Broadcast Spawning: What Is It And How Does It Work?

Broadcast spawning is a form of sexual reproduction used by many aquatic invertebrate animals. Billions of gametes (the combination of sperm and egg) are spewed into the surrounding environment of the ocean.

Acropora tenuis broadcast spawning
Acropora tenuis releasing egg sperm bundles; photo by Jamie Craggs

Most of these animals are either sessile or they move very slowly. Sessile means they are fixed to the ground in either a burrow, shell, integument, or some other kind of casing that prevents them from moving around. Adult movement is not a major factor for these creatures. Since they live in an aquatic environment which serves as a convenient medium, broadcast spawning is a successful strategy for sexual reproduction.

Successful Broadcast Spawning

Spewing one’s genetic blueprints into the world without a care for whether they meet a suitable counterpart would be an unwise thing for any mom to do. Maternal duties are minimal in broadcast spawners, but the timing of gamete release is among the most important aspects for any individual’s reproductive success.

Successful broadcast spawning involves attentiveness to a wide range of environmental characteristics. The best strategies take many factors into consideration:

  • the availability of potential mates who are also ready to reproduce
  • currents
  • tides
  • predator density
  • water temperature
  • density of potential partners
  • density of potential competitors

Broadcast spawning certainly sounds like an effective way to get the job done. However, reproduction is less successful if gamete densities become too high. Eggs that come into contact with too much sperm could be susceptible to polyspermy, or multiple fertilization, which renders them useless. On the other hand, eggs that do not come into contact with the appropriate sperm will go unfertilized.


This post is an excerpt from my book “Wild Moms: Motherhood in the Animal Kingdom.”

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