''Rhynchactis'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of deep-sea

anglerfish The anglerfish are fish of the teleost order Lophiiformes (). They are bony fish named for their characteristic mode of predation, in which a modified luminescent fin ray (the esca or illicium) acts as a lure for other fish. The luminescence ...

in the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

Gigantactinidae, containing three species found worldwide at depths greater than . Adult female ''Rhynchactis'' reach a

standard length Fish measurement is the measuring of individual fish and various parts of their anatomies. These data are used in many areas of ichthyology, including taxonomy and fisheries biology. Overall length * Standard length (SL) is the length of a fish m ...

(SL) of and have a dark-colored, streamlined body and a relatively small head bearing a very long illicium (the "fishing rod" formed by the first

ray Ray may refer to: Fish * Ray (fish), any cartilaginous fish of the superorder Batoidea * Ray (fish fin anatomy), a bony or horny spine on a fin Science and mathematics * Ray (geometry), half of a line proceeding from an initial point * Ray (g ...

of the

dorsal fin A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through conv ...

). Unlike almost all other deep-sea anglerfishes, the illicium bears no

bioluminescent Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some Fungus, fungi, microorganisms including ...

esca (the "lure") at the tip. The mouth is almost devoid of teeth, and the inside of both jaws are covered by numerous white glands that are unique to this genus. The lack of an esca, greatly reduced

dentition Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, it is the characteristic arrangement, kind, and number of teeth in a given species at a given age. That is, the number, type, and morpho-physiolo ...

, and glands inside the mouth all point to ''Rhynchactis'' having a highly specialized mode of feeding, the nature of which has yet to be deciphered. As in other deep-sea anglerfishes, there is enormous

sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...

with males being much smaller than females and lacking an illicium, though they do not appear to be parasitic as in some families. Reproduction is

oviparous Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, most reptiles, and all pterosaurs, dinosaurs (including birds), and ...

, with the

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...

e having a rounded shape and enlarged

pectoral fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as ...

Taxonomy and phylogeny

British

ichthyologist Ichthyology is the branch of zoology devoted to the study of fish, including bony fish ( Osteichthyes), cartilaginous fish (Chondrichthyes), and jawless fish (Agnatha). According to FishBase, 33,400 species of fish had been described as of Octobe ...

coined the genus ''Rhynchactis'' and described its first species, ''R. leptonema'', in a 1925 issue of ''Annals and Magazine of Natural History''.Eschmeyer, W. N. (ed.
''Rhynchactis''
Catalog of Fishes electronic version (February 19, 2010). Retrieved on March 29, 2010. The generic name comes from the Greek ''rhynchos'' ("snout") and ''aktis'' ("ray"), while the

specific epithet In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...

comes from the Greek ''leptos'' ("thin") and ''nema'' ("thread"). Regan based his account on a single metamorphosed female collected by the research vessel ''Dana'' in the western central

Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe an ...

. In 1998, Erik Bertelsen and Theodore Pietsch published a revision of the genus in the

scientific journal In academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually by reporting new research. Content Articles in scientific journals are mostly written by active scientists such as s ...

''Copeia'' and recognized two new species: ''R. macrothrix'' and ''R. microthrix''. These two specific epithets are derived from the Greek ''makros'' ("long"), ''mikros'' ("small"), and ''thrix'' ("hairs"), both referring to characteristics of the illicium. ''Rhynchatis'' is the more

derived Derive may refer to: * Derive (computer algebra system), a commercial system made by Texas Instruments * ''Dérive'' (magazine), an Austrian science magazine on urbanism *Dérive, a psychogeographical concept See also * *Derivation (disambiguatio ...

genus in its family, exhibiting the secondary reduction of many morphological elements such as bones, fin rays, and teeth.Pietsch, T.W. (2005)
Gigantactinidae. Whipnose Seadevils
The Tree of Life Web Project. Retrieved on March 29, 1010.

Species

There are currently three recognized species in this genus: * '' Rhynchactis leptonema''

Regan The family name Regan, along with its cognates O'Regan, O Regan, Reagan, and O'Reagan, is an Anglicized form of the Irish surname Ó Riagáin or Ó Ríogáin, from Ua Riagáin. The meaning is likely to have originated in ancient Gaelic ''ri'' ...

, 1925 * '' Rhynchactis macrothrix'' Bertelsen & Pietsch, 1998 * '' Rhynchactis microthrix'' Bertelsen & Pietsch, 1998

Distribution and habitat

''Rhynchactis'' species have been caught between the

latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...

s of 32°N and 12°S in all three major

ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...

s. Of the specimens that could be identified to species, the two ''R. leptonema'' were caught in the western central Atlantic and from off

Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...

, ''R. macrothrix'' has been collected from near

Bermuda ) , anthem = "God Save the King" , song_type = National song , song = " Hail to Bermuda" , image_map = , map_caption = , image_map2 = , mapsize2 = , map_caption2 = , subdivision_type = Sovereign state , subdivision_name = , e ...

and

Madagascar Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Africa ...

, and the one ''R. microthrix'' was caught in the western

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by th ...

. This genus occurs deeper than , to at least down.

Description

Like in the related genus ''

Gigantactis ''Gigantactis'' is a genus of deep-sea fish of the family Gigantactinidae, first described in 1902 by August Brauer. The species in this genus are poorly known and found in all oceans, at depths of . The most striking feature of these fish is ex ...

'', metamorphosed female ''Rhynchactis'' are slender and streamlined, with a relatively small head comprising about a quarter of the standard length. The illicium originates at the front of the blunt head and exceeds the length of the body. In contrast to most other deep-sea anglerfishes the illicium does not bear a

esca at the tip. The upper jaw extends past the lower; the

is greatly reduced, particularly in larger females, consisting of only 0–2 tiny, curved teeth on each side at the tip of the upper jaw, and sometimes also fine teeth in the lower jaw. Uniquely, the inside of both jaws are densely covered by white, papilla-like

gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). Structure De ...

s, each containing a short, pigmented tube at the center lined by glandular cells. Compared to ''Gigantactis'', several bones in the

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...

are reduced or absent. The

opercle The operculum is a series of bones found in bony fish and chimaeras that serves as a facial support structure and a protective covering for the gills; it is also used for respiration and feeding. Anatomy The opercular series contains four bon ...

(gill covering) is also reduced in size, with a forked posterior margin. There are 17–18 fin rays in each

, 3–4 (rarely 5) in the

, and 3–4 in the

anal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

. The

pelvic fin Pelvic fins or ventral fins are paired fins located on the ventral surface of fish. The paired pelvic fins are homologous to the hindlimbs of tetrapods. Structure and function Structure In actinopterygians, the pelvic fin consists of two en ...

s are absent, though unlike in ''Gigantactis'' the bones at the base remain. The

caudal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

is slightly forked, with 4 rays in the upper half and 5 rays in the lower half. The second and seventh rays are longest, measuring 34–45% of the standard length. The skin is dark brown to black, smooth in small fish and becoming densely covered by tiny spines in females over long. As in other deep-sea anglerfishes, the adult male is much smaller than the female and has no illicium; the eyes are minute while the olfactory organs are highly developed, measuring 8–10% of the standard length. The species differ from each other as follows: *''R. leptonema'' The illicium of this species measures 143–158% of the standard length, and entirely lacks filaments. The larger of the two specimens is long (SL). *''R. macrothrix'' The illicium of this species measures 109–144% of the standard length. There are 11–20 relatively long filaments along the distal 28–57% of its length, which are darkly pigmented in the largest fish and may be forked and/or have a tiny swelling at the tip. There are also 3–4 short, unpigmented filaments at the tip of the illicium. The largest known female is long (SL). *''R. microthrix'' The illicium of this species measures 210% of the standard length, bearing 19 short filaments along the distal 14% of its length and 4 tiny filaments at the tip. The single known specimen is long (SL).

Biology and ecology

Without a luminous esca to attract prey or large teeth to secure it, ''Rhynchactis'' seems to have

evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation t ...

a completely novel, and as yet unknown, mode of feeding. Like other anglerfishes, it is

with a distinct

l stage. The larvae have short, stout bodies and highly inflated skin that give them an almost spherical shape; their pectoral fins are very large, measuring about half as long as the standard length. There is no evidence that the males become permanently parasitic on the females.

References

{{Taxonbar, from=Q3189561 Lophiiformes Marine fish genera Taxa named by Charles Tate Regan Gigantactinidae