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Class Agnatha (jawless fish)

 Subclass Cyclostomata (hagfish and lampreys)

Class Chondrichthyes (cartilaginous fish)

 Subclass Elasmobranchii (elasmobranchs)Selachimorpha (sharks)Batoidea (skates, rays, guitarfish, sawfish)

 Subclass Holocephali (chimaeras)

Class Osteichthyes (bony fish)

 Subclass Sarcopterygii (fleshy‐finned fish: lungfish, coelacanths)

 Subclass Actinopterygii (ray‐finned fish)Order Acipenseriformes (sturgeons, paddlefish)Order Polypteriformes (bichirs, reedfish)Infraclass Holostei (gars, bowfins)Infraclass Teleostei (teleosts)

Figure A1.1 Coeliotomy in a porcupinefish (Diodon holocanthus) showing the cut roots of the spines (*).

Source: Image courtesy of Catherine Hadfield, National Aquarium.

Some fish have segmented bony plates within the dermis rather than scales, e.g. seahorses and pipefish (Syngnathidae), shrimpfish (Centriscidae), trunkfish and boxfish (Ostraciidae), and armored catfish or plecostomus (Loricariidae). In these fishes, injections and incisions should be made in softer areas to ensure success, for example on the tail of a seahorse or the peduncle of a boxfish. If plates must be cut, the area should be sealed with a wax product. Pufferfish (Tetraodontidae) have erectable spines derived from scales that have overlapping roots (Helfman et al. 2009); these limit the image quality on radiography and must be cut through for a coelomic incision (Figure A1.1).

Some fish produce copious mucus, including many eels (Anguilliformes), catfish (Siluriformes), and rays (Myliobatiformes). This can be a challenge for handling as they are quite slippery; a chamois or flannel cloth can help restraint without stripping the protective mucus layer. Parrotfish can also create a mucoid casing (cocoon) at night for protection; hand‐net catches of parrotfish are much easier at night than in the daytime.

Epithelial hyperplasia is a common, nonspecific response of fish skin to irritants. This may be multifocal, e.g. the white spots caused by Cryptocaryon irritans. In some fancy goldfish (Carassius auratus), overgrowth of the head epithelium (called the wen or hood) has been genetically selected. This growth can obscure ocular and oral features to the detriment of the animal. It consists of non‐ciliated epithelium with goblet cells covering a mucinous stroma. Surgical management has been described (Angelidis et al. 2009).

The lateral line lies along the body wall of fish. It is a canal within the integument that has pores along its length. In the canal are neuromasts that each have a sensory hair surrounded by gelatinous material. These organs are sensitive to water displacement and vibration (Roberts and Ellis 2012). Lateral line depigmentation (also known as head and lateral line erosion) is a common problem in teleosts.

Transparency is a feature of some fish species. The arrangement of collagen fibers allows light to pass through without reflection. An example that may be seen in the aquarium trade is the glass catfish (Kryptopterus vitreolus).

Bacterial luminescence is seen in the Beryciformes group, e.g. pinecone fish (often Monocentris spp.) and flashlight fish (e.g. Anomalops katoptron) (Hoar et al. 1983). These fish have a single species of bioluminescent bacteria (Photobacterium fischeri) in an organ under the eye (Morin et al. 1975). The bacteria can be obscured by being pulled into or covered by a fold of skin. The biggest clinical implication is that antibacterials can damage the bioluminescent bacteria.