Influences of temperature and salinity on asexual budding by hydromedusa Proboscidactyla ornata (Cnidaria: Hydrozoa: Proboscidactylidae)

Abstract

At the head of Tanabe Bay, on the Pacific coast of Japan, dense populations of Proboscidactyla ornata medusae were observed at 24℃, 34 psu from July 2001 to September 2002 (maximum 472.4 individuals m−3). In the laboratory, 288 immature medusae of P. ornata collected from Tanabe Bay in May–June 2006 were cultured individually and fed with asphyxiated Artemia nauplii under 12 experimental conditions (four different temperatures (17℃, 20℃, 23℃ and 26℃) combined with three different salinities (28 psu, 31 psu and 34 psu)). For 30 days, 94% of the medusae survived at all conditions. The maximum number of clones produced was 0.32 clones medusa−1 day−1 at 20℃, 34 psu: the number of medusae increased 10.5 times in 30 days. The maturation percentage at 23℃ (54.2–58.3%) was higher than at 17℃ (12.5–25.0%) and was positively correlated with the number of nauplii ingested. Temperature-specificity of energy usage was observed: 20℃ for asexual budding and 23℃ for sexual reproduction. The direct relationship between budding and temperature is demonstrated among hydromedusae for the first time in this study. Additionally, the influence of salinity on budding suggests a hypothesis on an advantage of budding at offshore and high saline waters.