Amiiformes (Bowfin)
The bowfin (Amia calva) is the only extant species of this order. They are freshwater predatory fish with long, sharp teeth, and two protruding tube‐like nares. They have spiracle remnants and their skeleton is predominantly cartilaginous. They are physostomous and the swim bladder can function as a lung. There is a rudimentary spiral valvular intestine. Scales are cycloid rather than the ganoid scales typical of similar “primitive” fish. Kidney tubules open directly into the coelomic cavity. They are often mistaken for burbot (Lota lota) or snakeheads (Channidae) (Helfman et al. 2009; Nelson et al. 2016).
Anguilliformes (Eels)
Most eels are within the Anguilliformes. Important families include freshwater eels (Anguillidae) and moray eels (Muraenidae). Some freshwater eels are important food fish, e.g. European eel (Anguilla anguilla), short‐finned eel (Anguilla australis), Japanese eel (Anguilla japonica), and American eel (Anguilla rostrata). These have complicated life cycles including freshwater and marine life stages (catadromous, moving from freshwater to saltwater to spawn) and long migrations. Some moray eels are common in display aquaria, e.g. green morays (Gymnothorax funebris) and zebra morays (Gymnomuraena zebra). They lack a pelvic girdle and most also lack a pectoral girdle. Scales are typically absent, but if present, they are cycloid and embedded. Moray eels have two sets of jaws: oral and pharyngeal (Figure A1.8). Eels can inflict a painful bite on handling. The gill opening is narrow and visualization of gills is difficult without endoscopy. The swim bladder has a single lobe (Trischitta et al. 2013). Radiographs have been used for diagnosis of swim bladder nematodes in Anguillidae (Palstra et al. 2007).
Beryciformes (Soldierfish, Squirrelfish, Pinecone Fish, Flashlight Fish)
These marine fish often have deep bodies and large eyes. They are prone to ocular disease, specifically gas bubbles. They tend to be nocturnal or cave dwellers. Soldierfish and squirrelfish are often dark red in color. Pinecone (Monocentridae) and flashlight fish (Anomalopidae) have bioluminescent bacteria (Photobacterium fischeri) in pockets of skin or in light organs near the eyes. The bioluminescent bacteria are susceptible to antibacterials. Some species have spinoid scales, where the trailing edges of scales have spines (Hoar et al. 1983; Nelson et al. 2016).
Characiformes (Piranha, Tetras, Hatchetfish)
Of these, tetras and hatchetfish are common in public and home aquariums, e.g. neon tetras (Paracheirodon innesi). Characiformes, along with Cypriniformes, Siluriformes, and Gymnotiformes have a Weberian apparatus (bones which connect the swim bladder to the inner ear). They have ctenoid or ctenoid‐like scales, apart from the naked tetra (Gymnocharacinus bergii) which lacks scales. Feeding specializations of this group are diverse, including detritivory, herbivory, planktivory, predatory; some have well‐developed teeth (Helfman et al. 2009).
Clupeiformes (Herrings, Anchovies)
This group includes some of the most important forage and food fish. They tend to be silvery with a consistent body plan. They have an unusual swim bladder that extends from the inner ear (otophysic) to the anus and into the lateral lines. This increases sensitivity to low‐frequency sounds from other fish (Helfman et al. 2009). They are schooling and may be seen in aquaria as “balls” of fish.
Cypriniformes (Carp, Goldfish, Koi, Minnows, Zebrafish, Algae‐Eaters, Loaches)
This large group of mostly freshwater fish includes several species that are common in the home aquarium or pond including goldfish (Carassius auratus) and common carp (Cyprinus carpio), of which koi are a subspecies (Cyprinus carpio koi). Carp are also important foodfish. Zebrafish (Danio rerio) are common aquarium and research animals. The mouth, jaws, and palate are toothless but there are pharyngeal teeth in most (except algae‐eaters) with a basioccipital grinding pad or “carp stone” on the dorsal pharynx. These stones are shed periodically and are often noted by owners. Algae‐eaters, loaches, and suckers have oral modifications that allow them suck onto rocks and scrape algae. There is no stomach; the esophagus appears to go straight into the intestine. They have a Weberian apparatus. They are physostomous with two‐chambered swim bladders, although many fancy goldfish have lost the caudal lobe (Roberts and Ellis 2012; Nelson et al. 2016; Roberts‐Sweeney 2016).
Elopiformes (Tarpon, Ladyfish)
Elopiformes have an upturned mouth with a long gular plate between the lower jaws. Scales are cycloid and particularly large and thick in tarpon (Megalops spp.), making phlebotomy and injections difficult (Helfman et al. 2009). The swim bladder is lung‐like and tarpon are facultative air‐breathers. Tarpon have additional cranial outgrowths of the swim bladder that contact the auditory region and are partially enclosed in bony bullae, which presumably improve their sense of hearing (Fletcher and Crawford 2001).
Gobiiformes (Gobies, Mudskippers)
Previously classified within Perciformes, gobies are predominantly marine and benthic or burrowing. They lack swim bladders. Mudskippers are amphibious fish. Adaptations include a thickened epidermis with mucus cells, large gill chambers that can hold water, modified fins to allow terrestrial locomotion, and air‐breathing across the cutaneous and pharyngeal epithelium. They are also able to simultaneously see out of water as well as in water.
Lepisosteiformes (Gars)
These are large, carnivorous, predominantly freshwater fish. The body and jaws are long with mouths that have needle‐like teeth. They have heavy rhomboid, interlocking ganoid scales. The swim bladder is heavily vascularized for air‐breathing (Figure A1.14) (Nelson et al. 2016). The vertebral centra are opisthocoelous (concave on their caudal surface, convex on the cranial, creating a ball‐and‐socket joint) rather than the amphicoelous centra of most other fish (concave on both surfaces) (Helfman et al. 2009).
Lophiiformes (Frogfish, Anglerfish)
This group shows a specific feeding mechanism where the first dorsal spinous process is modified into a line/rod and lure/bait (illicium and esca) to capture prey. The illicium and esca can be regenerated if damaged. Their bodies are stocky with large upward‐pointed mouths to accommodate surprisingly big prey. They lack an operculum and instead have an opening behind the pectoral fins which facilitates propulsion. The swim bladder is physoclistous in some and missing in other species (Pietsch 1984; Nelson et al. 2016). Under managed care, they can be particularly hard to convert onto frozen‐thawed foods and many individuals will only feed on live foods.
Osteoglossiformes (Arowana, Arapaima, Freshwater Butterflyfish, Elephantfish, Knifefish)
Known as the “bony tongues”, they have well‐developed teeth on the tongue and roof of the mouth that provide a shearing bite. The intestine curls around to the left of the esophagus and stomach; in most fish, it passes to the right (Nelson et al. 2016). They possess one or two pyloric ceca.
Arapaima (Arapaima spp.) are some of the largest freshwater fish in the world and have large, corrugated scales which complicate venipuncture and injections. They are obligate air‐breathers.
Freshwater butterflyfish (Pantodon buchholzi) have greatly enlarged pectoral fins just dorsal to their large pelvic fins. They have a swim bladder and are obligate air‐breathers. They are not related to marine butterflyfish.
Elephantfish