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 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!