Multiplicity of sperm receptacles for securing the offspring genetic variability in a pelagic squid

Female eumetazoans often develop sperm storage organs (SSOs). Although the processes of sperm storage may influence sperm competition and cryptic female choice in polyandrous species, the significance of developing multiple SSOs is not well understood. In contrast to coastal squids (which develop no more than two SSOs), the female Japanese common squid Todarodes pacificus, a more oceanic pelagic species, develops more than 20 SSOs, which take the form of specialized pockets, called seminal receptacles (SRs), on the buccal membrane. We investigated the sperm storage pattern of SRs by paternity analysis of hatchlings obtained after artificial insemination using sperm retrieved from 6 arbitrarily selected SRs. The results showed that females were capable of storing sperm contributed by 9 to 23 males, indicating that females are broadly promiscuous. In the pattern of sperm storage, the number of males and the proportion of their sperm present in the SRs varied widely among SRs, and sperm storage was biased towards particular males at the individual SR level. However, when calculated as a proportion of all the SRs within a female, the number of sires increased and the paternity bias towards any particular male weakened. These results suggest that one function of having multiple SRs in T. pacificus may be associated with ensuring higher genetic diversity of the offspring.