Corpuscles of Stannius are endocrine cells in the caudal kidney of bony fish. They act like parathyroid glands and secrete teleocalcin (also called hypocalcin) which blocks calcium absorption across the gills (Roberts and Ellis 2012). Ultimobranchial bodies are endocrine cells located ventral to the esophagus or in the septum that separates the heart from the coelom. They also act like parathyroid glands and produce calcitonin for calcium regulation (Stoskopf 1993; Roberts and Ellis 2012)
It is reported that the pseudobranch may have some endocrine functions (see above). The gonads can also be considered part of the endocrine system as they secrete androgens and estrogens.
Urogenital System
Urinary System
The kidneys have hemopoietic, reticuloendothelial, endocrine, and excretory functions. They are located retrocoelomically and vary from parallel, paired structures to a fused single structure. Freshwater fish have larger kidneys (Helfman et al. 2009). In bony fish, kidneys are divided into a cranial (head) kidney and a caudal (excretory) kidney (Figure A1.15). Lymphoid tissue predominates in the cranial kidney; thyroid tissue may also be found (Stoskopf 1993). Excretory function is primarily in the caudal kidney. Ureters move urine from the collecting ducts to either urinary papilla or a urinary bladder where water resorption occurs. This bladder is not homologous to the mammalian bladder as it develops from the distal ureter and is not mesodermal in origin (Stoskopf 1993).
Internal body fluid composition and maintenance is complex and environmental circumstances play a huge role in how fish manage osmoregulation. Fluid exchange occurs at the gill, gastrointestinal tract, and kidney. In freshwater teleosts, osmoregulation is marked by a large influx of water from the environment and production of a large volume of dilute urine (Table A1.2). Sodium is taken up by the gills in exchange for protons in a process that is pH‐dependent. Marine teleosts do drink water but do not absorb water through the gastrointestinal tract; instead sodium and chloride tend to diffuse into the fish causing salt overload. The salt is mostly excreted by the gills; the kidneys are more involved in excretion of magnesium and sulfate. Marine teleosts have fewer glomeruli than freshwater teleosts and some marine teleosts are aglomerular, e.g. some seahorses (Syngnathidae), toadfish (Opsanus spp.), and goosefish (Lophiidae). Marine teleosts are also missing the distal segments of the nephron, including the loop of Henle. They cannot concentrate their urine above their blood osmolality and are prone to dehydration. In euryhaline species, the urinary bladder changes permeability based on environmental osmolality and regulates sodium and chloride removal. Some fish migrate between freshwater and marine environments. Fish that move from freshwater to saltwater to spawn are catadromous, e.g. true eels (Anguillidae). Fish that move from saltwater to freshwater to spawn are anadromous, e.g. salmonids (Salmonidae), sturgeon (Acipenseridae). For a full review, readers are directed to Hoar and Randall (1969), Hoar et al. (1983), Stoskopf (1993), and Evans et al. (2004).
Figure A1.15 Kidneys in a deacon rockfish (Sebastes diaconus): cranial kidney (Cr Kd), caudal kidney (Ca Kd), opened swim bladder (SB) showing the rete, and liver (L).
Source: Image courtesy of Catherine Hadfield, Seattle Aquarium.
Table A1.2 Fluid and electrolyte balance in freshwater and marine bony fish.
| Freshwater bony fish | Marine bony fish |
|---|---|
| Hypertonic compared to the environment | Hypotonic compared to the environment |
| Do not drink water | Drink water |
| Active excretory process | Passive excretory process |
| Excrete large volumes of dilute urine | Excrete small volumes of urine |
| Ions maintained by gill and gastrointestinal uptake | Gills excrete Na and Cl. Urine is similar to plasma |
| Easily become dehydrated |
Urine collection is possible but not easy. In most species, urine flow is low and continuous, limiting opportunities for collection via traditional veterinary techniques (e.g. percutaneous or catheter passage). Catheter implantation or surgically fitted collection devices are possible, but the diagnostic value of these samples is not understood (Stoskopf 1993).
Reproductive System
The reproductive system is highly variable and complex. Fish usually show separate sexes (producing sperm or ova), but hermaphroditism and parthenogenesis occur (Sloman 2011). Some bony fish show sexual dimorphism (Table A1.3). Some of the skin changes can be mistaken for pathology. Hermaphroditism is particularly common within Perciformes, namely parrotfish (Scaridae), wrasses (Labridae), damsels (Pomacentridae), and gobies (Gobiidae) (Sloman 2011). Hermaphroditism may be sequential or simultaneous. Sequential hermaphroditism can be divided into protandry (males become females, e.g. gilthead seabream [Sparus aurata] and clownfish [Amphiprioninae]), and protogyny (females become males, e.g. Indo‐Pacific cleaner wrasse [Labroides dimidiatus]). Simultaneous hermaphroditism is defined by capacity to release viable eggs or sperm during the same spawning, e.g. hamlets (Serranidae). Parthenogenesis is less common, but examples are found, e.g. Amazon molly (Poecilia formosa) where all individuals are females.
Typical teleost testes are elongated, paired, and either lobular (most common) or tubular (Roberts and Ellis 2012). They are supported in the coelom by a mesorchium and surrounded by a tunica albuginea. They can be hard to differentiate grossly from ovaries in young animals, but wet mounts of the tissue can be diagnostic. The ducts can serve for both sperm transport and storage (Schulz and Nobrega 2011).
Teleost ovaries vary from clusters of follicles to a more organized organ that may be paired, fused, or coiled. They are supported within a thin mesovarium. Mature ovaries can take up a substantial amount of the coelom (up to 70%). Ovaries in bony fish are typically paired, but they are fused in some species, e.g. lampreys and hagfish (Agnatha), mollies and guppies (Poecilia spp.), and medaka (Oryzias sp.). There are three types of ovary (Table A1.4). In gars (Lepisosteidae) and most teleosts, the ovary is continuous with the oviduct, while in trout and salmon (Salmonidae), the oviduct is diminished or even absent and ova enter the coelomic cavity before exiting the genital pore (Helfman et al. 2009; Roberts and Ellis 2012). In live‐bearing species, embryos develop in either the oviduct or the uterus (Turner 1947).
Fertilization occurs externally or internally (Table A1.5). Under human care, normal reproduction may be inhibited or altered which may result in health problems such as egg retention and oophoritis.
The reproductive systems of fish are enormously varied and important additional details can be found elsewhere (Stoskopf 1993; Farrell 2011; Wootton and Smith 2014).