This brief introduction to fish families is intended to provide veterinarians new to fish medicine with information on anatomy and pathogens common to those families. Fishes can be first split into two major groups: the cartilaginous fishes such as sharks and stingrays, and the bony fishes. To further divide them, bony fishes commonly kept as pets or display animals usually belong to one of the two subcohorts of Ostariophysi or Neoteleostei.
For an overview of common parasitic and viral diseases of aquarium fish, see Table: Common Parasitic Diseases of Selected Fish Families and see Table: Common Viral Diseases of Selected Fish Families. Susceptibility of specific families of fish to specific pathogens is implied; however, the absence of mention of a specific pathogen does not mean that other families of fish are refractory.
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Fishes that belong to this subcohort are considered to be basal fishes; this does not mean they are primitive. They have a unique chemical alarm system associated with their skin. If the epithelium is damaged, a material called schreckstoff is released by specialized club cells in the epidermis. It induces a flight response when detected by nearby related fishes. These fish also have cellular bone, which lends itself well to fracture stabilization techniques.
Ostariophysians are physostomous, which means they have a pneumatic duct that connects the anterior lobe of the gas bladder to the dorsum of the esophagus.
Another unique feature of ostariophysians is the presence of a Weberian apparatus, which amplifies sound. This apparatus consists primarily of a small series of auditory bones connected to the gas bladder.
Ostariophysi includes popular aquarium fishes that belong to the families Cyprinidae and Loricaridae and the order Characiformes.
The family Cyprinidae contains more fish species than any other family of freshwater fishes, and many are popular as pet fishes. Cyprinids have a widespread distribution, including Asia and North America. Representative cyprinids include goldfish, koi, barbs, danios, most of the freshwater sharks (eg, redtail, rainbow, and bala), and rasboras.
The primary distinguishing anatomic features of this family are the lack of a stomach and a dual-lobed gas bladder. They also often have a diffuse hepatopancreas, and the posterior kidney is usually positioned between the two lobes of the gas bladder. The absence of a stomach means attention must be given to feeding practices, and the bioavailability of some oral drugs may be lower. Cyprinids benefit from several feedings per day. Related to feeding, buoyancy control is a common problem in fancy goldfish such as orandas, ryukins, etc, due to aerophagia. This can be avoided by feeding sinking feeds.
Mycobacterial disease is a frequent problem in zebrafish (danio) research colonies but can also be seen in other cyprinids. Columnaris disease, caused by Flavobacterium columnare, is frequently a source of high mortality in koi and zebrafish. Edwardsiella ictaluri is also a problem in zebrafish.
Microsporidia (Pseudoloma) is a fungal parasite of concern in zebrafish.
The primary representatives of this order are the tetras, such as neon, cardinal, etc. Unlike the cyprinids, characins have a stomach and pyloric caeca. They usually have a single-chambered gas bladder. Although they are similar to the smaller cyprinids externally, most members of Characiformes have a small adipose fin.
Mycobacteriosis and columnaris are common bacterial diseases of characins.
Microsporidia (Pleistophora) is a fungal parasite of concern in tetras.
Originating from South America, this family contains the popular armored catfishes, the plecos (short for plecostomus) and the small Otocinclus commonly kept in planted tanks. Their scales are modified into plate-like scutes. They usually have a ventrally located mouth. Most are omnivorous, with some being herbivorous. Although some plecos such as the genus Panaque are said to be obligate wood-eaters in the hobby literature, some sources indicate this is not true. Accordingly, these fishes have a long intestine. Many of the plecos are able to obtain oxygen through their stomach after swallowing air at the water surface.
Another group of popular armored catfishes, including the genus Corydoras, belongs to this family. Like the loricariids, the scales of these fishes are modified into armor-like scutes. They have small barbels on either side of the mouth. Axillary glands are located beneath the skin of the shoulder and are the source of the venom that causes punctures by their pectoral fins to be painful. These fishes often scurry to the water surface to ingest air; oxygen is then absorbed via the intestine.
Corydoras spp appear to be very susceptible to infection with F columnare.
This subcohort of fish families contains the more advanced fishes. Unlike the ostariophysians, the members of this superorder do not have alarm signal cells in their epidermis. In addition, these fishes have acellular bone and lack the Weberian apparatus. Most have a single-chambered gas bladder and are physoclistous, which means the gas bladder does not have a connection to the esophagus except at hatching.
Many popular aquarium fishes belong to families in Neoteleostei, such as Cichlidae, Poeciliidae, and Osphronemidae.
This large family contains many popular aquarium fishes, including the freshwater angelfish, discus, oscar, and many more. Most cichlid species originate from Central or South America or Africa. These fish are often very colorful, particularly those originating from the African Rift Lakes. Many cichlid species are aggressive and territorial, requiring substantial cover to minimize injury caused by interspecific aggression. Wild-caught Amazonian cichlids (eg, freshwater angelfish and discus) generally prefer soft water and slightly acidic environments, whereas most African Rift Lake species thrive under hard water and slightly basic conditions.
Cichlids have a single-chambered gas bladder, which often is severely reduced in riverine species from Africa.
Cichlids are usually hardy and well adapted to aquariums. Many species are prone to intestinal parasitism from the flagellate Spironucleus. African cichlids may be infected with Cryptobia iubilans, which causes a granulomatous gastritis, and some affected fish become extremely emaciated. Monogeneans are common findings on the gills of tank-raised discus.
This is another popular family of aquarium fishes, which includes guppies, swordtails, platies, and mollies. Some are euryhaline, such as the guppy and the molly, whereas the swordtail and platy are stenohaline to freshwater. These fishes are adapted to feed primarily at the water surface.
Poeciliids are susceptible to the bacterial diseases F columnare and epitheliocystis.
This family, which includes gourami, paradise fish, and bettas, is well-adapted to life in shallow, slow-moving to stagnant water. This group of fishes has a unique air-breathing apparatus often called a labyrinth organ that consists of plates covered in respiratory epithelium. It is located above the gills in the branchial cavity and is deliberately filled with air when these fishes rise to the air-water interface. The bulk of the oxygen required is obtained via the labyrinth organ, and if these fishes are held so they cannot reach the air-water interface, they will drown.
The endocommensal Protoopalina is often seen in the intestine.
These fishes are susceptible to F columnare.
For an overview of common parasitic and viral diseases of aquarium fish, see Table: Common Parasitic Diseases of Selected Fish Families and see Table: Common Viral Diseases of Selected Fish Families. Susceptibility of specific families of fish to specific pathogens is implied; however, the absence of mention of a specific pathogen does not mean that the other families of fish are refractory.
Last full review/revision October 2015 by Barbara D. Petty, DVM; Ruth Francis-Floyd, DVM, MS, DACZM