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The swim (gas) bladder of bony fish shows extensive variations. Its primary function is buoyancy, but it can also be important in sound production and pressure reception (Roberts and Ellis 2012). It is absent in cartilaginous fish (chimaeras, skates, rays, sharks), some bottom‐dwelling teleosts (e.g. flounder, Pleuronectiformes), weather loaches (Misgurnus spp.), and some highly pelagic teleosts like tuna (Thunnini). The swim bladder is filled with oil or fat in some bathypelagic species, e.g. lanternfish (Myctophidae) and orange roughy (Hoplostethus atlanticus) (Blaxter and Batty 1990; Phleger 1998). The volume of the swim bladder compared to body weight is typically under 5% in saltwater fish and under 7% in freshwater fish (Blaxter and Batty 1990). The gas in the bladder is composed of carbon dioxide, oxygen, and nitrogen, but not in the same percentages as air (Helfman et al. 2009).

Two types of swim bladder exist: physostomous and physoclistous. In physostomous fish, there is a pneumatic duct that connects the swim bladder to the esophagus. The gas is maintained by swallowing air. This anatomy has its disadvantages in that foreign bodies or gavaged food may enter the swim bladder (Stoskopf 1993). Common physostomous fish are salmon and trout (Salmonidae), catfish (Siluriformes), koi and goldfish (Cyprinidae), and tetras (Characidae), although some of these species also have a rete mirabile for some gas absorption.

Physoclistous fish lack a connecting duct. Inflation occurs via blood gases diffusing along one or more rete mirabile (gas glands) or vessels in the swim bladder, typically located cranially and ventrally (Figure A1.4) (Pelster 2011; Roberts and Ellis 2012). Physoclists include most marine teleosts, cichlids (Cichlidae), bass and sunfish (Centrarchidae). Some of these species also possess a capillary plexus caudodorsally (the oval or oval window) that helps resorb gases (Pelster 2011; Roberts and Ellis 2012).

Familiarity with normal swim bladder appearance is important to evaluate abnormalities on diagnostic imaging, endoscopy, coeliotomy, or necropsy. These may include hyperinflation, hypoinflation, displacement, and fluid or parasites. If injecting air to manage hypoinflation, a knowledge of normal volume is also important to prevent hyperinflation.

 One lobe is most common. It may be U‐shaped (e.g. some pufferfish, Tetraodontidae).

 Two lobes are found in various species (Figure A1.5a), including goldfish (Carassius auratus) and common carp and koi (Cyprinus carpio), although some goldfish breeds have lost the caudal lobe.

 Three lobes are found in cod (Gadus spp.), channel catfish (Ictalurus punctatus), and some pufferfish (Arothron spp.) (Figure A1.5b).

 Extensions are common. The swim bladder may connect to the inner ear (e.g. herrings and anchovies, Clupeiformes), extend into vertebrae (e.g. freshwater angelfish, Pterophyllum spp.), or extend down the tail (e.g. electric eel, Electrophorus electricus, and arowana, Osteoglossidae) (Figure A1.5c).

 The swim bladder may be modified into lungs or lung‐like tissues (e.g. garfish, tarpon, arapaima, and lungfish, see below).

Figure A1.4 Intact (a) and incised (b) swim bladder of a bluestriped grunt (Haemulon sciurus) showing the rete mirabile.

Source: Image courtesy of Carlos Rodriguez, Disney’s Animals, Science and Environment.

Figure A1.5 Swim bladder variations including two lobes in a spangled grunter (Leiopotherapon unicolor) (a), three lobes in a map puffer (Arothron mappa) (b), and extension down the tail in an electric eel (Electrophorus electricus) (c).