Sperm competition risk affects ejaculate strategy in terms of sperm number but not sperm size in squid

In polygamous species, the mode of sperm storage in females influences
evolution of sperm quantitative and qualitative traits because it provides
the arena for sperm competition, cryptic female choice and fertilization
processes. In this study, we compared ejaculate traits of two squid
species, Heterololigo bleekeri and Loligo
reynaudii. Both species show dimorphic sperm traits associated with
alternative reproductive tactics where consort and sneaker males transfer
sperm to different storage sites within a female (on the oviduct and near
the mouth, respectively). Due to differences in reproductive behaviours
and sperm placement, sperm competition risk is expected to be higher in
sneakers than in consorts of both species, and higher overall
in L. reynaudii. Our results demonstrate that the instantaneous
number of released sperm is adjusted to the expected sperm competition
risk via an elaborate sperm package. Consort sperm are similar in size,
however, sneaker sperm have a significantly longer flagellum
in H. bleekeri than in L. reynaudii, most
likely due to intra-tactic conflicts associated with sperm storage
conditions. From consideration of the different mating tactics,
we suggest that while levels of sperm competition determine quantitative
traits, sperm quality traits are determined more by the mode of sperm
storage and fertilization.