Fish Taxonomy

The family Cyprinidae contains more fish species than any other family of freshwater fish, and many are popular as pet fish. 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.

Internal anatomy, koi

Image

Courtesy of Dr. Denise Petty.

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 and ryukins 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 cause of high mortality in koi and many other cyprinids. Edwardsiella ictaluri is also a problem in zebrafish. Erysipelothrix sp is a frequent cause of bacterial stomatitis in some cyprinids.

Both microsporidia (Pseudoloma), a fungus, and Pseudocapillaria tomentosa, a nematode, are parasites of concern in zebrafish.

Characiformes is an order that contains many important families of fish that are kept as aquarium fish or in public displays. The primary representatives of this order are the tetras (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, columnaris, and Erysipelothrix spp are common bacterial diseases of characins.

Microsporidia (Pleistophora and others) are fungal parasites of concern in many tetras.

Originating from South America, this family contains the popular armored catfish, 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 fish 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 catfish, including the genus Corydoras, belongs to this family. Like the loricariids, the scales of these fish 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 fish 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.

The Cichlidae is a large family that contains many popular aquarium fish, including the freshwater angelfish, discus, and oscar. 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 granulomatous gastritis, and some affected fish become extremely emaciated. Monogeneans are common findings on the gills of tank-raised discus.

The Poeciliidae are another popular family of aquarium fish, 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 fish are adapted to feed primarily at the water surface.

Poeciliids are susceptible to the bacterial diseases F columnare and epitheliocystis.

The Osphronemidae family, which includes gourami, paradise fish, and bettas, is well-adapted to life in shallow, slow-moving to stagnant water. This group of fish 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 fish rise to the air-water interface. The bulk of the oxygen required is obtained via the labyrinth organ, and if these fish are held so they cannot reach the air-water interface, they will drown.

Spironucleus and the endocommensal opalinid Protoopalina are often seen in the intestine.

These fish are susceptible to F columnare.