Abstract
The life history of the myodocopid ostracod Vargula hilgendorfii was investigated by size-frequency distribution analysis in Akita Port, Japan, from April to December, 2003. Vargula hilgendorfii individuals were collected using baited traps, and the carapace length was measured by computer analysis of a digital photograph of each individual taken under a stereomicroscope. The population was divided into seven groups based on carapace length. These groups corresponded to the five juvenile stages, the adult male stage, and the adult female stage. Each peak in the number of V. hilgendorfii individuals is related to the growth of the population. Our field data clearly suggest that the Akita Port population of V. hilgendorfii produces two generations per year. Adults that survived the winter became sexually active in May and released free-living juveniles in July. After reproducting, they died without having entered a second reproductive cycle. The first-generation juveniles reached the adult stage after five molts and released second-generation juveniles in September. By winter, these second-generation juveniles reached maturity without releasing any new juveniles; they become sexually active in the following spring.
