Megamouth shark

The megamouth shark (Megachasma pelagios) is a species of deepwater shark. Rarely seen by humans, it measures around 17 ft (5.2 m) long and is the smallest of the three extant filter-feeding sharks alongside the relatively larger whale shark and basking shark. Since its discovery in 1976, fewer than 100 specimens have been observed or caught.[2] Like the other two planktivorous sharks, it swims with its mouth wide open, filtering water for plankton and jellyfish. It is recognizable from its large head with rubbery lips. The megamouth is so unlike any other type of shark that it is usually considered to be the sole extant species in the family Megachasmidae, though some scientists have suggested it may belong in the family Cetorhinidae.[3]

Megamouth shark
Display at Toba Aquarium
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Order: Lamniformes
Family: Megachasmidae
Genus: Megachasma
Species:
M. pelagios
Binomial name
Megachasma pelagios
Range of megamouth shark (in blue)

Description
Frontal view of a preserved specimen at Marine World Uminonakamichi
Individual captured off the coast of Japan

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.[4] 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.[5][6][7] Mature males average at 4 m (13 ft) and females at 5 m (16 ft). Weights of up to 1,215 kg (2,679 lb) have been reported.[8] A 2019 study suggested that it would have reached 6 m (20 ft) in maximum length.[9] Megamouth sharks can be found as far northward as northern Japan; southern California (LACM 43745-1) and near Punta Eugenia, Baja California, and Hawaii. Megamouth sharks can be found at a depth of up to 1,000 m (3,280 ft). Megamouth sharks are dark blue, brownish-black, or gray above, lighter below; with a white band along the upper jaw; while the posterior margin of its fins are white.

As their name implies, megamouths have a large mouth with small teeth, and a broad, rounded snout, causing observers to occasionally mistake the 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 was 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.[5] In 2020, a study concluded that this species of shark does not in fact produce any light; the white band was found to merely be highly reflective of light.[10] 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.[4] Their mouths can reach up to 1.3 m (4 ft 3 in) in width. Megamouth sharks have up to 50 rows of teeth in their upper-jaw and up to 75 rows of teeth in their lower-jaw.[11]

Taxonomy and evolution

Researchers have predicted the feeding patterns of megamouth sharks in relation to the other two planktivorous sharks; the three planktivorous 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.[12] In addition to the living M. pelagios, however, two extinct megamouth species – the Priabonian M. alisonae[13] and the Oligocene–Miocene M. applegatei – have also recently been proposed on the basis of fossilized tooth remains.[14][15] An early ancestor of the recent species Megachasma pelagios was reported from the early Miocene (Burdigalian) of Belgium.[16] 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.[17] 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.[18]

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.[19] 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.[5] 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 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.[2]

Discovery

The first megamouth shark was captured on November 15, 1976, about 25 miles northeast of Kahuku, Hawaii, when it became entangled in the sea anchor of United States Navy ship AFB-14 at a depth of about 165 m (541 ft) in water about 4,600 m (15,092 ft) deep.[20] The species was identified as being of a new genus within the planktivorous shark species.[21] 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.[22] The pectoral fin of the megamouth shark was studied, along with its skeletal and muscular system, to show its phylogenetic relationship to the other two sharks.[21]

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

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,[2] and possibly Vietnam.

See also
For a topical guide, see Outline of sharks.

References
  1. Kyne, P.M., Liu, K.M. & Simpfendorfer, C. (2019). Megachasma pelagios. The IUCN Red List of Threatened Species 2019. doi:10.2305/IUCN.UK.2019-1.RLTS.T39338A124402302.en
  2. Liu, Shang-Yin 'Vanson'; Joung, Shoou Jeng; Yu, Chi-Ju; Hsu, Hua-Hsun; Tsai, Wen-Pei; Liu, Kwang Ming (March 5, 2018). "Genetic diversity and connectivity of the megamouth shark". PeerJ. 6: e4432. doi:10.7717/peerj.4432. PMC 5842762. PMID 29527411.
  3. C. Knickle; L. Billingsley & K. DiVittorio. "Biological profiles: basking shark". Florida Museum of Natural History. Archived from the original on August 21, 2006. Retrieved December 16, 2014.
  4. "Megachasma pelagios :: Florida Museum of Natural History". www.floridamuseum.ufl.edu. May 10, 2017. Retrieved July 25, 2017.
  5. Compagno, Leonard J. V. (2002). Sharks of the world: an annotated and illustrated catalogue of shark species known to date (PDF). Rome: Food and Agriculture Organization of the United Nations. pp. 74–78. ISBN 978-9251045435.
  6. Swaminathan, Nikhil (April 9, 2009). "What is a megamouth shark? Is it still a scientific mystery?". Scientific American.
  7. "Deep Sea: Megamouth Shark". www.elasmo-research.org. Retrieved July 25, 2017.
  8. "Megamouth shark washes up in Philippines and is only the 60th we've ever seen". wordpress.com. January 28, 2015. Retrieved April 3, 2018.
  9. Pimiento, C.; Cantalapiedra, J.L.; Shimada, K.; Field, D.J.; Smaers, J.B. (2019). "Evolutionary pathways toward gigantism in sharks and rays" (PDF). Evolution. 73 (3): 588–599. doi:10.1111/evo.13680. ISSN 1558-5646. PMID 30675721. S2CID 59224442.
  10. Duchatelet, Laurent; Moris, Victoria C.; Tomita, Taketeru; Mahillon, Jacques; Sato, Keiichi; Behets, Catherine; Mallefet, Jérôme (2020). "The megamouth shark, Megachasma pelagios, is not a luminous species". PLOS ONE. 15 (11): e0242196. Bibcode:2020PLoSO..1542196D. doi:10.1371/journal.pone.0242196. PMC 7688146. PMID 33237911.
  11. Parker, Steve (2008). The Encyclopedia of Sharks. Firefly. p. 54. ISBN 978-1-55407-409-9.
  12. Tomita, Taketeru; Sato, Keiichi; Suda, Kenta; Kawauchi, Junro; Nakaya, Kazuhiro (May 4, 2011). "Feeding of the megamouth shark (Pisces: Lamniformes: Megachasmidae) predicted by its hyoid arch: a biomechanical approach". Journal of Morphology. 272 (5): 513–524. doi:10.1002/jmor.10905. PMID 21381075. S2CID 25286745.
  13. Shimada, Kenshu; Ward, David (2016). "The oldest fossil record of the megamouth shark from the late Eocene of Denmark, and comments on the enigmatic megachasmid origin". Acta Palaeontologica Polonica. 61. doi:10.4202/app.00248.2016.
  14. Gallardo, Christopher; Shimada, Kenshu; Schumacher, Bruce A. (2013). "A New Late Cretaceous Marine Vertebrate Assemblage from the Lincoln Limestone Member of the Greenhorn Limestone in Southeastern Colorado". Transactions of the Kansas Academy of Science. 115 (3–4): 107–116. doi:10.1660/062.115.0303. S2CID 86258761.
  15. Shimada, Kenshu; Welton, Bruce J.; Long, Douglas J. (2014). "A new fossil megamouth shark (Lamniformes, Megachasmidae) from the Oligocene-Miocene of the western United States". Journal of Vertebrate Paleontology. 34 (2): 281–290. doi:10.1080/02724634.2013.803975. S2CID 83949683.
  16. De Schutter, Pieter J.; Everaert, Stijn (2020). "A megamouth shark (Lamniformes: Megachasmidae) in the Burdigalian of Belgium". Geologica Belgica. 23 (3–4): 157–165. doi:10.20341/gb.2020.001.
  17. Shimada, Kenshu; Popov, Evgeny V.; Siversson, Mikael; Welton, Bruce J.; Long, Douglas J. (September 3, 2015). "A new clade of putative plankton-feeding sharks from the Upper Cretaceous of Russia and the United States". Journal of Vertebrate Paleontology. 35 (5): e981335. doi:10.1080/02724634.2015.981335. S2CID 83793715.
  18. Mitchell, Michaela; Ciampaglio, Charles; Jacquemin, S.J. (November 1, 2018). "Convergent evolution in tooth morphology of filter-feeding lamniform sharks". Southeastern Geology. 53: 63–80.
  19. Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
  20. Leighton R. Taylor; L. J. V. Compagno & Paul J. Struhsaker (1983). "Megamouth – a new species, genus, and family of lamnoid shark (Megachasma pelagios, family Megachasmidae) from the Hawaiian Islands". Proceedings of the California Academy of Sciences. 43 (8): 87–110.
  21. Tomita, Taketeru; Tanaka, Sho; Sato, Keiichi; Nakaya, Kazuhiro (January 21, 2014). "Pectoral Fin of the Megamouth Shark: Skeletal and Muscular Systems, Skin Histology, and Functional Morphology". PLOS ONE. 9 (1): e86205. Bibcode:2014PLoSO...986205T. doi:10.1371/journal.pone.0086205. PMC 3897653. PMID 24465959.
  22. Berra, Tim M. (1997). "Some 20th century fish discoveries". Environmental Biology of Fishes. 50 (1): 1–12. doi:10.1023/A:1007354702142. S2CID 23665964.
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