Local gamete competition explains sex allocation and fertilization strategies in the sea

Authors: Jonathan M Henshaw, Dustin J Marshall, Michael D Jennions and Hanna Kokko

Published in: The American Naturalist, volume 184, number 2 (August 2014)


Within and across taxa, there is much variation in the mode of fertilization, that is, whether eggs and/or sperm are released or kept inside or on the surface of the parent’s body.

Although the evolutionary consequences of fertilization mode are far reaching, transitions in the fertilization mode itself have largely escaped theoretical attention.

Here we develop the first evolutionary model of egg retention and release, which also considers transitions between hermaphroditism and dioecy as well as egg size evolution. We provide a unifying explanation for reported associations between small body size, hermaphroditism, and egg retention in marine invertebrates that have puzzled researchers for more than three decades.

Our model, by including sperm limitation, shows that all these patterns can arise as an evolutionary response to local competition between eggs for fertilization. This can provide a general explanation for three empirical patterns:

  • sperm casters tend to be smaller than related broadcast spawners,
  • hermaphroditism is disproportionately common in sperm casters, and
  • offspring of sperm casters are larger.

Local gamete competition also explains a universal sexual asymmetry: females of some species retain their gametes while males release theirs, but the opposite (“egg casting”) lacks evolutionary stability and is apparently not found in nature.


Henshaw JM, Marshall DJ, Jennions MD, Kokko H (2014) Local gamete competition explains sex allocation and fertilization strategies in the sea. The American Naturalist, 184 E32–E49 PDF 737 KB DOI: 10.1086/676641

Supplementary information: Data for robustness analysis XLSX 1 MB