A2 Marine Science - life cycles + internal/external fertilisation

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

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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

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

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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

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

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

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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

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

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External fertilisation

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

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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)