be distinctly separate. At copulation, sperm ejaculate through the urogenital papilla(e) and travel to the dorsal groove of each clasper. Batoids have clasper glands with many proposed functions (Hamlett et al. 2005). There are subdermal siphon sacs in sharks that are either meant for propulsion of sperm or to wash rival sperm from a female's tract. These can be prominent on ultrasound.
Surgical amputation of the claspers has been performed when necessary. This may not impact reproductive capacity as it is a transmission organ and sperm would still be active and available at the cloaca.
Ovaries in elasmobranchs are typically paired but may be single. When single, or where one is much larger than the other, the left ovary is dominant in batoids and the right in viviparous sharks. The ovaries are attached to the dorsal wall of the body cavity by a mesovarium. Ovaries have developing follicles of various sizes, follicles undergoing atresia, and corpora lutea‐like tissue, all embedded together in connective tissue. The ovary can be within or separate from the epigonal organ (Hamlett et al. 2005). Hormones and functionality of the organs are highly varied and often differ from the mammalian model; see other references for details (e.g. Awruch 2016).
Table A1.6 Reproductive methods of elasmobranchs
Sources: Hamlett et al. (2005) and Castro et al. (2016). © John Wiley & Sons.
| Reproductive method | Nutrition | Examples | |
|---|---|---|---|
| Single: one egg per oviduct | Oviparous | Lecithotrophic | Bullhead sharks (Heterodontiformes), most skates (Rajidae), chimaeras (Holocephali) |
| Multiple: several eggs in oviduct | Oviparous | Lecithotrophic | Some catsharks, e.g. Halaelurus spp. |
| Yolk‐sac (ovoviviparous) | Viviparous | Lecithotrophic | Most sharks |
| Limited (mucoid) histotrophy | Viviparous | Matrotrophic | Dogfish and lanternsharks (Squaliformes), angelsharks (Squatiniformes), sawsharks (Pristiophoriformes) |
| Lipid histotrophy | Viviparous | Matrotrophic | Rays (Myliobatiformes) |
| Oophagy (ovatrophy) | Viviparous | Matrotrophic | Some ground sharks, e.g. Gollum, Pseudotriakis spp. White, mako, and mackerel sharks (Carcharodon, Isurus, Lamna spp.) |
| Embryotrophy | Viviparous | Matrotrophic | Tiger sharks (Galeocerdo cuvier) |
| Adelphotrophy | Viviparous | Matrotrophic | Sand tiger sharks (Carcharias taurus) |
| Placental (placentotrophy) | Viviparous | Matrotrophic | Higher requiem sharks (Carcharhiniformes) |
Each oviduct is differentiated into an ostium (that receives the ovum), oviducal (nidamental or shell) gland, and isthmus (in some species) which then leads to the uterus, cervix, and a common urogenital sinus. The uterus can be single or paired. When single, it is usually located on the left side of the body. In oviparous species, the uterus hardens the egg capsules and holds them until oviposition. In yolk‐sac viviparous species, the uterus creates an intrauterine milieu, supplying oxygen, water, and nutrients to the embryo and regulating wastes. The uterine wall is vascularized and folded. In stingrays, there are secretory cells within uterine trophonemata (large villous projections) that also produce a milieu (histotroph) which provides nutrients. All placental species also have a limited histotrophic stage after absorption of the yolk sac and before placental implantation where the uterus provides nutrients (Hamlett et al. 2005). Since placentation is a complex subject, the reader is directed elsewhere (Hamlett et al. 2005).
Neurologic System
The elasmobranch cranium encloses the brain, olfactory bulbs, and optic and otic organs. Similar to bony fish, the brain is composed of the telencephalon (forebrain); diencephalon (epithalamus, thalamus, and hypothalamus); and mesencephalon (with the cerebellar body and auricles and the medulla) (Northcutt 1977; Hofman 1999; Shuttleworth 2012).
The brains of elasmobranchs vary widely in shape and size. Some groups show an obvious asymmetry within the cerebellum, including rays (Myliobatiformes), requiem sharks (Carcharhinidae), and hammerhead sharks (Sphyrnidae). This asymmetry is notable on gross examination and should not be assumed to be pathological (Northcutt 1977). Cranial nerves are similar to other vertebrates.
The choroid plexus forms the cerebrospinal fluid (CSF); there is a large space rostral to the brain where this fluid can be collected. The electrolyte composition of CSF and plasma is different and regulated independently. The blood–brain barrier is different from other vertebrates in that the endothelium is permeable but there are tight junctions between glial cells (Bradbury 1979; Shuttleworth 2012).
Electric Organs
Electric (torpedo) rays (Torpediniformes) have large electric organs (electroplaques) on either side of the gill arches derived from branchial musculature that can produce moderate electric discharges (Shuttleworth 2012). Skates (Rajiformes) can produce weak electric discharges from small bilateral electric organs in the tail (Smeets et al. 2012). Both groups are capable of intermittent and focused discharges (Sisneros and Tricas 2002). As with teleosts capable of electric discharges, the co‐ordination of this discharge originates in the medulla (Sisneros and Tricas 2002; Smeets et al. 2012). The relatively low amplitude of 20–40 mV from skates contrasts with 30–60 V from electric rays (Sisneros and Tricas 2002).
Taxonomy
There is limited literature in the veterinary field on fish taxonomy, yet taxonomy relates directly to the anatomy, physiology, and disease susceptibility of these species. The following is a brief description of some of the more important fish orders and their key features. Details on classification can be found elsewhere; this section will focus on attributes that may impact clinical interpretation of disease processes and diagnostic findings. See Nelson et al. (2016) or Helfman et al. (2009) for additional details.
Taxonomy of Bony Fish (Osteichthyes)
Acipenseriformes (Sturgeons, Paddlefish)
Sturgeons and paddlefish are large, long‐lived, freshwater fish. They have a heterocercal fin with a large dorsal lobe similar to sharks and a reduced or absent preopercle (part of the operculum) such that some of the gills are grossly visible. They have spiracles, similar to some elasmobranchs. The skeleton is mostly cartilaginous. They have a spiral valvular intestine. Gills are often brown in color and this should not be mistaken for methemoglobinemia. Paddlefish have gill rakers. Their skin is largely scaleless although sturgeons have solid dermal plates. They have a prominent pneumatic duct to the swim bladder, so care should be taken when passing a gavage tube. They have a single, simple swim bladder