Subdivision Otocephala (= Ostarioclupeomorpha), Superorder Ostariophysi
Freshwater habitats worldwide are dominated in terms of numbers of both species and individuals by ostariophysans, which account for about 68% of all freshwater species. Ostariophysans include such disparate taxa as milkfish, minnows, carps, barbs, suckers, loaches, piranhas, tetras, catfishes, and electric eels, but two unique traits characterize most members of this massive taxon. With the exception of the gonorhynchiforms, ostariophysans possess a unique series of bones that connect the gas bladder with the inner ear, anotophysic condition. This Weberian apparatus, named after the German anatomist who first described it, involves a set of bones derived from the four or five anterior (cervical) vertebrae and their neural arches, ribs, ligaments, and muscles (see Fig. 6.4). The superorder gets its name from this complex structure (ostar =small bone, physa =a bladder; “otophysic” basically means “ear” and “bladder”); ostariophysans with the apparatus are referred to as the Otophysi. When sound waves contact the fish, the gas bladder vibrates, and this vibration is passed anteriorly to the inner ear, being amplified by the intervening Weberian ossicles (see Hearing). Unrelated taxa have convergently evolved connections between the gas bladder and the inner ear, either by an otophysic extension of the gas bladder anteriorly (elephantfishes, clupeoids, cods, Roosterfish, porgies, some cichlids); by a bony connection involving the pectoral girdle or skull (squirrelfishes, triggerfishes); or, in chaetodontid butterflyfishes, by connections between anterior extensions of the bladder and the lateral line canal system (Webb et al. 2006). Many of these families are known sound producers and it is assumed they derive auditory advantages via their specialized structures. Gonorhynchiforms, the Anotophysi, possess a primitive homolog of the Weberian apparatus consisting of three modified anterior vertebrae associated with cephalic rib bones.
The second shared derived trait that helps define the Ostariophysi is the alarm response, which involves: (i) the production of analarm substance (Schreckstoff ); and (ii) a behavioral alarm reaction to the presence of the substance in the water (Schreckreaktion) (see Discouraging capture and handling). The alarm substance is given off when specialized dermal club cells are ruptured, as when a predator bites down on a prey fish. Nearby individuals, most likely schoolmates, sense the chemical in the water and take a variety of coordinated escape actions, depending on the species. Possession of the alarm response was a factor contributing to the inclusion of the gonorhynchiforms within the Ostariophysi.
Some ostariophysans lack one or both parts of the response for apparently adaptive reasons. Piranhas lack the alarm reaction, which makes sense as many of their prey are also ostariophysans and it would be counterproductive for a predator to fl ee each time it bit into prey. Some nocturnal, non-schooling, or heavily armored ostariophysans lack both parts of the alarm response, including Blind Cave Characins, electric knifefishes, and banjo and suckermouth armored catfishes. An interesting seasonal loss of the production end of the response occurs in several North American minnows. Nest building and courtship in these fishes often involves rubbing by males against the bottom and between males and females, during which time the skin and its breeding turbercles may be broken. It would be less than helpful to the male if he produced a substance that frightened females away during these activities. Males resume the production of alarm substance in the fall, after the breding season. As with the Weberian apparatus, convergent evolution of alarm substances and responses have evolved in other teleostean groups, including sculpins, darters, and gobies (Smith 1992).
Ostariophysans encompass two series, the Anotophysi with one order and the Otophysi with four orders. The most primitive order is the Gonorhynchiformes, which includes the Milkfish, Chanos chanos (series Anotophysi, family Chanidae), and three other relatively small tropical families. Milkfish are an important food fish in the Indo-Pacific region and are often cultured in brackish fishponds, where juveniles are raised to edible size on an algae diet. C. chanos grows to almost 2 m and 25 kg and is a popular sportfish in some areas.
The series Otophysi contains the bulk of freshwater fish species globally. The Cypriniformes constitutes the largest order and probably contains the most familiar species of the superorder. The Cyprinidae, the largest family of freshwater fishes and the second largest family (after the gobies) of all fishes, contains 2200 of the >3200 cypriniform species. Among the better known cyprinids are the minnows, shiners, carps, barbs, barbels, gudgeons, chubs, dace, pikeminnows, tench, rudd, bitterlings, and bream and such popular aquarium fishes as the Southeast Asian “sharks” (Redtail Black Shark, Bala Shark), Goldfish, Koi (domesticated common carp), Zebra Danios, and rasboras. The Zebrafish or Zebra Danio, Danio rerio, has become a standard laboratory animal in developmental genetics, toxicology, and medical research (Westerfield 2000; Gong & Korzh 2004; see also Zebrafish Information Network, http://zfin.org). Zoogeographically, cyprinids are most diverse in Southeast Asia, followed by Africa, North America (where there are 300 species according to Berra (2001)), and Europe. Cyprinids are absent from Australia and South America, their ecological roles filled largely by osmeriforms and atheriniforms in the former and by characins in the latter.
It is in the cyprinids that we see the first real development of pharyngeal dentition, a second set of jaws in the throat region that are derived from modified, tooth-bearing pharyngeal arches (see Pharyngeal jaws). Specifically, the fifth ceratobranchial (=pharyngeal) bone occludes against an enlarged posterior process of the basiocciptal bone to form the pharyngeal bite. Cyprinids are also the first teleosts to develop a highly protrusible upper jaw and to eliminate the maxillary bones from the biting bones and gape of the mouth, both trends that are increasingly developed in more advanced teleostean taxa (see Division Teleostei). Exclusion from the gape of the maxilla is a characteristic of all fishes higher than the salmoniforms, although the bite of salmoniforms and their relatives involves the maxilla. The exclusion versus inclusion of the maxilla in cyprinids versus salmoniforms has led to some controversy over which group is more advanced. The bulk of the evidence favors salmoniforms as the more advanced clade (“minnows before trout”; Smith 1988); maxilla inclusion in salmoniforms may be a secondarily evolved trait.
Some cyprinids have chromosomes in the polyploid condition, an unusual occurrence among fishes. The normal diploid 2N condition of most cyprinids is 48 or 50, although tetraploid (2N =100), hexaploid, and even octaploid species occur, as is the case for the goldfish (Buth et al.1991). Polyploidy is linked with large size in minnows; the world’s largest species are the Southeast AsianCatlocarpio siamensis, a tetraploid (Fig. 14.7), and the Indian Mahseer, Tor putitora, both of which reach 2.5–3 m in length. The largest minnow in North America is the piscivorous Colorado Pikeminnow, Ptychocheilus lucius. Exceptional size in cyprinids is also often accompanied by predatory habits, as implied by the scientific names of such large (>2 m) species as Elopichthys bambusa andBarbus esocinus. Most cyprinids, however, are quite small (<5 cm), and the smallest freshwater fish, and perhaps vertebrate, in the world is an Indonesian cyprinid, Paedocypris progenetica, that matures at 7.8 mm (see Fig. 10.9).
The superfamily Cobitoidea includes the disparate families of algae eaters, suckers, loaches, and river loaches.
“Minnows.” The giant and imperiled Asian Carp, Catlocarpio siamensis, native to the Mekong River basin.
Gyrinocheilid algae eaters are interesting because of modifications to the mouth and gill apparatus that allow them to scrape algae from rocks in areas of strong current. The mouth is modified into a sucking organ that helps them cling to rocks while scraping off algae. The fish breathes by inhaling water dorsally and exhaling it ventrally through small apertures in the gill opening. Suckers (Catostomidae) include about 72 species of relatively large (50–100 cm), chiefly North American fishes (Figs 14.8, 14.9). One species, the Chinese Highfin “Shark”, Myxocyprinus asiaticus, occurs in eastern China, and another species, the Siberian Longnose Sucker, Catostomus catostomus rostratus, occurs in Alaska and northeastern Siberia (Scharpf 2006). Species include the buffaloes, quillback, carpsuckers, blue sucker, redhorses, jumprocks, and the extinct Harelip (Fig. 14.8) and Snake River Suckers. Most suckers are benthic feeders in flowing water with inferior mouths and plicate or papillate lips. Exceptional are some lake suckers in the western USA (e.g., Chasmistes) that are midwater planktivores with more terminal mouths. Suckers can be quite confusing taxonomically because they frequently hybridize. Potential confusion is not helped by the scientific and common name combinations in this family, such as the Quillback, Carpiodes cyprinus, and the River Carpsucker, Carpiodes carpio, which can be mixed up with the Common Carp, Cyprinus
carpio, which is a cyprinid.
The extinct Harelip Sucker, Moxostoma lacerum. Once abundant in 13 eastern US states, this may have been the first American fish driven to extinction, around 1900 (see Jenkins & Burkhead 1993). Actual coloration is not known because no live fish were ever drawn or photographed; only one adult was preserved. After Trautman (1981).
A juvenile Robust Redhorse, Moxostoma robustum. Growing to large size (80 cm, 8 kg), this rare catostomid endemic to Atlantic slope rivers of the southeastern USA went unrecorded for over 120 years.Rediscovered in 1991, a cooperative effort among government, corporate, and nongovernmental organizations succeeded in captive propagation, release into the wild, and establishment and reproduction by propagated fish (see Helfman 2007; www. robustredhorse.com/h/reportpubs.html). Photo by G. Helfman.
Loaches (Cobitidae) constitute 177 species of predominantly Eurasian fishes that have their highest diversity in Southeast Asia. Included are such popular aquarium fishes as the Kuhli, Clown, and Skunk loaches, the weatherfishes, and the Golden Dojo. Weatherfishes (Misgurnus) obtained their name because they become restless when atmospheric pressure drops preceding a storm. Their sensitivity to barometric fluctuations may somehow relate to their air-breathing abilities, which involve gulping atmospheric air and passing a bubble to the intestine where gaseous exchange occurs. Balitorid river or hillstream loaches are a highly diverse family (590 species), many of which are specialized for life in fast-flowing mountain streams of India and Southeast Asia. Their paired fins tend to be oriented horizontally, are enlarged, and have adhesive pads on their ventral surfaces. In addition, their bodies are depressed dorsoventrally and their mouths are ventral, all anatomical adaptations to life in swift or turbulent water. One large subfamily of balitorids, the Nemacheilinae, includes several cave-dwelling species.
Piranhas, Serrasalmus spp., are representative of the speciose tropical order of characiform fishes. Photo by G. Helfman.
The characiforms are another large order (c. 1670 species) of primarily tropical otophysans characterized (usually) by an adipose fin, well-armed mouths and replacement dentition (e.g., piranhas; Dentition), and peripheral ctenoid as opposed to the cycloid scales found in most lower groups (Fig. 14.10). This is a remarkably diverse order anatomically and ecologically, including predators, zooplanktivores, scale eaters, detritivores, and herbivores; the latter category includes fishes that feed on seeds, leaves, and fruits. Characiforms may be surface, water column, or benthic dwellers, although most species are found in midwater, many in shoals. Body sizes range from very small (13 mm adult tetras) to quite large (e.g., 1.5 m long tigerfishes) and body shapes range from long, slender, almost darterlike benthic fishes (e.g., South American darters, Characidium) to deep-bodied, compressed piranhas and hatchetfishes (Gasteropelecus). Numerous popular, colorful aquarium fishes belong to this order, including Distichodus, Prochilodus, headstanders, spraying characins, freshwater hatchetfishes, blind characins, pencilfishes, tetras (Cheirodon, Hemigrammus,Micralestes,Paracheirodon), and silver dollars, as do important food fishes (Prochilodus, Colossoma, Brycon). Currently, 18 families are recognized, although past classifications have recognized as few as one. The great majority of species (c. 1300) are South American, about 200 are African, a small number live in Central America, and one species, the Mexican Tetra, Astyanax mexicanus, extends naturally into southwestern Texas. Another 10 species, including piranhas, have been introduced into the USA. Because of its large size and tropical nature, the order has undergone considerable taxonomic revision, much of which is still in progress. Perhaps as many as 88 genera and 620 species are of unknown taxonomic position (see Nelson 2006).
The most primitive characiforms are the 100 species of African citharinoids (Distichodus, Citharinus), attesting to an African origin for the order and the connection between Africa and South America prior to the break up of Gondwanaland in the Mesozoic. African characiforms also include the advanced alestiids (about 100 species). Two of the largest characiforms are predaceous African species. The Pike Characin, Hepsetus odoe (Hepsetidae), is an impressive predator that reaches 65 cm in length and has fanglike teeth. It is remarkably convergent with the alestiid tigerfishes, Hydrocynus spp., which reach almost 2 m in length and over 50 kg in mass. Not far behind in the dentition and size categories are the cynodontid wolf characins or payara of South America (Hydrolycus: 1.2 m, 18 kg), a much-sought sportfish. Apart from various popular aquarium species, certainly the best known or at least most notorious characids are the piranhas (Fig. 14.10). This subfamily, the Serrasalminae, contains about 80 species, some of which are predatory (Serrasalmus), others which are scale-eating opportunists and specialists (Pygocentrus, Catoprion), and some that are largely herbivorous, such as the pacus and silver dollars (Colossoma, Metynnis) (Sazima 1983; Nico & Taphorn 1988). Despite their reputation and potential for doing damage, many purported attacks on humans by piranhas actually result from postmortem scavenging on rowning victims (Sazima & Guimaraes 1987). In recent years, however, this picture has changed as a result of a proliferation of dams in southeastern Brazil that created ideal piranha spawning habitat.
More than 85 attacks on bathers by nest-guarding piranhas have been reported, many resulting in serious injury (Haddad & Sazima 2003). The large, herbivorous serrasalmine species are important food fishes in the Amazon basin and are also important dispersers of seeds during the wet season, particularly because they use their massive dentition to husk seeds, which may aid germination (Goulding 1980). The Characidae overall includes 12 subfamilies, 165 genera, and at least 960 described species.
The diversity of catfishes (Siluriformes) amazes most everyone (Burgess 1989; Arratia et al. 2003) (Fig. 14.11). Approximately 35 families and 2900 species of catfishes are recognized and it is not surprising that catfish systematics remains active, controversial, and unsettled (see the All Catfish Species Inventory, http://silurus.acnatsci.org for regular updates and photos). Commonalities among the families include: fusion, reduction, or loss of a number of skull bones found in lower teleosts, including the maxilla; teeth on the roof bones of the mouth (vomer, pterygoid, palatine); an adipose fin, sometimes with rays or a spine; an unsegmented, spinelike ray at the front of both the dorsal and pectoral fins that in some families is covered by a toxin produced by epidermal glandular cells; the dorsal spine is often preceded by a shorter spine that helps lock the larger spine in the erect position; a lack of scales, often combined with the presence of bony plates or tubercles; small eyes (and nocturnal, benthic foraging habits); and one to four pairs of barbels associated with both the upper and lower jaws that serve both chemosensory and tactile functions.
Catfishes are known from all continents, including Antarctica during the Oligocene. They reach their greatest diversity in South America, where the largest families occur (loricariid suckermouth armored catfishes with 684 species, trichomycterid pencil catfishes with 201 species, and callichthyid armored catfishes with 177 species); the bagrids of Africa/Asia and the African mochokid upside-down catfishes are close behind with 170 to 180 species.
Selected catfishes, showing some of the array ofbody types and shared characteristics among the 35families. Drawings by John Quinn, in Burgess (1989),used with permission of TFH Publications.
The most primitive catfishes are the South American diplomystids,which have a well-developed, toothed maxillary bone, atrait they share with the extinct Eocene hypsidorids. Aswith the vast majority of ostariophysans, most catfishes areconfined to fresh water, including many cave species (e.g.,Proudlove 2006). However, two families of catfishes areprimarily marine, the widespread sea catfishes (Ariidae)and the highly venomous Indo-Pacific eeltail catfishes(Plotosidae). The plotosids have diversified in severalrespects. Juvenile lined catfishes, Plotosus lineatus, aretightly schooling, diurnally active fishes in contrast to thesolitary, nocturnal behavior of most other families. Bothfamilies contain members that occur in fresh water, such asthe plotosid tandan catfishes of Australia (Tandanus), whichprobably represents secondary evolution of the use of freshwaterhabitats (i.e., freshwater ancestor gave rise to amarine species which then reinvaded fresh water). Marineplotosids have highly venomous spines and relatively boldcoloration, suggesting an aposematic warning function (see Evading pursuit), which does not appear todeter their chief predators, the equally venomous seasnakes(Voris et al. 1978).
Most catfishes are naked, lacking true scales. But in somefamilies, different parts of the body are covered with individualor overlapping bony plates (armorhead catfishes,loach catfishes, sea catfishes, thorny catfishes, callichthyidarmored catfishes, suckermouth armored catfishes), orthornlike projections, tubercles or “odontodes” (sisorids,thorny catfishes, aspredinid banjo catfishes, spiny dwarfcatfishes). The popular “plecostomus” catfishes of theaquarium trade are loricariid suckermouth armored catfishes; most are in the genus Hypostomus because, regrettably,Plecostomus is no longer a valid scientific name.Relatives in the genus Pterygoplichthys have become anuisance introduction in Mexico and Florida where theyundergo population explosions.
Some of the world’s largest freshwater fishes are catfishes, including the predatory European wels (Siluris glanis,Siluridae) at 5 m and 330 kg; the herbivorous, CriticallyEndangered, Mekong Giant Catfish (Pangasianodon gigas,Pangasiidae) at 3 m and 300 kg; and the 2.8 m, 150 kglong-whiskered pimelodid Piraiba (Brachyplatystoma filamentosum) of South America (Fig. 14.12). Piraiba areimportantly economically and ecologically in the Amazonbasin and are legendary for their annual migrations(Barthem & Goulding 1997). The largest catfishes in NorthAmerica are the Flathead and Blue catfishes, Pylodictis olivarisand Ictalurus furcatus, which reach about 1.5 m and50–68 kg. Very small catfishes such as spiny dwarf catfishes(Scoloplacidae) and whalelike catfishes (Cetopsidae) areonly 20–25 mm long as adults. Some small catfishesare notably unpleasant. The pencil or parasitic catfishes(Trichomycteridae) of South America include species thateat mucus and scales from other fishes or that pierce theskin or gill cavities of other fishes and feed on blood. Atleast one species, a candiru, Vandellia cirrhosa, is known toswim up the urethra of bathers and lodge itself there withits opercular spines, necessitating surgical removal (Burgess1989; Spotte 2002). This behavior results in part becausetrichomycterids are positively rheophilic, which means theytend to swim “upstream” into gill cavities, or occasionallyinto a urethra.
Large catfishes. (A) The Mekong Giant Catfish, one of the world’s largest catfishes. This specimen was caught from a stocked population in Bung Sam LanLake, Thailand. (B) Two c. 20 kg Flathead Catfishes, North America’s second largest catfish. (A) photo courtesy of Jean-Francois Healias, www.anglingthailand.com; (B) photo by G. Helfman.
Most catfishes are benthic, but some silurid sheatfishes,schilbeids, ageneiosid bottlenose catfishes, and Hypopthalmuslookdown catfishes normally swim above the bottom(the relatively bizarre lookdowns are probably filter feeders).Occupation of the water column has produced some strikingconvergences in species that hover in open space. For example, the Glass Catfish, Kryptopterus bicirrhus, ofSoutheast Asia is a 10 cm long, transparent silurid with onepair of long barbels that protrude outward from its head,a long anal fin, no adipose fin, a forked tail, and only a single small ray in its dorsal fin (hence the name kryptopterus=“hidden fin”).
It tends to hover tail down in thewater column, often in shoals. Parailia pellucida, the AfricanGlass Catfish, is a 10 cm long transparent schilbeid withfour pairs of long barbels that protrude outward from itshead, a long anal fin, a small adipose fin, a forked tail, andno dorsal fin. It also tends to hover tail down in the watercolumn, often in shoals.
A number of unique modifications occur in the differentfamilies, far too many to detail here. Amongthese traits are accessory air-breathing structures andterrestrial locomotion in airbreathing catfishes, airsaccatfishes, and callichthyid armored catfishes; generation of electric impulsesin the African electric and upside-down catfishes (see Electroreception); climbing ability in climbingcatfishes and jet propulsion in banjo catfishes (see Locomotion: movement and shape); use of lures inangler catfishes (see Pursuit); and mouthbroodingof large eggs in sea catfishes (see Parental care).
The most advanced ostariophysans are the gymnotiforms,which show internal anatomical similarities to thesiluriforms and probably share a common ancestor.However, gymnotiforms are distinct from catfishes and allother ostariophysans, as well as from all other teleostsexcept osteoglossiform mormyrids and gymnarchids, inthat they produce and receive weak electric impulses. Gymnotiformsare known collectively as South American knifefishes because of their strong resemblance to the Africanknifefishes (Notopteridae); the latter have electrogenic relativesamong the mormyrids and gymnarchids but do notproduce electricity themselves.
Gymnotiforms are restricted to Central and SouthAmerica and consist of 134 species in five families. Anatomically,they are characterized by an elongate, compressedbody, an extremely long anal fin that reaches from thepectoral fin to the end of the body, no dorsal or caudal fin,small eyes (and nocturnal foraging), and electrogenic tissuecombined with modified lateral line organs for detectingweak electric fields. The electrogenic tissue is derived frommodified muscle cells in four families, and, curiously, fromnerve cells in the apteronotid ghost knifefishes (Alves-Gomes 2001). Apteronotids also depart from the rest ofthe order by having a distinct tail fin, and include sometruly bizarre species (Fig. 14.13). Electrical output in gymnotiformsis continual at high frequencies, as compared to the pulsed, low-frequency output of mormyrids. The electricaloutput is very weak, on the order of fractions of avolt, except in the Electric Eel (Electrophorus), which putsout a weak field for electrolocation purposes and strongpulses upwards of 600 volts for stunning prey or deterringpredators (Fig. 14.14). Gymnotiforms range in size from9 cm long hypopomid bluntnose knifefishes to the 2.2 mlong Electric Eel.
Two individual Orthosternarchus tamandua, an apteronotid knifefishfrom the Amazon basin. The small black dot on the head is the greatlyreduced eye. These predators occur at depths of 6–10 m where theyfeed on insect larvae (Fernandes et al. 2004). Photo courtesy of C. CoxFernandes.