Sperm allocation in relation to female size, male traits and copulation behaviour in a freshwater crayfish

Sperm competition is a well-recognised agent in
the evolution of sperm and ejaculate structure, as well as
variation in female quality. Models of the evolution of
ejaculate expenditure predict that male body condition,
female fecundity and the risk and intensity of sperm competition
may be the ultimate factors shaping optimal
ejaculate size. We investigated sperm allocation in Austropotamobius
italicus, a freshwater crayfish exhibiting a
coercive mating system and external fertilisation, in
relation to male and female traits and copulation behaviour
under laboratory conditions. We found that mating males
were sensitive to female size and produced larger ejaculates
when mating with larger females, which were more fecund
in terms of number of eggs produced. We found no
evidence for female egg production being sperm-limited, as
the number of eggs was not dependent on male sperm
expenditure. Copulation duration and number of ejaculations
reliably predicted the amount of sperm transferred,
and both these behavioural measures positively covaried
with female body size. These results indicate that male
freshwater crayfish can modulate their sperm expenditure
in accordance with cues that indicate female fecundity. In
addition, a novel finding that emerged from this study is the
decrease in sperm expenditure with male body size, which
may either suggest that large, old male crayfish are better
able than small males to economise sperm at a given
mating to perform multiple matings during a reproductive
season, or that they experience senescence of their reproductive
performance.