Life history traits could be affected by seasonal environmental factors. Caprellid amphipods are small marine crustaceans inhabiting a wide variety of substrata. They are iteroparous and generally reproduce all year round. Since caprellids life span extends a few months, the life history traits of caprellid species may sequentially vary among seasons.

We collected Caprella bispinosa from Hakodate city, Hokkaido, Japan in February, April, June and October of 2010 and compared female life history traits, such as mature size and the number of eggs among seasons.

Female mature size differed among seasons, and the average size of mature females in February and April was larger than in June and October. The number of eggs increased with body length, and relationships between the number of eggs and body size also differed among seasons.

We discuss why such seasonal variation in egg number occurs by considering relationships with other characters, such as maturity size, egg size and interval of breeding and environmental factors. Results are compared with life history traits of other caprellid species.