Many fish species, particularly freshwater teleosts, are also able to show aquatic surface respiration (ASR). When dissolved oxygen is low, they come to the surface to skim the air/water interface because of its higher oxygen content.
Cardiovascular System
All bony fish have a two‐chambered heart with four distinct anatomical regions. Blood flows through the sinus venosus into the atrium, then the ventricle, and out the bulbus arteriosus to the ventral aorta, the gills, the dorsal aorta, and the organs (Stoskopf 1993). The atrium and ventricle have partitions in the lungfish (Dipnoi) and coelacanth (Latimeria spp.). Teleosts have a pericardial sac filled with serous fluid.
Renal and hepatic portal systems are present in most fish although the proportion of blood that passes through the portal system varies by species. This may impact pharmacokinetics and toxicity if drugs that may be modified or excreted by the liver or kidneys are given in the caudal half of the body (Stoskopf 1993).
The lymphatic system is curious. In some teleosts, there is a fluid system separate from the primary circulation that is considered a lymphatic system (with leukocytes and devoid of erythrocytes). A secondary vascular system (SVS) that is not a lymphatic system has also been well‐described (Steffensen and Lomholt 1992). This SVS is larger in volume than the primary circulation and has similar constituents to plasma but lacks most cellular components. Circulation rates are lower in the SVS, sometimes by hours (Roberts and Ellis 2012). Stress, hypoxia, and exercise alter the volume and cellularity in each system, most importantly resulting in a hematocrit change of the primary system (Rummer et al. 2014). The primary and secondary systems are connected with anastomoses, in contrast to other vertebrate classes. The secondary system has a role in gas and ion exchange.
Venipuncture in bony fish usually makes use of the ventral tail vessels, via a lateral or ventral approach. However, size, anatomy, or disease may necessitate other sampling sites. Other possible sites that can be used in fish are listed below.
Figure A1.14 Air‐breathing structures: modified pharyngeal mucosa in an electric eel (Electrophorus electricus) (a), modified swim bladder of a longnose gar (Lepisosteus osseus) (b), and a true lung seen on lateral radiograph of an African lungfish (Protopterus annectens) (c).
Sources: Image (a) courtesy of Catherine Hadfield, National Aquarium. Image (b) courtesy of Carlos Rodriguez, Disney’s Animals, Science and Environment.
Gill arch (Suedmeyer 2006)
Peduncular notch
Retro‐orbital sinus via oral cavity (Zang et al. 2013)
Cardiac
Cloaca superficial vessels (especially elasmobranchs)
Dorsal fin sinus (elasmobranchs)
Pectoral fin/radial vessels and mesopterygial vein (elasmobranchs)
Venipuncture is described in more detail in Chapter A6.
The gross appearance of blood varies in some groups. Blood and serum are blue‐green due to biliverdin in humphead wrasse (Cheilinus undulatus) and Japanese eels (Anguilla japonica) (Fang and Bada 1990). The blood of icefish (Channichthyidae) is clear as they lack hemoglobin (Sidell and O'Brien 2006). Pale tan to brown blood can be due to methemoglobinemia, often caused by high nitrites (Mirghaed et al. 2017).
Lymphomyeloid System
Fish lack lymph nodes, Peyer's patches, and bone marrow. Hematopoiesis in bony fish occurs primarily in the spleen and the cranial kidney with some activity in the liver, thymus, and pericardium in some species (Roberts and Ellis 2012). Additional lymphoid activity occurs in the mucosa‐associated lymphoid tissues (MALT). The gastrointestinal tract, gills, and skin act as pathogen barriers and contain local leukocyte populations. Another cell, the melanomacrophage, is present ubiquitously and increases in the presence of antigen stimulation. Pigmented macrophage aggregates (PMA), also known as melanomacrophage centers (MMC), are solid foci of these cells and found in high numbers in the liver, spleen, and cranial kidney (Roberts and Ellis 2012). They can be seen on wet mounts of these organs examined under direct microscopy and can increase with chronic inflammatory diseases.
The spleen is dark red to black and has sharp edges. There are multiple spleens in some fish and no distinct spleen in hagfish and lampreys (Agnatha).
The thymus can be very hard to find. Presence and size vary greatly among teleosts and it does not follow the mammalian involution pattern (Roberts and Ellis 2012). It may be found subcutaneously at the dorsal edge of the operculum, at the base of the gill arches, associated with pharyngeal epithelium, or within the cranial kidney (Stoskopf 1993). One pair of organs is typical, but some fish (e.g. clingfish, Gobiesocidae) have two pairs (Bowden et al. 2005).
Endocrine System
The endocrine system resembles that of other vertebrate classes but has some features with no mammalian counterparts such as the urophysis and the corpuscles of Stannius.
The pituitary (hypophysis) has many endocrine functions including the secretion of hormones (GnRH, TSH, vasopressin, etc.). A saccus vasculosus is associated with the hypothalamus and helps detect seasonal changes (Nakane et al. 2013). The pineal organ (epiphysis) is light‐sensitive and lies between the midbrain and dorsal forebrain; it can be seen through the cartilage in young fish. The urophysis is a thickening of the caudal spinal cord and has neurosecretory cells with an osmoregulatory function (Stoskopf 1993; Roberts and Ellis 2012).
The thyroid gland is usually diffuse, with follicles along the ventral aorta, branchial arteries, pharyngeal cavity, and retro‐orbital tissues. Thyroid hyperplasia (goiter) is common in fish and can cause oral or respiratory obstruction.
Pancreatic tissue (exocrine and endocrine) in bony fish is typically diffuse throughout the adipose tissue or along the portal veins (Caruso and Sheridan 2011). It is occasionally seen as white nodules (Brockmann bodies) in the mesentery around the bile ducts and portal veins (Caruso and Sheridan 2011). In some fish, the pancreas is associated with the venous system or capsule of the spleen. It is a discrete organ in the lungfish (Dipnoi) and some catfish (Stoskopf 1993).
In most fish, interrenal tissue is the equivalent of the mammalian adrenal cortex. It is found within the cranial kidney or around the posterior cardinal veins