Anoplogaster: Characteristics, Habitat, Feeding

Anoplogaster is a genus ofray-finnedmeso- and bathy pelagic fish (Actinopterigii) that inhabits tropical and subtropical waters around the world. The fish of this genus are small in size, with narrow eyes and huge teeth.

The name comes from the Greek ‘ἀν’ = sin, ‘ oplon ‘ = weapon, and ‘ gaster ‘ = stomach, and refers to the absence of scales in the form of shields or armor in the gastric region of these fish. This characteristic is opposed, for example, to that of the Trachichthyidae fish, which do have gastric armor.

Anoplogaster cornuta, seen under X-rays. Taken and edited from Sandra Raredon / Smithsonian Institution [Public domain].

The genus comprises only two species; the first described in 1833 and the second more than 150 years later, in 1986. These are fish that are of no commercial importance and are rarely caught accidentally.

They are commonly known by various names, including long-fanged fish, ogre fish, and saber-toothed fish. These fish, despite being able to live in deep waters, are unable to generate light, so the true usefulness of their eyes is unknown.

Article index

  • one

    Characteristics

  • two

    Taxonomy and classification

  • 3

    Habitat and distribution

  • 4

    Feeding

  • 5

    Reproduction

  • 6

    References

Characteristics

Among the characteristics that describe this genus of abyssal fish are:

– Small fish, not exceeding 16 cm, with a large head provided with very deep mucous cavities, which are separated by elevations armed with small spines like saws.

– The mouth is proportionally large compared to the size of the body, and it is armed with long fangs, which are curved inwards to better catch their prey. Additionally, these fangs are so long that they must lodge in cavities in the mouth when it is closed.

– When the organisms are in the larval stage, they have eyes that are larger than the snout, however, when they reach adulthood, the proportion is reversed, as the eyes become very small.

– The fins are simple, they do not have spines, only rays. The dorsal is elongated at the base, while the scales are small, cup-shaped, embedded in the skin and not overlapping each other.

– The lateral line is only partially covered with scales, and appears as a well-defined groove on the sides of the body.

Taxonomy and classification

Anoplogaster is the only genus of ray-finned fish in the family Anaplogastridae, of the order Beryciformes. The first species of this genus was first described by the French zoologist Achille Valenciennes in 1833, under the name Hoplostethus cornutus.

Later, in 1859, the German zoologist Albrecht Carl Ludwig Gotthilf Günther erected the taxon Anoplogaster as a monospecific genus to house the species described by Valenciennes, which was renamed Anoplogaster cornuta .

That was the only species in the genus until 1986, when a careful review of the fish in the genus allowed Aleksandr Kotlyar to define a new species, which he named Anoplogaster brachycera. To date, these remain the only valid species of the genus.

Juveniles and adults, as well as males and females, present certain differences between them. Due to this, the Caulolepsis longidens species which was erected based on adult specimens, must have been considered invalid when it was discovered that it was the same Anoplogaster cornuta.

To date, the adult forms of Anoplogaster brachycera are unknown .  The description of the species was made only based on juvenile specimens, so it is not ruled out that some other species described is actually the adult form of this species.

Drawing of Anoplogaster brachycera. Taken and edited from https://hr.wikipedia.org/wiki/Datoteka:Anoplogaster_brachyceraZ.gif#filelinks.

Habitat and distribution

The species of the genus Anoplogaster are pelagic, that is, they are found in the water column. Although they are sometimes defined as abyssal fish, they can inhabit a wide range of depths, which can go from 500 to 5,000 meters deep, in temperate and tropical zones.

The species A. cornuta is pan-oceanic, that is, they are found in all seas, while A. brachycera is limited to the tropical waters of the Atlantic and Pacific.

There are also differences in the bathymetric distribution of both species.  Anoplogaster brachycera  is known for relatively shallow waters, reaching only up to 1,500 m depth, while A. cornuta  has been recorded up to 3,200 m in juvenile stages and up to 5,000 meters in adult stages.

Feeding

Fish of the genus Anoplogaster are predatory carnivores. These organisms vary their diet throughout their development, feeding on crustaceans in their juvenile stages and on fish in their adult stage.

The small eyes of these fish, according to some authors, help to confuse their prey and catch them by surprise, while the long fangs, curved inwards, help to catch them and prevent them from escaping.

Some authors suggest that they are aggressive predators, as they have high mobility compared to other abyssal species. However, other authors believe that they capture their prey using the surprise factor (they are ambushers).

The large mouth, which extends far behind the eyes, allows Anoplogaster to ingest large fish, even up to a third of its size. To do this, they are also helped by distending the operculum, and by separating the branchial arches.

Due to this form of feeding, these fish have had to undergo adaptations to be able to breathe while feeding. These adaptations consist of using the pectoral fins to fan the water over the gills while the gills are distended and the prey not completely ingested.

This form of breathing is called reverse ventilation.

Anoplogaster cornuta. Taken and edited from: © Citron

Reproduction

The reproductive biology of these fish, as in many deep-sea fish, is poorly understood. The Anoplogaster are dioecious fish, ie having separate sexes. They reach sexual maturity at a size close to 13 cm, the females being larger than the males.

It is unknown if they have breeding areas, but it is believed that they do not show reproductive migrations to particular areas.

The species of this genus are oviparous, with external fertilization, producing eggs that hatch into a planktonic larva. This larva will move to shallower waters for its development, less than 50 meters deep. There is no type of parental care in these fish.

References

  1. AN Kotlyar (2003). Family Anoplogastridae Gill 1893. Annotated checklists of fishes. California Academy of Sciences.
  2. R. Froese & D. Pauly, Editors (2019). FishBase. Anoplogaster Günther, 1859. World Register of Marine Species. Recovered from marinespecies.org
  3. J. Davenport (1993). Ventilation of the gills by the pectoral fins in the fangtooth Anoplogaster cornutum: how to breathe with a full mouth. Journal of Fish Biology.
  4. Anoplogaster . Recovered from en.wikipedia.org.
  5. Anoplogaster cornuta. On Animal Diversity Web. Recovered from animaldiversity.org.
  6. R. Paxton (1999). Order Beryciformes. Anoplogastridae. Fangtooths. In KE Carpenter and VH Niem (eds.) FAO species identification guide for fishery purposes. The living marine resources of the WCP. Vol. 4. Bony fishes part 2 (Mugilidae to Carangidae). FAO, Rome.

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