  Competition among gametes for fertilization imposes strong selection. For
external fertilizers, this selective pressure extends to eggs for which
spawning conditions can range from sperm limitation (competition among eggs) to
sexual conflict (overabundance of competing sperm toxic to eggs). Yet existing
fertilization models ignore dynamics that can alter the functional nature of
gamete interactions. These factors include attraction of sperm to eggs, egg
crowding effects or other nonlinearities in per capita rates of sperm-egg
interaction. Such processes potentially allow egg concentrations to drastically
affect viable fertilization probabilities. I experimentally tested whether such
egg effects occur using the urchin $\textit{Strongylocentrotus purpuratus}$ and
parameterized a newly derived model of fertilization dynamics and existing
models modified to include such interactions. The experiments revealed that at
low sperm concentrations, eggs compete for sperm while at high sperm
concentrations eggs cooperatively reduce abnormal fertilization (a proxy for
polyspermy). I show that these observations are consistent with declines in the
per capita rate at which sperm and eggs interact as eggs increase in density.
The results suggest a fitness trade-off of egg release during spawning: as
sperm range from scarce to superabundant, interactions among eggs transition
from highly competitive to cooperative in terms of viable fertilization
probabilities.
