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!