Regalecus russelii

Regalecus russelii, or Russell's oarfish, is a species of oarfish in the family Regalecidae.[1] It is a broadly-distributed marine fish, found in waters in the bathypelagic zone.[2] R. russelii is a scaleless, elongate and ribbonlike fish, growing up to 8 meters in length.[2]

Regalecus russelii
Taxidermied specimen of R. russelii in Academia Sinica, Taipei
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Actinopterygii
Order:
Lampriformes
Family:
Regalecidae
Genus:
Regalecus
Species:
R. russelii
Binomial name
Regalecus russelii
Cuvier, 1816

Taxonomy

Regalecus russelii is a member of the fish genus Regalecus and oarfish family Regalecidae. The genus currently includes only one other species of oarfish, Regalecus glesne. [2][3] R. russelii is part of the order Lampriformes, which represents tube-eyes and ribbonfishes, and is part of the larger class of ray-finned fishes called Actinopterygii.[2]

Anatomy and morphology

Regalecus russelii can grow up to 8 meters in length, and it has two dorsal fin crests that can reach 1 meter high.[2] The species can be distinguished by its red dorsal fin crests, light brown head, and scaleless, silver body.[2][3] The body is covered by dermal tubercles, which are concentrated along the ventral and lower side regions of the body.[3] Each dorsal fin has 333 to 371 rays, which shorten and then become longer at the middle of the body.[3] [2] Its pelvic fin contains more than three membranous appendages and is a single elongated ray.[2] The stomach has a long caecum, beginning posterior of the abdomen and extending to the end of the body.[2] Muscle is contained in intermuscular septa, with dorsal and ventral septa along with teleost's characteristic horizontal, vertical, and transverse septa.[2] Older R. russelii often have a posterior stump-like tail, which is a consequence of self-amputation but show no signs of regeneration.[2] In some specimens, this tail appears jagged and unhealed, consistent with a fresh self-amputation.[2]

The majority of these fish have toothless jaws, but small vestigial teeth have been observed in some.[2] There are 113 to 122 vertebrae present in the spine.[2] The skeleton contains distinct areas of hyperostosis, or hyper-ossified bones, that are most prominent on the dorsal pterygiophores, but is also present on the cleithrum and along the supraoccipital bone.[4] The rest of the skeleton is cartilaginous.[4] The hyperostosis provides additional support to the pterygiophores during movement.[4] The presence of hyperostosis varies among R. russelii of different ages and sizes; it is present in most large adult fish, and many smaller fish lack these regions of swollen bones.[4] R. russelii's flaccid skeleton lacks mineralization, which is advantageous in maintaining buoyancy in deep waters.[4]

One of the few biological structures that has been studied is the otolith, which is a structure in the inner ear that is involved in sensing movement and gravity.[5] R. russelii have very small sagittal otoliths that are difficult to observe; their small size may indicate that they play an insignificant role in sensing.[5] Researchers have been able to perform CT scans and rare, invasive studies on this structure of a deceased R. russelii to better understand its physiological significance.[5]

Distribution

Regalecus russelii lives in deep waters near areas such as Japan, California, and Baja California, in waters such as the Pacific, Atlantic, and Indian oceans.[2] Since 1901, there have been 19 verified sightings and strandings along the coast of California waters.[6] The R. russelii is found around the world equatorially, while the Regalecus glesne is found with antitropical distribution.[7] The lack of live sightings of oarfish has made it difficult to determine the precise distribution of the Regalecus, and further research is needed.

Feeding and diet

The species uses a feeding stance to see the silhouette of its prey. It feeds on euphausiid crustaceans, small fishes and squid, and uses its protrusile jaws to suck in prey.[8] The oarfish mostly consumes a diet of krill[9] as its energy source, using its jaw to fill its oro-branchial cavity with the crustaceans, that will then be held in the gullet and passed through.[10]

Conservation

There are no specific conservation measures for R. russelii, and it occurs in at least one marine protected area. It has been listed as 'Least concern' by the IUCN Red List.[8]

Mythology

Two deep sea oarfish were caught live in nets off the coast of Okinawa, Japan January 28, 2019. Both died before making their way into an aquarium inland in the town Motobu. One fisher described the two oarfish squirming in the nets as looking like "real dragons."[11] A belief surrounds the species that they are "harbingers of earthquakes", but there is no scientific evidence or an association established between recorded oarfish sightings and earthquakes.[6] Spread through Japanese myth, the oarfish has been described as "messenger from the Sea God's Palace." Kiyoshi Wadatsumi, an expert in ecological seismology and director of e-PISCO, an organization which studies earthquakes, states that deep sea fish are more sensitive to the tectonic movements or tremors from active faults than fish closer to the surface of the ocean.[12]

Parasites
  • Gymnorhadinorhynchus mariserpentis Steinauer, Garcia-Vedrenne, Weinstein & Kuris, 2019
  • Spinitectus gabata Poinar, Weinstein, Garcia-Vedrenne & Kuris, 2014[1]

Despite the relatively limited number of oarfish researched, in 2014 scientists were able to discover a new species of nematode called Spinitectus gabata n. sp. (Spirurina: Cystidicolidae) within the gastrointestinal tract of a R. russelii off the coast of Japan. The S. gabata likely uses the krill the oarfish consumes as an intermediate host, since known Spinitectus species often involve crustaceans as intermediate hosts.[13]

References
  1. "WoRMS - World Register of Marine Species - Regalecus russelii (Cuvier, 1816)". www.marinespecies.org. Retrieved 2019-03-28.
  2. "Regalecus russelii summary page". FishBase. Retrieved 2019-03-28.
  3. Angulo, A.; López-Sánchez, M. I. (2017). "New records of lampriform fishes (Teleostei: Lampriformes) from the Pacific coast of lower Central America, with comments on the diversity, taxonomy and distribution of the Lampriformes in the eastern Pacific Ocean". Zootaxa. 4236 (3): 573–591. doi:10.11646/zootaxa.4236.3.11. PMID 28264321.
  4. Paig-Tran, E.W.; Barrios, A.S.; Ferry, L.A. (2016). "Presence of repeating hyperostotic bones in dorsal pterygiophores of the oarfish, Regalecus russellii". Journal of Anatomy. 229 (4): 560–7. doi:10.1111/joa.12503. PMC 5013060. PMID 27296623.
  5. Midway, S.R.; Wagner, T. (2015). "The first description of oarfish (Regalecus russellii Cuvier 1816) (Regalecidae) ageing structures". Journal of Applied Icthyology. 32 (1): 113–116. doi:10.1111/jai.12967.
  6. Feeney, Richard F.; Lea, Robert N. (December 2018). "California Records of the Oarfish,Regalecus russelii(Cuvier, 1816) (Actinopterygii: Regalecidae)". Bulletin, Southern California Academy of Sciences. 117 (3): 169–179. doi:10.3160/3294.1. ISSN 0038-3872. S2CID 91283288.
  7. Chakrabarty, Prosanta (2013). "Systematics, Biology, and Distribution of the Species of the Oceanic Oarfish Genus Regalecus (Teleostei, Lampridiformes, Regalecidae)". Copeia. 2013 (2): 336. doi:10.1643/OT-12-165. ISSN 0045-8511. JSTOR 24635733. S2CID 85635714.
  8. "The IUCN Red List of Threatened Species". IUCN Red List of Threatened Species. Retrieved 2019-03-28.
  9. Poinar, George; Weinstein, Sara; Garcia-Vedrenne, Ana; Kuris, Armand (2014). "First description of a nematode, Spinitectus gabata n. sp. (Spirurina: Cystidicolidae), from the deep sea oarfish, Regalecus russelii (Regalecidae) in Japan". International Journal of Nematology. 24 (2): 117–123.
  10. "Regalicus russelii". Retrieved 2020-03-11.
  11. "Okinawan fishermen reel in two live oarfish, eat portion of one". The Japan Times Online. 2019-02-17. ISSN 0447-5763. Retrieved 2020-03-11.
  12. Yamamoto, Daiki (2010-03-06). "Sea serpents' arrival puzzling, or portentous?". The Japan Times Online. ISSN 0447-5763. Retrieved 2020-03-11.
  13. Poinar, George; Weinstein, Sara; Garcia-Vedrenne, Ana; Kuris, Armand (2014). "First description of a nematode, Spinitectus gabata n. sp. (Spirurina: Cystidicolidae), from the deep sea oarfish, Regalecus russelii (Regalecidae) in Japan". International Journal of Nematology. 24 (2): 117–123.
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