Polypteriformes

(Bichirs)

Class Actinopterygii

Order Polypteriformes

Number of families 1

Evolution and systematics

Polypteriforms are the most basal or "primitive" living actinopterygian group, according to many recent authors. There are two living genera: Polypterus (bichirs), and Erpetoichthys (reedfish or ropefish), with 11 to 16 species presently recognized (Erpetoichthys has a single species). Their sketchy fossil record suggests that the group has never been particularly diverse. Fossils have been found in both Africa and South America, indicating that they were in existence before the breakup of Gondwana in the early Cretaceous, some 118 million years ago (mya); living forms are restricted to Africa. However, if the most accepted evolutionary scheme is correct, then polypteriforms have been around since much earlier (from at least the late Devonian), as indicated by stratigraphic correlations with fishes more closely related to the remaining actinopterygians (e.g. Mimia and Moythomasia, from the late Devonian of Australia). The few fossil polypteriform occurrences, usually dermal remains, are from the late Cretaceous (Cenomanian, 100 mya) of Morocco (Serenoichthys), Niger (?Campanian, 84 mya) and Bolivia (Maastrichtian, 71 mya), and Paleocene (63 mya) of Bolivia (Dagetella). Hence, there is a tremendous gap in our knowledge of polypteriforms, as fossils are as yet unknown from the late Devonian to the Cenomanian, a period spanning some 270 million years. On the other hand, there is molecular evidence suggesting that polypteriforms are more closely related to neopterygians (gars, bowfins, and teleosts), which, if confirmed, slightly reduces the discrepancy with the fossil record.

Uncertainty regarding the evolutionary affinities of polypteriforms is not new, as they have been interpreted as being more closely related to either sarcopterygians or actinopterygians, or even lying somewhere in between (such as in Erik A. Stensiö's Brachiopterygii), at least until the influential 1928 study by Edwin S. Goodrich. Even though the current consensus is to place them among the actinopterygians (following Goodrich), there is much room for refinement. The disagreement over their ancestry stems from their enigmatic amalgam of anatomical features. Some of these features are present in sarcopterygians (lobefins), such as fleshy pectoral fin bases (not the internal pectoral fin skeleton, however), feathery external larval gills, larval cement organs, and paired, vascularized swimbladders (lungs) arising from a ventral esophageal pneumatic duct; the latter three features are present in lepidosireniform lungfishes and tetrapods. Other features are similar to those present in sharks and rays (e.g., intestinal spiral valve, pectoral fin skeleton). But all of these traits probably evolved independently in the Polypteriformes, which share various derived characters with actinopterygians (e.g., scales with ganoin, dermohyal, gill arch musculature), as summarized by British paleoichthyologist Colin Patterson in 1982. The structure of their eggs (with a single opening for the entry of sperm cells) also supports their evolutionary affinity with actinopterygians.

Evolutionary relationships among polypteriform species, as well as the taxonomic status of many of these (along with their respective subspecies) are in need of further evaluation. Species of Polypterus are usually identified by their color pattern and meristics (such as numbers of scales along the lateral line, number of dorsal finlets), but there is overlapping in many features among certain species. Both genera are easily separated, as Erpetoichthys lacks pelvic fins and is very elongate, eel-like, with posteriorly positioned, small, and widely separated dorsal finlets.

Physical characteristics

Polypteriforms are morphologically unusual, and as a result their anatomy has been intensely studied over the past 100 years. They are moderately large, ranging from 15.7 to 47.2 in (40 to 120 cm) in total length, and are readily identified,

presenting a slender body with a depressed head, wide terminal mouth with fleshy lips, and unique, subdivided spiny dorsal fins (finlets). There is a tubular pair of nostrils extending anteriorly beyond the mouth. The teeth are sharp, small, and numerous. The gill opening is large, with an extended skin covering ventrally; four functional gill arches are present. The arrangement of dermal bones of the head and cheek are visible externally. The pectoral fin is greatly rounded posteriorly, with a fleshy base. Each dorsal finlet is composed of a strong, sharp spine attached posteriorly to a dermal fold, which in turn is attached to the base of the succeeding spine. Spines vary from seven to 18 among species, and are bifid (doubleedged) at their tips. The dorsal fin originates either shortly after the pectoral fins or farther posteriorly, and is confluent with the caudal fin origin. The caudal fin is posteriorly elongate and distally rounded, composed only of soft rays. Pelvic fins (Polypterus) are situated at the posterior third of the body, followed by the anal fin (in both genera) which is very close to the caudal fin (and is functionally correlated with it while swimming); the anal fin, unlike the dorsal fin, is separated from the caudal fin by a notch. The dorsal finlets are the only fins with spines.

Polypteriforms have a compact, dense covering of trapezoidal, shiny (ganoid) scales, arranged in numerous diagonal series, which give them a rigid texture (similar to gars). Scales along the lateral line vary from about 55 to 70. Internally, polypteriforms have paired, asymmetrical (right lobe larger than left), and highly vascularized swim bladders that function as airbreathing organs. Coloration is olive-brown to dark brown dorsally and laterally, and over the head, but creamy white ventrally. Numerous dark or clear spots and blotches and irregular stripes are present in many species, sometimes over pectoral fins (e.g., P. ornatipinnis), but others are more uniform in color (P. senegalus). The heads of most species have a mottled or reticulated appearance.

Distribution

Present in western and central tropical Africa, with three species also occurring in the Nile River. They are absent from rivers that drain into the Indian Ocean.

Habitat

Commonly found in both fast and slow moving rivers, floodplains, swamps, lakes, and pools. Because they are able to breathe air directly, bichirs and the reedfish are capable of living in stagnant waters. They enter rivers with associated marginal vegetation during the spawning season.

Behavior

Not many studies documenting polypteriform behavior have been conducted. They are reported to "walk" over land for small distances to feed on insects, as they are able to absorb oxygen directly from the air for at least a few hours. However, air breathing is not obligatory, as it is in lepidosirenid lungfishes. In aquaria their behavior varies from remaining motionless on the bottom for short periods to swimming about vigorously. Their pectoral fins function as paddles.

Feeding ecology and diet

Polypteriforms are carnivorous, feeding on invertebrates such as insect larvae, snails, earthworms, and freshwater crustaceans, as well as fishes and amphibians; they are primarily nocturnal predators. Polypteriforms are preyed upon by crocodiles and large, fish-eating birds.

Reproductive biology

Reproduction has been observed in aquaria for a few bichir species as well as for the reedfish. Males may compete for the attention of a female. The anal fins are sexually dimorphic, as males have a pronounced bulge at the anal fin origin (anal fin is broader and more muscular). This modification develops gradually with sexual maturity; otherwise the fin is identical in both sexes. The anal fin is important during spawning, as the male will use his anal and caudal fins to envelop the genital opening of the female, thereby forming a receptacle in which he will fertilize her eggs. Eggs are then released by the male, through vigorous shaking of the anal fin, and quickly adhere to vegetation. This behavior has been described for both polypteriform genera. The larvae have feathery external gills. Polypterids do not practice any form of parental care of their eggs or fry.

Conservation status

No species are presently threatened or protected under CITES legislation, and none are listed in the IUCN database.

Significance to humans

Imported with frequency in the ornamental fish trade. The larger bichirs are highly regarded food fishes in West Africa. Their firm, white flesh tastes very much like the freshwater prawns that constitute an important part of the human diet in this region. These are very long-lived fishes, with records of large bichirs living 50 years in captivity.

Species accounts

List of Species

Bichir

Polypterus ornatipinnis

family

Polypteridae

taxonomy

Polypterus ornatipinnis Boulenger, 1902, Congo River.

other common names

None known.

physical characteristics

Maximum length 23.6 in (60 cm). Body protected by an armor of large, rhombic, bony scales. Moderately elongate, with nine to 10 independent dorsal finlets. Pelvic fins located posteriorly. White belly with dark mottling on head, flanks and dorsum, with continuous parallel bands on fins.

distribution

Central and East Africa, found in the Congo Basin and in Lake Tanganyika.

habitat

Lakes, rivers, floodplains, and swamps, including waters with low oxygen content.

behavior

Often sits motionless on the bottom, resting on its pectoral fins such that the head and anterior portion of the body are slightly elevated. Periodically gulps air from the surface in stagnant water.

feeding ecology and diet

Carnivorous; feed mostly at night on a variety of prey, including other fishes, frogs, insects, and crustaceans.

reproductive biology

During courtship, their usual inactivity is abandoned, and both male and female engage in energetic twisting, turning, and darting movements. The male subsequently envelops the female's genital opening with his anal and caudal fin, fertilizing the eggs and then scattering them by thrashing his tail.

conservation status

Not listed by IUCN.

significance to humans

Found in markets as a food fish; also captured for the aquarium trade.

Reedfish

Erpetoichthys calabaricus

family

Polypteridae

taxonomy

Erpetoichthys calabaricus Smith, 1866, Old Calabar, West Africa.

other common names

English: Ropefish.

physical characteristics

Maximum length 35.4 in (90 cm). Shares with Polypterus rhombic bony scales and distinct dorsal finlets. Unlike Polypterus, Erpetoichthys lacks pelvic fins and is very elongate and eel-like in appearance. Uniform brown-olive color dorsally, with white underside and black spot on pectoral fins. Sexually active individuals develop an orange-red flush on the venter.

distribution

Coastal drainages of West Africa, from Nigeria to the Republic of the Congo.

habitat

Areas with aquatic vegetation in swamps and along rivers.

behavior

Hunts along the bottom, moving in serpentine fashion.

feeding ecology and diet

Nocturnal predator, feeds on worms, crustaceans, and insects.

reproductive biology

Very similar to that of Polypterus, in which the males wraps his anal fin around the female's genital pore, fertilizes the eggs, and then scatters them into the surrounding vegetation with vigorous tail thrashing. In Benin and Nigeria, ripe individuals undertake mass movements overland into seasonally flooded swamp pools in order to spawn.

conservation status

Not listed by IUCN.

significance to humans

A popular aquarium fish

Resources

Books

Berra, Tim M. Freshwater Fish Distribution. San Diego, CA: Academic Press, 2001.

Boulenger, George A. Les Poissons du Bassin du Congo. Bruxelles, Belgium: Publication de l'État Indépendant du Congo, 1901.

——. The Fishes of the Nile. London, England: Hugh Rees, 1907.

Gosse, J.-P. "Polypteridae." In Check-list of the Freshwater Fishes of Africa (CLOFFA), edited by J. Daget, J.-P. Gosse, and D. F. E. Thys van den Audenaerde. Paris, France and Tervuren, Belgium: Orstom and MRAC, 1984.

——. "Polypteridae." In Faune des Poissons d'eaux douces et saumâtres de l'Afrique de l'Ouest, edited by Christian Lévêque, Didier Paugy, and Guy G. Teugels. Paris, France and Tervuren, Belgium: Orstom and MRAC, 1990.

Graham, Jeffrey B. Air-breathing Fishes: Evolution, Diversity, and Adaptation. San Diego, CA: Academic Press, 1997.

Janvier, P. Early Vertebrates. New York: Oxford University Press, 1996.

Kerr, J. Graham. "The Development of Polypterus senegalus Cuv." In The Work of John Samuel Budgett, edited by J. Graham Kerr. Cambridge, England: Cambridge University Press, 1907.

Nelson, J. S. Fishes of the World. 3rd ed. New York: John Wiley & Sons, 1994.

Patterson, C. "Bony Fishes." In Major Features of Vertebrate Evolution, edited by Donald R. Prothero and Robert M. Schoch. Knoxville, TN: Paleontological Society, 1994.

Stensiö, E. A. Triassic Fishes from Spitzbergen. Vol. I. Vienna, Austria: A. Holzhausen, 1921.

Wiley, E. O. "Bichirs and Their Allies." In Encyclopedia of Fishes, edited by John R. Paxton and William N. Eschmeyer. San Diego, CA: Academic Press, 1995.

Periodicals

Azuma, H. "Breeding Polypterus endlicheri." Tropical Fish Hobbyist 44, no. 2 (1995): 116–128.

Bartsch, P., and S. Gemballa. "On the Anatomy and Development of the Vertebral Column and Pterygiophores in Polypterus senegalus Cuvier, 1829 ("Pisces," Polypteriformes)." Zool. Jb. Anat. 122 (1992): 497–529.

Bartsch, P., S. Gemballa, and T. Piotrowski. "The Embryonic and Larval Development of Polypterus senegalus Cuvier, 1829: Its Staging with Reference to External and Skeletal Features, Behavior and Locomotory Habits." Acta Zoologica 78 (1997): 309–328.

Bartsch, P., and R. Britz. "Zucht und Entwicklung von Polypterus ornatipinnis." Datz 1 (1996): 15–20.

——. "A Single Micropyle in the Eggs of the Most Basal Living Actinopterygian Fish, Polypterus (Actinopterygii, Polypteriformes)." Journal of Zoology 241 (1997): 589–592.

Britz, R., and P. Bartsch. "On the Reproduction and Early Development of Erpetoichthys calabaricus, Polypterus senegalus, and Polypterus ornatipinnis (Actinopterygii, Polypteridae)." Ichthyological Exploration of Freshwaters 9, no. 4 (1998): 325–334.

Daget, J. "Révision des affinités phylogénétiques des polyptéridès." Mem. L'Inst. Fran. D'Afr. Noire 11 (1950): 1–178.

Dutheil, D. B. "First Articulated Fossil Cladistian: Serenoichthys kemkemensis, gen. et spec. nov., from the Cretaceous of Morocco." Journal of Vertebrate Paleontology 19 (1999): 243–246.

Gardiner, B. G., and B. Schaeffer. "Interrelationships of Lower Actinopterygian Fishes." Zoological Journal of the Linnean Society 97 (1989): 135–187.

Goodrich, E. S. "Polypterus a Paleoniscid?" Palaeobiologica 1 (1928): 87–92.

Nelson, G. J. "Subcephalic Muscles and Intracranial Joints of Sarcopterygians and Other Fishes." Copeia 1970, no. 3(1970): 468–471.

Patterson, C. "Morphology and Interrelationships of Primitive Actinopterygian Fishes." American Zoologist 22 (1982): 241–259.

Poll, M. "Les tendances évolutives des polyptères d'après l'étude systématique des espèces." Ann. Soc. R. Zool. Belg. 72, no. 2 (1941): 157–173.

——. "Contribution à l'étude systématique des Polypteridae (Pisc.)." Rev. zool. Bot. Afr. 35 (1941): 143–179.

——. "Contribution à l'étude systématique des Polypteridae (Pisces)." Rev. zool. Bot. Afr. 35 (1942): 269–317.

Swinney, G. N., and D. Heppell. "Erpetoichthys or Calamoichthys: The Correct Name for the African Reedfish." Journal of Natural History 16 (1982): 95–100.

Marcelo Carvalho, PhD

Robert Schelly, MA