a proboscine snout. Many generate weak electric fields to locate objects and for communication (up to 10 V at low frequency) (Roberts 2011). The electric organ is derived from muscle and located in the peduncle. Their cerebellum is large and they show excellent learning skills. They have an otic gas‐bladder that is separate from the swim bladder. Eyes are small and show adaptations to low light, including small reflective cups on the retina. There is only a left gonad. Uniquely, the sperm have no flagellum (Hopkins et al. 2007).
The African and Asian knifefish (Notopteridae) lack many fins (anal, caudal, and pelvic) but have one long dorsal fin. They also generate weak electric fields. They have small eyes and poor eyesight (Nelson et al. 2016). They are not related to the South American or neotropical knifefish (Gymnotiformes).
Perciformes (Perch‐Like Fish)
This is the largest order of fish with a high level of diversity and confusing taxonomy (see Helfman et al. 2009; Nelson et al. 2016). These are “typical fish”. Species that may be commonly seen in display aquariums and aquaculture include the following:
Acanthuridae: tangs, doctorfish, surgeonfish. These have one or more scalpel‐sharp spines bilaterally on the tail that can present a significant hazard during restraint.
Anabantidae: gouramis, bettas. This group uses a suprabranchial organ as the primary breathing apparatus; they need an air space to survive. Males create bubble nests as part of their reproductive strategy. The lips have horny teeth used for scraping algae.
Blenniidae: blennies, cleaner wrasse. These animals often bury. They create water flow over the gills while buried using branchiostegal membranes that form small gill openings and function as a ventral pump, rather than the lateral opercular pumping of other fish. They also have finger‐like projections that keep sand out of the gills.
Carangidae: jacks, dolphinfish, cobia, remoras, lookdowns. Remoras lack a swim bladder and have a unique disc on their head for adhering onto other animals. In some jacks, the swim bladder extends to the end of the peduncle.
Chaetodontidae: marine butterflyfish. These have a spine at the angle of the preopercle. The gastrointestinal tract is coiled many times. The swim bladder has two cranially directed processes.
Cichlidae: cichlids, freshwater angelfish, discus, tilapia. Tilapia are important foodfish (Oreochromis spp.). These fish have a single nare on either side of the face, unlike the paired, rostral nares of most species. These fish have unique and highly organized breeding activities and may be mouthbrooders or substrate brooders. Female discus secrete a milk‐like substance from the skin to feed their fry.
Pomacanthidae: marine angelfish. These have a spine at the angle of the preopercle that can be a hazard during handling (Figure A1.21).
Scorpaeniformes: gurnards, scorpionfish, stonefish, rockfish, lionfish, sculpins. Most lack scales, but scales are ctenoid when present. The swim bladder is absent in some genera (Plectrogenium and Sebastolobus spp.). The gurnards (Triglidae) have huge pectoral fins which expand as they walk along the substrate using their pelvic fins. Most have spines projecting from the head including a caudally directed spine derived from a bone below the eye. In scorpionfish (Scorpaenidae, which includes the lionfish Pterois spp.) and stonefish (Synanceiidae), there are venomous spines along their dorsal, anal, and pelvic fins. Lumpfish (Cyclopteridae) are round with bony tubercles arranged in rows around their body and a sucking disc made up of modified pelvic fins. Most show internal fertilization and some show live births (e.g. Sebastes spp.) (Helfman et al. 2009; Nelson et al. 2016).
Figure A1.21 Preopercular spine on a marine angelfish.
Source: Image courtesy of Catherine Hadfield, National Aquarium.
Pleuronectiformes (Flounder, Sole, Halibut)
Flatfish start their lives with a typical fish body plan (laterally compressed and bilaterally symmetrical) but as they mature, one eye migrates to the top of the skull and the body becomes dorsoventrally compressed. The most typical orientation is dextral (both eyes on the right side, lying on the left side) but even within the same species, sinistral orientation can occur. The eyeless side is ventral and light‐colored while the side with eyes is pigmented. The dorsal fin extends onto the head at the neurocranium in most species, except spiny turbots (Psettodes spp.). There is no swim bladder. Other adaptations include the asymmetrical location of the mouth and protrusible eyes.
Salmoniformes (Salmon, Trout, Char)
Extant species are all within the family Salmonidae. Salmonids are important in aquaculture and fisheries. They are physostomous with small cycloid scales. They have pyloric ceca. There is obvious sexual dimorphism in mature adults, with modified jaws (kypes) seen in mature males (Figure A1.22). Many species migrate between freshwater and saltwater, although not all rely on migration, e.g. rainbow trout (Oncorhynchus mykiss) typically remain in freshwater but some populations, known as steelheads, migrate back to freshwater after spending two to three years in saltwater.
Siluriformes (Catfish)
This group lacks scales but some have bony plates or tubercles. They can produce abundant mucus which is used for cutaneous respiration (Bruton 1996). They have small eyes and up to four pairs of barbels that are chemosensory and tactile (Helfman et al. 2009). In some catfish, the spine‐like rays at the front of the dorsal and pectoral fins have glandular cells that secrete a toxin with variable levels of potency. The pancreas is visible as white nodules (Brockmann bodies). They are physostomous with a bilobed swim bladder, but they are generally negatively buoyant due to the small swim bladder and heavy skull. Males in some families have an anal fin modified into an intromittent organ (Melo et al. 2011). Electric catfish (Malapteruridae) have bilateral electric organs in the skin that can emit up to 300 V at high frequency (Johnels 1956; Helfman et al. 2009).
Syngnathiformes (Seahorses, Sea Dragons, Pipefish, Trumpetfish, Shrimpfish)
These were historically classified within the Gasterosteiformes. The seahorses, sea dragons, and pipefish are within the family Syngnathidae. They have thick dermal scutes or plates. Seahorses have prehensile tails and lack caudal fins. Gills are unique with lobate gill filaments and a spherical appearance; there is a small opening on the top of the head instead of an operculum, limiting access to the gills. The oral cavity is a tube with an upturned mouth. The esophagus makes a sharp curve downward at the neck which requires gentle manipulation to get past when gavaging. Some species are aglomerular, others have some glomeruli (Stoskopf 1993; Koldewey 2005). The male maintains the brood in a pouch or on a patch at the base of the tail (Helfman et al. 2009). In the male, the elongated genital opening caudal to the anal fin represents the opening to the brood pouch; the anal opening is cranial to the anal fin. In females, no patch or pouch is present and the reproductive and anal openings are both cranial to the small anal fin.
Tetraodontiformes (Pufferfish, Triggerfish, Filefish, Ocean Sunfish)
The order is named for the four “teeth” in the outer jaws of pufferfish; these teeth are actually sharp‐edged jaw bone. They lack ribs. The skin is thick, tough, and covered by scales that are modified into spines, bony plates, or ossicles. The first dorsal spine is prominent, particularly in the triggerfish and filefish. The base of the second spine fits into a groove in the first spine, locking it. They can create grunting noises via vibration of the swim bladder by the pectoral spines. The eyes can rotate independently. Puffers can