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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 end, or can 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).