immunoglobulins, lysozymes, and free fatty acids to create the mucosal defense system (Roberts and Ellis 2012). The epidermis consists of epithelial cells and mucous glands. It is thicker in fish that do not have scales (e.g. true eels, Anguilliformes) (Roberts and Ellis 2012). It differs from mammals in that it lacks keratin and all layers are capable of mitosis, including the squamous layer. The dermis includes scales in most bony fish, as well as chromatophores and mast cells. The chromatophores are clinically important, as color changes in fish can imply different physiologic states (Hoar et al. 1983). Generalized dark coloration may indicate stress or disease, color change on only one side of the body may indicate a visual problem on that side, and color change caudally can help localize the site of a spinal problem such as a vertebral fracture.
The scales are embedded within pockets of the dermal tissue and oriented toward the tail. Scales come in several types that vary in size, shape, and thickness, including placoid, cosmoid, ganoid, cycloid, and ctenoid. Cycloid and ctenoid scales are the most common in teleosts. Some scales are particularly large and thick, such as the ganoid scales of arowana (Osteoglossidae), arapaima (Arapaima spp.), sturgeon (Acipenser spp.), and tarpon (Megalops spp.). Scales can be an impediment for injections, vascular access, and surgical incisions. During injection or venipuncture, care should be taken to pass underneath scales; if a scale is penetrated, it is removed when the needle is pulled out, which can lead to osmoregulatory problems or infections by opportunists such as oomycetes and scuticociliates. For surgical incisions, scales along the incision line may be removed to prevent contamination of the surgical site. Some fish have very fine scales, e.g. some jacks (Carangidae) and tuna (Thunnini), while others are scaleless, e.g. some jacks, true eels (Anguilliformes), catfish (Ictaluridae), and elephantfish (Mormyridae). Scaleless fish may be more vulnerable to toxins in the water, to medications, and to direct trauma from handling (Stoskopf 1993). Some fish have deciduous scales that are shed regularly, e.g. herring and anchovies (Clupeiformes) (Helfman et al. 2009).
Box A1.1 Basic Taxonomy of Extant Fish
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.
Musculoskeletal System
Skeleton
Skeletal bone may be cellular or acellular. Cellular bone is more common in bony fish. Acellular bone (without osteocytes) is seen in perch‐like fish (Percidae) and bass and sunfish (Centrarchidae). In both types, bones are typically solid and calcium absorption cannot occur locally; this means that fractures lack a local calcium reserve for repair. Fish lack bone marrow but there may be some vascular canals and spaces in the bone (Roberts and Ellis 2012). Hyperostosis or pachyostosis (also known as Tilly bones) has been documented in 22 fish families; the most common affected species in aquaria is Atlantic spadefish (Chaetodipterus faber) (Figure A1.2) (Smith‐Vaniz et al. 1995). Hyperostosis is not typically considered pathologic, but if the lesions get large, they may form sequestrae with associated skin ulcerations. Removal or rongeuring the bone underneath has helped resolve signs in some patients.
Teleost skulls are a complicated series of bones; anatomy varies significantly between species. Vertebrae also vary across species. Radiographically, vertebrae usually have a prominent cross that represents the conical recesses enclosing the intervertebral pad, a neural spine, a hemal arch (or pleural ribs cranially), and a hemal spine (Roberts and Ellis 2012). Ribs are either pleural (attached to vertebrae) or intermuscular (within muscular tissue) as in salmonids (Helfman et al. 2009).
Fin shapes and locations vary between species. They may be embedded in musculature or bone. Firm fin spines are common, particularly along the dorsal fin, and present a human health hazard. Some fin spines also contain venom, e.g. lionfish (Pterois spp.) and stonefish (Synanceiidae). Some fins are modified into suckers, e.g. lumpfish (Cyclopteridae). The lobe‐finned fish, lungfish (Dipnoi) and coelacanths (Latimeria spp.), have muscular fins with an articulating