Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 16

The external layer of mucus is rich in mucopolysaccharides, 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).