''Rhynchactis'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

deep-sea The deep sea is broadly defined as the ocean depth where light begins to fade, at an approximate depth of or the point of transition from continental shelves to continental slopes. Conditions within the deep sea are a combination of low tempe ...

anglerfish The anglerfish are ray-finned fish in the order Lophiiformes (). Both the order's common name, common and scientific name comes from the characteristic mode of predation, in which a modified dorsal Fish fin#Ray-fins, fin ray acts as a Aggressiv ...

in the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

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 fish anatomy, their anatomies, for data used in many areas of ichthyology, including Taxonomy (biology), taxonomy and fishery biology. Overall length Standard length (SL) is ...

(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 of the

dorsal fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...

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

bioluminescent Bioluminescence is the emission of light during a chemiluminescence reaction by living organisms. Bioluminescence occurs in multifarious organisms ranging from marine vertebrates and invertebrates, as well as in some fungi, microorganisms inc ...

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-physiology ...

, 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 sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

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 reproduce by depositing fertilized zygotes outside the body (i.e., by laying or spawning) in metabolically independent incubation organs known as eggs, which nurture the embryo into moving offsprings kno ...

, with the

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

e having a rounded shape and enlarged

pectoral fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish aquatic locomotion, swim. Apart from the tail or caudal fin, fish fins have no direct connection with the vertebral column ...

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, 35,800 species of fish had been described as of March 2 ...

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 (biology), taxonomy, binomial nomenclature ("two-term naming system"), also called binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin gramm ...

comes from the Greek ''leptos'' ("thin") and ''nema'' ("thread"). Regan based his account on a single

metamorphosed Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

female collected by the research vessel ''Dana'' in the western central

Atlantic The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for se ...

. 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 designed to further the progress of science by disseminating new research findings to the scientific community. These journals serve as a platform for researchers, schola ...

''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 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, 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 geographic coordinate system, geographic 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 t ...

s of 32°N and 12°S in all three major

ocean The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as ''oceans'' (the Pacific, Atlantic, Indian Ocean, Indian, Southern Ocean ...

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 ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...

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

Bermuda Bermuda is a British Overseas Territories, British Overseas Territory in the Atlantic Ocean, North Atlantic Ocean. The closest land outside the territory is in the American state of North Carolina, about to the west-northwest. Bermuda is an ...

and

Madagascar Madagascar, officially the Republic of Madagascar, is an island country that includes the island of Madagascar and numerous smaller peripheral islands. Lying off the southeastern coast of Africa, it is the world's List of islands by area, f ...

, 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 approximately 20% of the water area of Earth#Surface, Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia (continent), ...

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

Description

Like in the related genus '' Gigantactis'',

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 A gland is a Cell (biology), cell or an Organ (biology), organ in an animal's body that produces and secretes different substances that the organism needs, either into the bloodstream or into a body cavity or outer surface. A gland may also funct ...

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

skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

are reduced or absent. The opercle (gill covering) is also reduced in size, with a forked posterior margin. There are 17–18

fin ray Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only b ...

s in each

, 3–4 (rarely 5) in the

, and 3–4 in the

anal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported o ...

. The

pelvic fin Pelvic fins or ventral fins are paired fins located on the ventral (belly) surface of fish, and are the lower of the only two sets of paired fins (the other being the laterally positioned pectoral fins). The pelvic fins are homologous to the hi ...

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

caudal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...

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 the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

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 Marine fish genera Taxa named by Charles Tate Regan Gigantactinidae