system (Muñoz‐Chápuli and Satchell 1999; Mylniczenko et al. 2006).
Hematopoetic and Immunologic System
Lymphomyeloid tissues of cartilaginous fish are the epigonal organ, Leydig organ, thymus, meninges of the brain, eye orbit, spleen, and gut‐associated lymphoid tissue (GALT) (Rumfelt 2014).
The epigonal organ is physically associated with the gonads. In some species, the gonads are enclosed within the epigonal organs, e.g. common guitarfish (Rhinobatos rhinobatos) and dogfish (Squalus spp.), and in others the two are attached, e.g. stingrays (Hypanus and Aetobatus spp.). Generally, where there is a prominent epigonal organ, the Leydig organ is unapparent or absent.
The Leydig organ is a lymphomyeloid organ found in some species; it differs from the Leydig gland which is involved in sperm maturation. It is sometimes identifiable as a lighter‐colored area within the dorsal (and occasionally ventral) submucosa of the esophagus. The area may be raised. Species with recognizable Leydig organs include skates, some rays, guitarfish, and some shark species, e.g. velvet belly lanternshark (Etmopterus spinax) (Rumfelt 2014).
All elasmobranchs have a bilateral thymus located dorsally near the gills (Luer et al. 1995; Rumfelt 2014). In some species, like the catsharks (Scyliorhinus spp.) and nurse sharks (Ginglymostoma cirratum), involution occurs with age and it cannot be found in adulthood. In others, it remains visible albeit small through life, e.g. bullhead sharks (Heterodontus spp.) and some rays (Rumfelt 2014).
The spleen is dark red and strap‐like to oval in shape. Unlike mammals, it lacks marginal zones and germinal centers, likely because there is no true lymphatic system. There are melanomacrophages in the spleen and liver, but they do not aggregate as they do in teleosts (Rumfelt 2014).
Endocrine System
Elasmobranch endocrinology is complex, but the organs, stimuli, and targets of action are similar to other vertebrates.
The pituitary is similar to teleosts, with slight anatomical variations. It produces growth, thyroid, reproductive, and vasopressin‐like hormones (Anderson 2015). The hypothalamus (within the diencephalon) is a well‐developed organ. It is important for feeding, reproduction, aggression, and likely migration (Hofman 1999).
The interrenal gland (equivalent of the mammalian adrenal cortex) is grossly visible as a section of yellow tissue between each kidney. It is long, thin strip in sharks and a smaller oval in skates and rays (Figure A1.20). The chromaffin cells of the suprarenal bodies are separate and located on the dorsal kidneys near the dorsal aorta (Anderson 2015). The hypothalamo‐pituitary‐interrenal axis is considered the endocrine stress axis, although this has not been fully characterized in elasmobranchs (Shuttleworth 2012).
The single thyroid gland is encapsulated and varies in size and shape across species. In batoids, the thyroid is usually located ventral to the pharynx near the bifurcation of the ventral aorta. In sharks, it is under the commissure of the mandible. The ultimobranchial bodies produce calcitonin, similar to the mammalian parathyroid system although the actions of calcitonin in cartilaginous fish are less well‐understood (Anderson 2015). In batoids, the ultimobranchial bodies are paired and found in the caudal wall of the pericardial cavity. In sharks, they are only on the left side of the body on the dorsal wall of the pericardial cavity (Stoskopf 1993).
Figure A1.20 Interrenal gland (arrow) between the kidneys removed from a southern stingray (Hypanus americanus).
Source: Image courtesy of Catharine Wheaton, Disney’s Animals, Science and Environment.
The bilobed pancreas and gastrointestinal tract produce the expected hormones, though detail on their function and activity are limited. The heart and kidney can also be considered endocrine glands as they produce several natriuretic peptides and have a renin–angiotensin system which regulates the renal and cardiovascular systems (Anderson 2015).
The pineal organ and gonads also have endocrine functions.
Urogenital System
Urinary System
The urinary system manages osmotic and ionic regulation. Marine elasmobranchs maintain a high plasma osmolality due to high levels of sodium, chloride, urea, and methylamine oxides. They are slightly hyperosmotic to their environment, which results in an uptake of water that balances fluid lost in the urine. In contrast to marine teleosts, drinking is negligible. The gills, kidney, and rectal gland are important for maintaining homeostasis, and the kidney is the main site for urea regulation (Lacy and Reale 1999). In freshwater elasmobranchs, urea, electrolytes, and osmolality are much lower; the kidneys are microscopically simpler; and the rectal gland is significantly reduced in size and function.
The kidneys are paired and firmly attached to the dorsal wall of the caudal coelom under a thick capsule. In sharks, they lie midway in the coelomic cavity and widen as they reach caudally. In skates and rays, they are more caudal in the coelomic cavity and more lobulated. In males, the ureters leave the kidneys medially and travel along the midline to meet the seminal vesicles and sperm sacs caudally and exit through a single (or sometimes double) urogenital pore or papilla. A small urine sample may be obtained by passing a catheter past the reproductive tract; large volumes are not possible since urine production is low. A semen sample can be collected more distally and where the sacs or ampullae are.
The kidneys of marine elasmobranch are unable to produce a concentrated urine; ionic concentrations cannot be higher than those of the plasma. Glomerular filtration rates (GFRs) vary from 0.2 to 4.0 mL/kg/h, higher than seawater teleosts but similar to freshwater teleosts. Exposure to dilute environments increases the urine flow rate and decreases reabsorption of urea. Freshwater stingrays have a GFR of 8–10 mL/kg/h. Details of renal function in elasmobranchs can be found elsewhere (Evans et al. 2004; Shuttleworth 2012).
Reproductive System
The reproductive anatomy and biology of elasmobranchs are diverse. The mode of reproduction is generally split by oviparity or viviparity and fetal nutrition (Table A1.6).
Synchrony (seasonal aggregation) is important for breeding activity in some species. Gestation varies from 4.5 months to 2 years (or up to 3.5 years if you include egg cases) (Hamlett et al. 2005). Embryonic diapause has been confirmed in three species but may be more common: Australian sharpnose sharks (Rhizoprionodon taylori), bluntnose stingrays (Hypanus say), and common stingrays (Dasyatis pastinaca) (Waltrick et al. 2012). Parthenogenesis is a phenomenon of many elasmobranch species. Polyandry (multiple paternity) also occurs.
Sperm storage occurs in both male and female elasmobranchs. Females of many shark species store sperm in the oviducal gland. Storage time varies but may be weeks to over a year. Males store sperm in the ampullae of the distal epididymis. Storage time is weeks to months, sometimes longer (Hamlett et al. 2005).
Fertilization is internal in elasmobranchs. All males have external paired claspers (intromittent organs) that are a part of the pelvic fins. As animals mature, these get larger and become internally calcified and able to rotate.
Internal paired organs include the testes (three types: diametric, radial, and compound); genital ducts; Leydig glands which produce sperm maturation substances; and alkaline glands (Marshall's glands) in skates which produce a seminal fluid. The genital ducts (efferent ductules, epididymis, ductus deferens, and seminal vesicle) are embedded in the dorsal abdominal wall and covered by coelomic membrane. Epigonal organs can embed the testis, incorporate the caudal