Secrets of Shark Reproduction: Evolutionary Strategies Unveiled

1. Natural History of Sharks, Skates, and Rays Reproductive Biology MARE 380 Dr. Turner

2. Reproductive Biology Reproductive adaptations have contributed to evolutionary success & genetic continuity Reproductive processes for most sharks remains unknown

3. Fish Reproduction Primitive model for fish reproduction – oviparity ↑ large numbers of eggs & sperm are shed into the water - where fertilization occurs Typical of bony fishes

4. Fish Reproduction Oviparity - primitive model for fish reproduction – typical of bony fishes ↑ large numbers of eggs & sperm are shed into the water - where fertilization occurs Embryos – small amount of yolk; hatch undeveloped Eggs & young highly vulnerable to predators & environmental factors

5. Elasmobranch Reproduction Depart from simple oviparity: Internal fertilization Small numbers of large young Young born active, fully developed Develop during critical stages within ♀ ↑ size young, ↓ # potential predators r-selected species many, smaller offspring K-selected species fewer, larger offspring

6. Elasmobranch Reproduction Mating associated with ∆ in temperatures Reproductive cycles exist; details unknown Some sharks reproduce: - all year round - annually - alternate between resting periods

7. Elasmobranch Reproduction Mating involves aggressive courtship - ♂ bites ♀ to hold her into position In small sharks: ♂ wraps body around ♀ In large sharks: ♂ parallel to ♀

8. Love Bites! Aggressive courtship leads to scars & adaptation in ♀ Immature ♀ Mature ♀ Mature ♂

9. Elasmobranch Reproduction Ladies need a thicker skin… ♂ ♀ To counter ♂ aggression, ♀ sharks are larger and have up to 2X thicker epidermis

10. Elasmobranch Reproduction All have internal fertilization Ensures energy-expensive eggs not eaten Energy passed to embryos; not wasted 2 Groups: Oviparous (egg-laying), Viviparous (live-bearing)

11. Elasmobranch Reproduction Male (♂) pathway: Testicle (sperm produced) – epididymis – vas deferens – seminal vesicle- sperm sac – cloaca

12. Elasmobranch Reproduction Male (♂) pathway: Clasper draws in water – enters cloaca/vagina – opens like umbrella (anchor) – siphon contracts – expels H2O & sperm

13. Elasmobranch Reproduction Female (♀) pathway: Ova produced in both or just right ovary – released into abdominal cavity – cilia draw into oviduct – reach nidamental (shell gland – secretes “shell”) where fert takes place – down to uterus until egg laid or hatch/birth

14. Elasmobranch Reproduction Female (♀) pathway: Ova produced in both or just right ovary – released into abdominal cavity – cilia draw into oviduct – reach nidamental (shell gland – secretes “shell”) where fert takes place – down to uterus until egg laid or hatch/birth

15. Oviparous Forms Enclose eggs in tough horny egg case Incubation – months to over a year Oviparous (egg-laying) elasmobranchs – benthic, 1° littoral or bathyal, rarely large*

16. Oviparous Forms Whale shark found in the Gulf of Mexico in 1953; 30x14x8 cm (12x51/2x3 inches)

17. Oviparous Forms Whale shark harpooned in Taiwanese fishery - 300 embryos developing inside thin egg cases within the mother Determined whales sharks - ovoviviparous; eggs hatch within the uterus of the mother and remain until development is complete

18. Oviparous Forms 3 families of sharks: Heterodontidae, Scyliorhinidae, & Orectolobidae & Skates: Rajiformes

19. Viviparous Forms Retain embryos in uterus during entire development Divided into placental & aplacental; depending upon placental connection b/w ♀ & offspring

20. Aplacental Viviparity No placental connection (formerly ovoviviparity) Modes of nourishment vary widely: 1) depend upon yolk reserves 2) are oophagous (egg-eating) 3) are nourished via placental analogues

21. Yolk Dependency Embryos depend solely upon yolk deposited in egg at time of fertilization – no supplemental Relatively small at birth Include Squaliformes, Hexanchiformes, Squantinaformes, etc

22. Compartmentalization Life History strategy – one embryo – one uterus; leaves only 2 offspring… therefore, subdivide… Eliminates - oophagy

23. Oophagy Ovaries grow to tremendous size Eggs small; most exist to nourish young First few weeks – utilize yolk form own egg After – ingest other eggs in the uterus Some produce single (thresher) or multiple embryos (sandtiger – 12) per oviduct

24. Thunderdome! Bigeye Thresher Shark – No compartmentalization One uteri – one surviving embryo Many embryo enter – one embryo leave…

25. Placental Analogues “Uterine milk” (embryotroph/histotroph) is produced by long villi (trophonemata) Common in rays

26. Placental Viviparity Embryos nourished by yolk in yolk sac during first few weeks Once exhausted – yolk sac elongates and distal surface vascularized – touches uterine wall Grows into placenta – nourished now directly from female

27. Success!