to pathogens and environmental toxins. Since gills have metabolic and excretory functions, damage can subject the fish to respiratory and osmoregulatory challenges (e.g. dehydration) (Hughes and Morgan 1973).
Figure A1.11 Ultrasound of the spiral intestine of an African lungfish (Protopterus annectens).
Gills have the capacity to regenerate, but the extent and time line are not well‐characterized in all fish. It is suggested to occur within one to two weeks of an insult and take about two more weeks (Tzaneva et al. 2014).
Bilateral pseudobranchs lie dorsocranially to the gill arches in most teleosts, with the exception of the catfish (Siluriformes), a few eels (Anguilliformes), African knifefish (Gymnarchus niloticus), and spiny loaches (Cobitis spp.) (Helfman et al. 2009). There are three types: free (exposed); covered (with subtypes, see Bertin 1958); and glandular. Exposed pseudobranchs look like a partial gill arch (Figure A1.13) and have direct contact with the water; they can be seen in some perch‐like fish (Percoidei) and many marine fish. Covered pseudobranchs resemble a gill arch but are covered with a membrane; they can be seen in cyprinids and salmonids. In the glandular type, the pseudobranch is deep within the connective tissue of the opercular cavity or buccopharynx. This is seen in some gouramis (Anabas spp.), snakeheads (Channa spp.), and featherbacks (Notopterus spp.) (Bertin 1958; Laurent and Dunel‐Erb 1984).
Figure A1.12 Normal gills seen during necropsy of a sweetlips (Plectorhinchus sp.) showing the gill rakers and primary filaments.
Source: Image courtesy of Carlos Rodriguez, Disney’s Animals, Science and Environment.
Figure A1.13 Exposed pseudobranch (arrow) in a soldierfish (Myripristis sp.).
Source: Image courtesy of Catherine Hadfield, National Aquarium.
The pseudobranch delivers oxygen to the choroid of the eye via the carbonic anhydrase pathway and thus is suspected of regulating intraocular oxygen and pressure; a process that depends on hydrostatic and osmotic pressure, pH, and pCO2 (Roberts and Ellis 2012). Some also deliver oxygen to the vascular rete of the swim bladder. There is a proposed osmoregulatory function (Na+ and Cl− secretion and excretion) and a glandular function that is poorly understood. Pseudobranch surgery can be performed where ocular gas is not responding to medical management. This may involve surgical removal, ablation, or cauterization (Harms and Lewbart 2000).
Most bony fish have opercula over the gills. Opercular appearance is variable, e.g. in sturgeons (Acipenser spp.) the operculum does not fully cover the gill filaments, while in marine angelfish (Pomacanthidae) the operculum has a spike that is a potential human health hazard. Opercular flaring or curling can be due to egg incubation temperature, genetics, trauma, or nutritional issues (Branson 2008). Consequences of this vary from purely cosmetic defects to gill damage. Some bony fish lack opercula with water instead flowing out from the gills through a slit in the skin. This is seen in triggerfish (Balistidae), eels (Anguilliformes), frogfish (Antennariidae), lumpfish (Cyclopteridae), and seahorses, sea dragons, and pipefish (Syngnathidae). This limits visibility of the gills; otoscopes or endoscopy are often needed for examination and biopsy.
Fish use pressure changes to move water over the gills from the mouth. Some fish need to swim to create the pressure gradient (e.g. pelagic sharks, tuna), and it is essential that additional ventilation is provided to these species when they are under manual or chemical restraint.
Air‐breathing can occur in bony fish. This allows fish to deal with low dissolved oxygen levels in water, but it also allows a few fish species to survive brief periods out of water (as long as hydration is appropriate) or to avoid other dissolved gases such as hydrogen sulfide. Two general strategies exist: obligate and facultative air‐breathing (Table A1.1). Obligate air‐breathers have rudimentary gills, but do not maintain enough gas exchange across the gills for respiration without access to air (Feder and Burggren 1985; Graham 1997). If air access is restricted (e.g. if kept underwater for anesthesia), these fish cannot ventilate. Facultative air‐breathers are able to maintain gas exchange across the gills and make use of air‐breathing when dissolved oxygen is low. Air‐breathing organs can include surfaces of the buccal/pharyngeal cavity, digestive tract, swim bladders, and lungs (Figure A1.14). The skin also serves a respiratory function in many fish, including many larval stages. Air is directly absorbed across the epithelium and into the bloodstream. For a detailed review of air‐breathing in fish, see Graham 1997.
Table A1.1 Obligate and facultative air‐breathers.
Source: Graham (1997). © 1997, Elsevier.
| Species | Type of air‐breathing | Site of gas exchange |
|---|---|---|
| Gouramis, bettas (Anabantoidei) | Obligate | Modified epibranchial and suprabranchial chambers (labyrinth organ) |
| African bichirs or reedfish (e.g. Polypterus spp.) | Obligate | Lung (modified swim bladder) |
| African knifefish or aba (Gymnarchus niloticus) | Obligate | Swim bladder |
| Freshwater butterflyfish (Pantodon buchholzi) | Obligate | Swim bladder |
| Arapaima (Arapaima gigas) | Obligate | Lung |
| Electric eel (Electrophorus electricus) | Obligate | Pharynx |
| Mudskippers (e.g. Periophthalmus spp.) | Obligate | Skin and pharynx |
| Snakeheads (Channidae) | Obligate | Labyrinth organ |
| Weather loach (Misgurnus spp.) | Obligate | Intestine |
| Swamp eel (Monopterus cuchia) | Obligate | Pharynx |
| Lungfish (Protopterus aethiopicus, Protopterus amphibius, Protopterus annectens, Protopterus dolloi, Lepidosiren paradoxa, Neoceratodus forsteri) |
Facultative in Australian lungfish (Neoceratodus forsteri);
|