BIOL3045 Lecture Notes - Lecture 14: Polyspermy, Sperm Competition, Sexual Selection
3 Jul 2018
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Animal Ecological Physiology BIOL3045 460381099
Lecture 14: Sexual selection in broadcast spawners.
• Sperm and eggs released into the ocean, most common form of marine mating.
• Wasteful, needs to release so much to ensure some enable fertilization.
• Generally synchronized, from a que in the environment. Example: Mass spawning event of coral,
after a full moon in October/November.
Does sexual selection occur in broadcast spawners?
• Sexual selection doesn’t stop after mating (competition of sperm), occurs through cryptic female
choice (choice of the eggs).
• Competition of sperm is:
o Proximity dependent: Depends how close the spawning individual is to the egg. Eggs
need enough sperm at a close distance to be fertilized.
o Density dependent: How many spawning at the same time. Maximum fertilization
success occurs at intermediate density. This is due to polyspermy (too much sperm
enters the eggs, not allowing it to fertilize). Bell shaped curve.
Which sex should compete for mating success in broadcast spawners?
• Females have lots of variation in their reproductive success, similar to males. Thus, both sexes
compete.
How do females compete?
• Large, fewer eggs (larger offspring) or small, more eggs (smaller offspring). If she believes that
her offspring will be beneficial in any environment, she will produce many small ones. If the
environment is harsh, better to produce few larger ones.
• Eggs size influences fertilization success: if few sperm, large eggs are favored. If many sperm,
leads to polyspermy in large eggs and successfully fertilized small eggs.
Is changing egg size likely to have any other consequences?
• Larger egg = more nutrients = larger offspring. Maybe better dispersal? If they run out of energy
they will just settle. Time for settlement increases as size of larvae increases. Larger offspring
tend to do better back out in the field, less of survival decline.
• Accessory structures: Large water filled cells without having to increase energy. This is greater in
eggs from higher density areas. Changing different components of egg to enhance offspring
quality.
How does sexual selection operate in males?
• Risk of sperm competition is increased, the more sperm males release. Will this work for
broadcast spawners? This won’t work due to polyspermy (all the eggs will die).
• Better to make the sperm fitter (better quality). Size, shape, longevity, and variation between
ejaculations.
• Males can adaptively change the phenotype of the sperm (plasticity). Increased sperm velocity,
example: Arctic charr. Dominants don’t need to compete as much, spends more time guarding
territory. In sea squirts (example), high density males produced larger, more mobile and greater
longevity sperm.
Can these changes in sperm effect offspring?
• Sperm transmit more than DNA, sperm may be influences by changes in male environment (pre-
zygotic) or after spawning (post-zygotic) – more resistant sperm survives through environment.
Thus, links between sperm and phenotypes could be more variability in offspring.
Document Summary
Example: mass spawning event of coral, after a full moon in october/november. Eggs need enough sperm at a close distance to be fertilized: density dependent: how many spawning at the same time. This is due to polyspermy (too much sperm enters the eggs, not allowing it to fertilize). Which sex should compete for mating success in broadcast spawners: females have lots of variation in their reproductive success, similar to males. Large, fewer eggs (larger offspring) or small, more eggs (smaller offspring). If she believes that her offspring will be beneficial in any environment, she will produce many small ones. If the environment is harsh, better to produce few larger ones: eggs size influences fertilization success: if few sperm, large eggs are favored. If many sperm, leads to polyspermy in large eggs and successfully fertilized small eggs. Larger egg = more nutrients = larger offspring. If they run out of energy they will just settle.
