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Anableps anableps

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Anableps anableps
at Caroni Swamp, Trinidad
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Cyprinodontiformes
Family: Anablepidae
Genus: Anableps
Species:
A. anableps
Binomial name
Anableps anableps
Synonyms[2]
  • Cobitis anableps Linnaeus, 1758
  • Anableps anableps Meuschen, 1778
  • Anableps tetrophthalmus Bloch, 1794
  • Anableps surinamensis Lacepède, 1803
  • Anableps gronovii Valenciennes, 1846
  • Anableps lineatus Gronow, 1854

Anableps anableps, the largescale four-eyes,[3] is a species of four-eyed fish found in fresh and brackish waters of northern South America and Trinidad. This species grows to a length of 30 centimetres (12 in) total length (TL). This fish can occasionally be found in the aquarium trade. The fish does not actually have four eyes. Instead, each eye is split into two lobes by a horizontal band of tissue, each lobe with its own pupil and separate vision. This allows the fish to see above and below the surface of the water at the same time.[2] This species has been documented to occasionally feed on land.[3] Both sexes exhibit asymmetrical genitalia.[4]

Taxonomy

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This fish was first described in 1758 by the Swedish naturalist Carl Linnaeus in the tenth edition of his Systema Naturae. Linnaeus gave it the name Cobitis anableps, but it was later transferred to the genus Anableps, becoming Anableps anableps.[5]

Description

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Anableps anableps is a slender elongate fish that can reach 30 cm (12 in) in maximum total length.[2] Juveniles and adult females possess an anal fin, while male fish have the anal fin modified into an intromittent organ with the tip angled to one side. This structure, also called a gonopodium, is used in reproduction. Adult females have 11 anal fin rays, where rays 1–3 and 11 are unbranched.[6] Female fish also have a modified scale called a foriculum covering either the left or right genital opening.[4] Females are larger and much heavier than males on average, reaching 18.5 cm (7.3 in) standard length (SL) and 80 g (2.8 oz) in weight, while males reach 14.5 cm (5.7 in) SL and 31 g (1.1 oz) in weight.[7] The most distinctive feature of this fish is the eyes, which project prominently from the head. Each eye is divided lengthwise and thus has two pupils; the fish positions itself in such a way that the dividing line between the two parts of the eye lies on the surface of the water, so that one pupil has an aerial view while the other is submerged.[8]

Anableps anableps can be distinguished from other species in the genus by the small number and large size of its scales. Another distinguishing characteristic of this species is the presence of thin blue or purple lateral stripes, which extend between the pectoral and pelvic fins. Typically, there are 3 prominent stripes but up to 5 may be present. Females of this species are the only ones in the genus to possess a foriculum.[9]

A side view of A. anableps showing typical morphology.

Distribution and habitat

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Anableps anableps is found in the tropical western Atlantic Ocean and the southern Caribbean Sea. Its range extends from Trinidad and Venezuela to the delta of the Amazon River in Brazil.[8] However, some sources report occurrences as far south as the Bay of All Saints in Brazil, and some occurrences very far inland.[1][10] It mostly occurs in estuaries and on coastal mudflats, where it shares its habitat with Anableps microlepis. Unlike A. microlepis, however, Anableps anableps can survive in freshwater environments for extended periods.[8][11]

Ecology

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A pair of A. anableps near the surface at Newport Aquarium.

Anableps anableps typically stays close to the surface of the water, utilizing its unique eyes to see above and below the surface of the water at the same time.[12] It rarely ever dives below the surface, though it must occasionally submerge its eyes to prevent desiccation.[13] This species forms schools of 10 to 50 individuals.[11]

Anableps anableps has a generalist diet, feeding on insects and other small invertebrates (such as small crabs),[14] small fish, and on algae. A large part of its diet consists of detritus, but the highest diversity of prey items comes from insects.[15] It has occasionally been observed catching insects by jumping in the air,[13] but otherwise, it ingests prey on or near the surface. It also ingests silt, consuming diatoms, mussels, and isopods found among the grains.[8][15] Sometimes it emerges from the water and feeds on mudbanks.[3][15]

In the water, the fish uses a combination of biting and suction to capture its prey. It feeds by depressing the lower jaw and protruding the upper jaw, and ingesting the prey by suction. If the prey is too large to swallow, it is gripped by the teeth, the upper jaw is retracted, and the fish bites the prey. This cycle of protrusion and biting can be repeated until the prey is able to be swallowed.[3] There is also evidence that this fish uses some degree of ram feeding for small prey items.[13]

When the fish beaches itself to feed on land, it uses a different feeding mechanism. It positions its head above the prey, rotating its lower jaw over 180 degrees. It then protrudes its upper jaw downwards to bite down on the prey. This mechanism is similar to the picking behavior observed in other Cyprinodontiforms.[3]

In mangrove areas of Brazil, it has a daily migration pattern synchronized with the tides; as the tide rises it enters the intertidal channels, feeding in the inundated mangroves at high water, and retreating to the main channels as the tide ebbs. Feed consumption was at its greatest around high water during daylight spring tides and was at its minimum at night-time neap tides, perhaps demonstrating the importance of its vision in feeding. Red intertidal algae (Catenella sp.) was the main food item, supplemented by insects and shore crabs (family Grapsidae).[12]

Anableps anableps is affected by the parasitic praniza larvae of isopods in the family Gnathiidae, which feed on its blood by attaching to the gills and skin. Prevalence may vary between populations, though one study reported that 42.3% of individuals in an area were infected.[16] This species is also affected by a species of parasitic copepod, Acusicola rochai, in the family Ergasilidae, with as many as 66% of individuals reported to be infected.[17]

Eyes

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A. anableps with half of its eyes poking out of the water.

Anableps anableps positions itself in the water column so that half of each eye is above water and half is below. It is able to detect tiny Drosophila species floating on the water, and can detect even slight movements above the waterline, making them difficult to catch.[13]

The eyes of Anableps anableps are similar to other species in the genus, with each spherical eye possessing double the amount of several structures, including two pupils, two corneas, and two retinas. The eyes of this fish still retain a single lens and optic nerve, however. The eyes are divided in half laterally by a band of tissue, which is kept at the water level. The iris and tissue band divide the vision by forming an aperture. This prevents two images from being formed due to refraction on the water-air boundary.[18]

In addition to the duplication of many structures, many are also modified. The upper pupil is larger than the lower pupil. The upper cornea is slightly more curved than the lower. The lower side of the retina is thicker and has different cell densities. The lower retina receives light signals from the air, while the upper retina receives signals from the water. Though there is only one lens present, it is asymmetrical, with the lower portion being more rounded.[18]

A diagram showing the eye of the genus Anableps, and how it is positioned in relation to the waterline. (1) Upper retina. (2) Lens. (3) Upper pupil. (4) Tissue band. (5) Iris. (6) Lower pupil. (7) Lower retina. (8) Optic nerve.

As embryos, the eyes of Anableps anableps are typical of other fish species, with a single pupil. As the fish develops, the middle of the pupil constricts laterally, dividing it in half. The eyes increase in size, and the surrounding bones expand upwards, resulting in the eyes protruding above the skull. Cartilage fills in empty space in the top of the skull between the eyes, which later hardens into bone. The two halves of the retina show differences in protein expression during development in the absence of light, suggesting the two halves are genetically independent.[19]

Reproduction

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Like other members of its family, Anableps anableps is viviparous, meaning it has internal fertilization and gives live birth (as opposed to laying eggs). Members of this species can reproduce multiple times throughout the year, though some evidence suggests the later months of the year are preferred.[7][11]

Females may carry between 1–37 eggs, which stay and develop as embryos in the ovarian follicles until the yolk sac is absorbed and the fish are born. The ovaries and follicles change dramatically in both size and structure to accommodate the embryos as they grow, but return to normal after birth.[6] All embryos are fertilized and develop at the same time, and fertilization of new eggs cannot occur until the current brood is born, unlike in A. microlepis.[6][7][11][20]

In males, the rays of the anal fin develop into a long bony structure called a gonopodium, which is used to deposit sperm. The rays of the anal fin curve to form an asymmetrical cylindrical structure that almost fully encloses a sperm duct (distinguishing it from the gonopodia in families Goodeidae and Poeciliidae). The tip of the gonopodium bends to either the left or right side of the fish. During the development of the gonopodium, an additional anteriormost anal fin ray forms that is not present in females or juveniles.[6]

Females of this species also exhibit asymmetrical genitalia, though symmetrical individuals do exist in low numbers.[21] The asymmetry in females comes from the foriculum, which is a modified scale that covers the oviduct, located behind the vent. The foriculum only opens to one side, meaning that right-handed males can only mate with left-handed females, and vice versa.[4]

Right and left-handed fish of both sexes are found in roughly equal proportions in the wild. Breeding experiments with this species may suggest that the handedness of the gonopodium is determined randomly, and is not heritable or dependent on environmental conditions.[21][22]

Human uses and impacts

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Largescale four-eyed fish at Hellabrunn Zoo

Researchers favor this species for its unique eye morphology. The unique appearance of this species has also attracted the attention of the public, as this species can be found in many films, lectures, and in both public and home aquariums.[2][23]

Despite its use in the aquarium trade, Anableps anableps was classified as Least Concern by the IUCN in 2020. No specific population trends are known, however.[1]

References

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Wikimedia Commons has media related to Anableps anableps.
  1. ^ a b c IUCN (2020-04-10). Anableps anableps: Frederico, R.G.: The IUCN Red List of Threatened Species 2022: e.T162693357A162693388 (Report). International Union for Conservation of Nature. doi:10.2305/iucn.uk.2022-2.rlts.t162693357a162693388.en.
  2. ^ a b c d Froese, Rainer; Pauly, Daniel (eds.). "Anableps anableps". FishBase. August 2019 version.
  3. ^ a b c d e Michel, Krijn B.; Aerts, Peter; Gibb, Alice C.; Van Wassenbergh, Sam (2015). "Functional morphology and kinematics of terrestrial feeding in the largescale foureyes (Anableps anableps)". Journal of Experimental Biology. 218 (18): 2951–2960. doi:10.1242/jeb.124644. hdl:10067/1324730151162165141. PMID 26400981.
  4. ^ a b c Garman, S. (1895). "Sexual Rights and Lefts". The American Naturalist. 29 (347): 1012–1014. ISSN 0003-0147. JSTOR 2452452.
  5. ^ Bailly, Nicolas (2020). "Anableps anableps (Linnaeus, 1758)". WoRMS. World Register of Marine Species. Retrieved 26 January 2021.
  6. ^ a b c d Turner, C. L. (March 1950). "The skeletal structure of the gonopodium and gonopodial suspensorium of Anableps anableps". Journal of Morphology. 86 (2): 329–365. doi:10.1002/jmor.1050860206. ISSN 0362-2525.
  7. ^ a b c Oliveira, Valéria de Albuquerque; Fontoura, Nelson Ferreira; Montag, Luciano Fogaça de Assis (2011). "Reproductive characteristics and the weight-length relationship in Anableps anableps (Linnaeus, 1758) (Cyprinodontiformes: Anablepidae) from the Amazon Estuary". Neotropical Ichthyology. 9 (4): 757–766. doi:10.1590/S1679-62252011005000042. ISSN 1679-6225.
  8. ^ a b c d Reis, Roberto E.; Kullander, Sven O.; Ferraris, Carl J. (2003). Check List of the Freshwater Fishes of South and Central America. EDIPUCRS. pp. 582–583. ISBN 978-85-7430-361-1.
  9. ^ Miller, Robert Rush (1979). "Ecology, Habits and Relationships of the Middle American Cuatro Ojos, Anableps dowi (Pisces: Anablepidae)". Copeia. 1979 (1): 82–91. doi:10.2307/1443732. ISSN 0045-8511. JSTOR 1443732.
  10. ^ "Anableps anableps (Linnaeus, 1758) - Ocean Biodiversity Information System". obis.org. Retrieved 2024-11-08.
  11. ^ a b c d do Nascimento, Francylenna Lima; Assunção, Maria Ivaneide da Silva (2008). "Ecologia reprodutiva dos tralhotos Anableps anableps e Anableps microlepis (Pisces: Osteichthyes: Cyprinodontiformes: Anablepidae) no rio Paracauari, ilha de Marajó, Pará, Brasil" [Reproductive ecology of the tralhotos Anableps anableps and Anableps microlepis (Pisces: Osteichthyes: Cyprinodontiformes: Anablepidae) in the Paracauari River, Marajó Island, Pará, Brazil]. Boletim do Museu Paraense Emílio Goeldi (in Portuguese). 3 (3): 229–240. doi:10.46357/bcnaturais.v3i3.677. ISSN 1981-8114.
  12. ^ a b Brenner, M.; Krumme, U. (2007). "Tidal migration and patterns in feeding of the four-eyed fish Anableps anableps L. in a north Brazilian mangrove" (PDF). Journal of Fish Biology. 70 (2): 406–427. Bibcode:2007JFBio..70..406B. doi:10.1111/j.1095-8649.2007.01313.x.
  13. ^ a b c d Greven, Hartmut; Götz, Albrecht (January 2002). "Beobachtungen zur Nahrungsaufnahme und zum Fluchtverhalten der Vieraugenfische Anableps microlepis und A. anableps (Anablepidae, Cypronodontiformes)". Verhalten der Aquarienfische (PDF) (in German) (2 ed.). Verlag, Bornheim: Birgit Schmettkamp. pp. 55–62.{{cite book}}: CS1 maint: date and year (link)
  14. ^ "Anableps anableps (Four-eyed Fish)" (PDF). Sta.uwi.edu. Retrieved 9 April 2022.
  15. ^ a b c Figueiredo, M. B.; Nunes, J. L. S.; Almeida, Z. S.; Paz, A. C.; Piorski, N. M.; Reis, M. R. (2019-09-15). "Feeding ecology of Anableps anableps (Actinopterygii: Cyprinodontiformes: Anablepidae) off the north-eastern coast of Brazil". Acta Ichthyologica et Piscatoria. 49 (3): 213–219. doi:10.3750/AIEP/02477. ISSN 1734-1515.
  16. ^ Diniz, Daniel G.; Varella, Jamille E. A.; Guimarães, Maria Danielle F.; Santos, Arthur F. L.; Fujimoto, Rodrigo Y.; Monfort, Karla C. F.; Pires, Marcus A. B.; Martins, Maurício L.; Eiras, Jorge C. (December 2008). "A note on the occurrence of praniza larvae of Gnathiidae (Crustacea, Isopoda) on fishes from Northeast of Pará, Brazil". Anais da Academia Brasileira de Ciências. 80 (4): 657–664. doi:10.1590/S0001-37652008000400007. ISSN 0001-3765. PMID 19039489.
  17. ^ Couto, João Victor; Nunes, Jorge Luiz Silva; Paschoal, Fabiano; Pereira, Felipe Bisaggio (January 2024). "Acusicola rochai n. sp. (Copepoda: Ergasilidae) parasitizing Anableps anableps (Anablepidae) from the Amazon Coast, with a key for Acusicola spp". Journal of the Marine Biological Association of the United Kingdom. 104: e64. Bibcode:2024JMBUK.104E..64C. doi:10.1017/S0025315424000560. ISSN 0025-3154.
  18. ^ a b Schwab, I. R.; Ho, V.; Roth, A.; Blankenship, T. N.; Fitzgerald, P. G. (2001). "Evolutionary attempts at 4 eyes in vertebrates". Transactions of the American Ophthalmological Society. 99: 145–156, discussion 156–157. ISSN 0065-9533. PMC 1359005. PMID 11797302.
  19. ^ Perez, Louise N.; Lorena, Jamily; Costa, Carinne M.; Araujo, Maysa S.; Frota-Lima, Gabriela N.; Matos-Rodrigues, Gabriel E.; Martins, Rodrigo A. P.; Mattox, George M. T.; Schneider, Patricia N. (2017-04-12). "Eye development in the four-eyed fish Anableps anableps : cranial and retinal adaptations to simultaneous aerial and aquatic vision". Proceedings of the Royal Society B: Biological Sciences. 284 (1852): 20170157. doi:10.1098/rspb.2017.0157. ISSN 0962-8452. PMC 5394668. PMID 28381624.
  20. ^ Turner, C. L. (March 1938). "Adaptations for viviparity in embryos and ovary of Anableps anableps". Journal of Morphology. 62 (2): 323–349. doi:10.1002/jmor.1050620208. ISSN 0362-2525.
  21. ^ a b Torres-Dowdall, Julián; Rometsch, Sina J.; Velasco, Jacobo Reyes; Aguilera, Gastón; Kautt, Andreas F.; Goyenola, Guillermo; Petry, Ana C.; Deprá, Gabriel C.; da Graça, Weferson J.; Meyer, Axel (2022-05-11). "Genetic assimilation and the evolution of direction of genital asymmetry in anablepid fishes". Proceedings of the Royal Society B: Biological Sciences. 289 (1974). doi:10.1098/rspb.2022.0266. ISSN 0962-8452. PMC 9091857. PMID 35538779.
  22. ^ Torres-Dowdall, Julián; Rometsch, Sina J.; Kautt, Andreas F.; Aguilera, Gastón; Meyer, Axel (2020-07-08). "The direction of genital asymmetry is expressed stochastically in internally fertilizing anablepid fishes". Proceedings of the Royal Society B: Biological Sciences. 287 (1930): 20200969. doi:10.1098/rspb.2020.0969. ISSN 0962-8452. PMC 7423465. PMID 32635868.
  23. ^ Robbins, C. Richard; Bailey, Reeve M.; Bond, Carl E.; Brooker, James R.; Lachner, Ernest A.; Lea, Robert N.; Scott, W. B. (1991). World Fishes Important to North Americans: Exclusive of Species from the Continental Waters of the United States and Canada (21 ed.). American Fisheries Society. p. 87. ISBN 0-913235-54-7. ISSN 0097-0638.
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