Download (3398Kb) - ePrints Soton - University of Southampton

In 1973, Vance proposed a theoretical model to predict optimal egg size with a
bimodal distribution corresponding to type of development of the larvae. Considering
a continuous reproductive effort, there is an important trade-off between fecundity
and energy content per egg; subsequently the model proposed an apparent selection
for extreme egg sizes with the production of many small eggs with minimal material
or production of very few large yolky eggs. Sewell & Young (1999) performed a reexamination
of asteroid and echinoid egg sizes and tested the prediction of bimodality
in holothuroids and ophiuroids. Eggs diameters in asteroid species were found to
range from 100 to 3500 µm, and the two modes are found in the ranges of 100 to 150
µm and 700 to 1000 µm. The ranges in egg diameter for planktotrophic and
lecithotrophic asteroids do not overlap, but egg sizes overlap considerably between
lecithotrophic and brooding species (Emlet et al., 1987).
Levitan (1993) proposed a hypothesis to explain the evolution of egg size in
marine invertebrates related to the probability of egg fertilization using a model of
fertilization kinetics developed by Vogel (1982). The hypothesis proposes that
conditions of sperm limitation can select for larger eggs. Consequently, variation in
such conditions can contribute to the observed patterns of interspecific variation in
egg size, concluding that larger eggs will be fertilized at a greater rate because they
provide a larger target for sperm. Podolsky and Strathmann (1996) concluded that the
results of Levitan (1993) led to incorrect inferences because he used an interspecific
comparison in which other gamete attributes such as egg fertilizability, sperm speed,
and sperm half-life co-vary with egg size. As an example they propose that greater
zygote production of the echinoid Strongylocentrotus droebachiensis relative to its
congeneric species results from interspecific differences in egg fertilizability and
sperm half-life, not from larger egg size. Podolsky and Strathmann (1996) also
74