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Natural History of Sharks, Skates, and Rays
Reproductive Biology
MARE 394Dr. Turner
Summer 2008
Reproductive BiologyReproductive adaptations have contributed to evolutionary success & genetic continuity
Reproductive processes for most sharks remains unknown
Fish ReproductionPrimitive model for fish reproduction – oviparity
↑ large numbers of eggs & sperm are shed into the water - where fertilization occurs
Typical of bony fishes
Fish ReproductionOviparity - 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
Elasmobranch ReproductionDepart from simple oviparity:Internal fertilizationSmall numbers of large youngYoung born active, fully developedDevelop during critical stages within ♀↑ size young, ↓ # potential predators
r-selected species many, smaller offspringK-selected species fewer, larger offspring
Elasmobranch ReproductionMating associated with ∆ in temperatures
Reproductive cycles exist; details unknown
Some sharks reproduce: - all year round - annually - alternate between resting periods
Elasmobranch ReproductionMating involves aggressive courtship - ♂ bites ♀ to hold her into position
In small sharks:♂ wraps body around ♀
In large sharks:♂ parallel to ♀
Love Bites!Aggressive courtship leads to scars & adaptation in ♀ Immature ♀
Mature ♂
Mature ♀
Elasmobranch ReproductionLadies need a thicker skin…
♂ ♀
To counter ♂ aggression, ♀ sharks are larger and have up to 2X thicker epidermis
Elasmobranch ReproductionAll have internal fertilization
Ensures energy-expensive eggs not eaten
Energy passed to embryos; not wasted
2 Groups: Oviparous (egg-laying), Viviparous (live-bearing)
Elasmobranch ReproductionMale (♂) pathway:
Testicle (sperm produced) – epididymis – vas deferens – seminal vesicle- sperm sac – cloaca
Elasmobranch ReproductionMale (♂) pathway:
Clasper draws in water – enters cloaca/vagina – opens like umbrella (anchor) – siphon contracts – expels H2O & sperm
Elasmobranch ReproductionFemale (♀) 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
Elasmobranch ReproductionFemale (♀) 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
Oviparous FormsEnclose eggs in tough horny egg case
Incubation – months to over a year
Oviparous (egg-laying) elasmobranchs – benthic, 1° littoral or bathyal, rarely large*
Oviparous FormsWhale shark found in the Gulf of Mexico in 1953; 30x14x8 cm (12x51/2x3 inches)
Oviparous FormsWhale 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
Oviparous Forms3 families of sharks: Heterodontidae, Scyliorhinidae, & Orectolobidae
& Skates: Rajiformes
Viviparous FormsRetain embryos in uterus during entire development
Divided into placental & aplacental; depending upon placental connection b/w ♀ & offspring
Aplacental ViviparityNo placental connection (formerly ovoviviparity)
Modes of nourishment vary widely:1) depend upon yolk reserves2) are oophagous (egg-eating)3) are nourished via placental analogues
Yolk DependencyEmbryos depend solely upon yolk deposited in egg at time of fertilization – no supplemental
Relatively small at birth
Include Squaliformes, Hexanchiformes, Squantinaformes, etc
CompartmentalizationLife History strategy – one embryo – one uterus; leaves only 2 offspring…therefore, subdivide…
Eliminates - oophagy
OophagyOvaries grow to tremendous size
Eggs small; most exist to nourish young
First few weeks – utilize yolk form own eggAfter – ingest other eggs in the uterus
Some produce single (thresher) or multiple embryos (sandtiger – 12) per oviduct
Thunderdome!Bigeye Thresher Shark – No compartmentalizationOne uteri – one surviving embryoMany embryo enter – one embryo leave…
Placental Analogues“Uterine milk” (embryotroph/histotroph) is produced by long villi (trophonemata)
Common in rays
Placental ViviparityEmbryos 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
Success!