two ex­tinct meg­amouth species – the Pri­abon­ian M. al­isonae and the Oligocene–Miocene M. ap­ple­gatei – have also re­cently been pro­posed on the basis of fos­silized tooth remains. An early an­ces­tor of the re­cent species Megachasma pela­gios was re­ported from the early Miocene (Bur­di­galian) of Belgium. How­ever, the Cre­ta­ceous-aged M. comanchensis has been re­cently re­clas­si­fied as an odon­taspid shark in the genus Pseudomegachasma, and is in fact un­re­lated to the meg­amouth shark de­spite sim­i­lar teeth morphology. The meg­amouth's fil­ter-feed­ing adap­ta­tions likely evolved in­de­pen­dently from other ex­tant fil­ter-feed­ing sharks, even the lam­ni­form bask­ing shark, mak­ing it an ex­am­ple of con­ver­gent evo­lu­tion.

Behavior
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 around 120–160 m (390–520 ft), but as the sun set, it would as­cend and spend the night at depths be­tween 12 and 25 m (39 and 82 ft). Both day and night, its progress was very slow, around 1.5–2.1 km/h (0.93–1.30 mph). This pat­tern of ver­ti­cal mi­gra­tion is seen in many ma­rine an­i­mals as they track the move­ment of plank­ton in the water col­umn. The shark cap­tured in March 2009 was re­port­edly net­ted at a depth of 200 m (660 ft).

Reproduction
Re­pro­duc­tion is ovo­vi­vip­a­rous, mean­ing that the young sharks de­velop in eggs that re­main within the mother's body until they hatch. Tis­sue sam­ples were ob­tained from twenty-seven meg­amouths caught in a two-year pe­riod off the Hualien coast (east­ern Tai­wan), and two caught in Baja Cal­i­for­nia, Mex­ico, and sam­ples taken from Gen­Bank in order to per­form a pop­u­la­tion ge­netic analy­ses of the meg­amouth shark; the re­sults in­di­cated no ge­netic di­ver­sity be­tween pop­u­la­tions found in dif­fer­ent ge­o­graph­i­cal lo­ca­tions, which in­di­cates the species forms a sin­gle, highly mi­gra­tory, in­ter­breed­ing population.

Discovery
The first meg­amouth shark 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. The species was iden­ti­fied as being of a new genus within the plank­tiv­o­rous shark species. 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. The pec­toral fin of the meg­amouth shark was stud­ied, along with the skele­tal and mus­cu­lar sys­tem of the meg­amouth shark to show its phy­lo­ge­netic re­la­tion­ship to the other two sharks.

Known specimens
Main article: List of megamouth shark specimens and sightings

As of 5 March 2018, only 99 meg­amouth spec­i­mens had been caught or sighted. 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, Puerto Rico, Ecuador, and pos­si­bly Viet­nam.

On March 30, 2009, off Burias Is­land in the Philip­pines, an 880- to 1,100-pound (400- to 500-kg) 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.

On June 12, 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 for Scripps In­sti­tu­tion of Oceanog­ra­phy and Mex­i­can re­searchers to study the struc­ture of its mus­cles and gills.

On May 7, 2014, a 4 m (13 ft), 680 kg (1,500 lb) fe­male shark was cap­tured at a depth of 800 m (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.

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, mounted along with the jaw, is on dis­play at the D' Bone Col­lec­tor Mu­seum in Davao.

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.

On April 14, 2016, a 5 m (16 ft) de­ceased meg­amouth shark was caught in a fish­er­man's net in Japan's Mie Pre­fec­ture, around three miles off Owase Port. The shark weighed an es­ti­mated 907 kg (2,000 lb). In Au­gust 2016 the shark was dis­sected be­fore an au­di­ence of stu­dents and academics. The event was or­ga­nized by Osaka Aquar­ium in Osaka, Japan.

On May 1, 2017, a nearly 3 m (9.8 ft) de­ceased meg­amouth shark washed up at Barangay Balu­arte in the Mis­amis Ori­en­tal, a re­gion in the North­ern Min­danao of the Philip­pines. The es­ti­mated 600 kg (1,300 lb) shark was buried to pre­vent lo­cals from con­sum­ing the fish, as au­thor­i­ties were un­sure of the cause of death.

On May 22, 2017, a live meg­amouth shark was found in a fish­ing net off Sunosaki light­house in Tateyama, Chiba Pre­fec­ture, Japan. The es­ti­mated 5 m (16 ft) to 6 m (20 ft) fe­male was filmed by an Asahi Shim­bun pho­tog­ra­pher and ex­am­ined by a TV celebrity ma­rine bi­ol­o­gist who calls him­self Sakana-kun, or Fish Kid. The shark died the fol­low­ing day.

As of July 2017, the pre­served and par­tially dis­sected head of a meg­amouth shark was on dis­play at Osaka Aquar­ium in Osaka, Japan.

On July 25, 2017, a meg­amouth shark was recorded on video at Gili Lawa Laut near Ko­modo is­land, In­done­sia, as it swam slowly past divers Penny Bielich and Heikki In­na­nen. The shark ap­peared to have at least one remora at­tached.

On Feb­ru­ary 11, 2018, a ju­ve­nile male mea­sur­ing 4.34 me­ters in length was ac­ci­den­tally en­tan­gled in a fish­ing net off the coast of Ne­gros Ori­en­tal in the Philip­pines. The body of the de­ceased shark was towed back and buried at Barangay Vil­lareal later that af­ter­noon. It was ex­humed the fol­low­ing day so sci­en­tists could con­duct a necropsy and get tis­sue sam­ples for research.