A2 Marine Science - life cycles + internal/external fertilisation

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Life cycles (salmon/tuna)

Life cycle of a salmon

  • Spawning takes place in fresh water (rivers)
  • Eggs laid in nest
  • Alevin feed on yolk sac in nest
  • Fry feed on small animals / plankton
  • Undergo physiological adaptations to live in sea water (smoulting)
  • Reach maturity over a period of 1-4 years in the sea, then return to river in which they hatched to spawn
  • Most adult salmon die after spawning


Life cycle of a tuna

  • Spawning occurs in seawater
  • Eggs float on ocean surface
  • Larvae are pelagic (form part of zooplankton)
  • Young tuna migrate to shallow water feeding grounds
  • Reach maturity after about a year
  • Adults spawn many times during their life
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Life cycles (oyster/shrimp)

Life cycle of an oyster

  • Male/female oysters release sperm/eggs into water
  • External fertilisation
  • Free-swimming planktonic larvae
  • Each larva develops a foot after about 3 weeks, sinks, and attaches to a substrate
    • Attached oyster larvae = 'spat'
    • Grow to maturity after 1-3 years


Life cycle of a shrimp

  • Adults are benthic
  • Larvae are planktonic
    • Undergo several stages of development
    • Juveniles move into estuaries
    • Settle at bottom + feed/grow
    • Eventually return to open sea


Difference in adult stages: adult oysters are sessile, while adult shrimp are free-living

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Life cycles (giant clam/grouper)

Life cycle of a giant clam

  • Adults are hermaphrodites
  • Release large numbers of sperm into water, then release large number of eggs
  • Spawning linked to lunar cycles
  • Fertilized eggs hatch after about 12 hours
  • Larvae are free-swimming / pelagic
  • Larvae undergo several stages of development (e.g. trochophores and veligers)
  • After 8-10 days: larvae become juvenile clams
    • Settle on rock/coral (do not say 'suitable substrate'!)
    • Mature in 2-3 years


Life cycle of a grouper

  • Adult groupers found in coral reefs
  • Usually solitary, but aggregate on reef edge to spawn
  • External fertilisation; fertilised eggs are planktonic
  • Larval groupers develop in sea grass beds and mangroves
  • Juveniles eventually travel offshore to join adult population
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Internal/external fertilisation + offspring care

Internal fertilisation

  • Male transfers sperm directly into female reproductive tract
  • Requires pairing of male/female of same species, but increases the chances of the fertilised egg surviving (e.g., due to higher protection from predators)
  • Less wasteful (less energy loss)
  • More likely to lead to fertilisation


External fertilisation

  • Gametes (eggs/sperm) released directly into water
  • No parental care of developing young
  • Useful for sessile organisms


Tuna / whales - offspring care

  • Tuna: produce millions of offspring via external fertilisation / no parental care / larvae are planktonic / only a small number survive (r selection)
  • Whales: produce one offspring via internal fertilisation / give birth to live offspring / high investment in one offspring (K selection)
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