MEPS prepress abstract  -  DOI:

Seasonal changes in reproductive traits and paternity in the Japanese pygmy squid Idiosepius paradoxus

Noriyosi Sato*


ABSTRACT: Sperm production in males and sperm storage in females change during their reproductive timespan. This implies that the strength of sperm competition may also change with time. The seasonal pattern of reproductive traits and paternity in the Japanese pygmy squid, which has two life-history types (large and small types having long and short reproductive periods, respectively), were compared among three reproductive periods: early and late reproductive period of large type and middle reproductive period of small type. Although the gonad somatic index (GSI) decreased during one of the reproductive periods in males, the number of spermatophores and the spermatophoric complex somatic index (SCSI) increased. The GSI was higher in large-type males, whereas the SCSI was higher in the small type. Multiple paternity was detected in all egg masses, and on average about 10 males were estimated to be sires in a single egg mass. Paternity numbers differed significantly between life-history types, whereas there were no significant differences between the early and late reproductive periods in the large type. These results imply that the Japanese pygmy squid is polyandrous and exposed to a very high level of sperm competition risk. Paternity numbers did not change during reproductive periods even when the reproductive traits changed between these periods. Seasonal changes in reproductive traits may have a minor effect on paternity in promiscuous animals. However, two life-history strategies were observed: large-type males invested in transferring more sperm to one female, whereas small-type males stored more spermatophores to copulate with many females.