Seasonal variations in abundance, size composition, biomass and production rate of Oikopleura dioica (Fol) (Tunicata: Appendicularia) in a temperate eutrophic inlet

Abstract

The seasonal variations in abundance, size composition, biomass and production rate of the appendicularian Oikopleura dioica Fol were investigated in a eutrophic inlet of the Inland Sea of Japan. It occurred throughout the year, being least abundant in early spring and mid-summer, and most abundant in early summer. The mid-summer decline of the population might be attributed to predation by the lobate ctenophore Bolinopsis mikado Moser. The composition of mature specimens, for which the sex was identified, varied at short intervals and did not show any seasonal trend (annual mean: 7.7%). Mean trunk length of mature animals was correlated negatively to water temperature. Change in population biomass showed a similar seasonal pattern observed in numerical abundance. The instantaneous growth rate of O. dioica was calculated from body carbon weights of newly hatched and mature specimens and the time from hatching to spawning. Estimated annual production rate was 953 mg C · m −3 · yr −1 (= 7.15 g C · m −2 · yr −1), 62% of which was attained between May and July. Another appendicularian species, Fritillaria haplostoma Fol, occurred only in early fall, at much lower densities than O. dioica.