Effects of Temperature and Salinity on the Egg Development and Larval Settlement of Ciona robusta (Ascidiacea, Phlebobranchia, Cionidae)

Abstract

Ascidians are marine benthic organisms, which attach to the surfaces of both natural and artificial substrates from shallow to deep waters around the world. Invasive ascidians are reported to have spread widely through maritime activities, especially as a fouling on ship hulls. Ciona robusta has been reported as an invasive ascidian species of the marine ecosystem in Korea since it was first recorded in 1967 and was designated as an organism causing marine ecological disturbance by the Ministry of Oceans and Fisheries, Korea in 2017. In the current study, in order to provide some basic ecological data for the management and control of C. robusta, we investigated the effects of temperature and salinity on egg development and larval settlement. The egg development and larval settlement of C. robusta were investigated at five constant temperatures, 10, 12, 16, 20, and 24°C, and at eight constant salinities ranging from 26 to 40 psu (increments of 2 psu). The highest hatching rate was 64.2 ± 7.8% at 16°C and 34 psu, and the lowest was 2.2 ± 2.0% at 20°C and 28 psu. No eggs hatched at salinities of 26 and 40 psu. The highest developmental rate (1/h) was 0.075 ± 0.015 at 24°C and 32 psu, and the lowest was 0.015 ± 0.000 at 20°C and 28 psu. The highest settlement rate was 47.1 ± 10.3% at 20°C and 34 psu, and the lowest was 0.7 ± 0.0% at 20°C and 28 psu. The highest rate of settlement time (1/h) was 0.034 ± 0.012 at 24°C and 34 psu, and the lowest was 0.011 ± 0.000 at 20°C and 28 psu. The approximate optimal temperature and salinity ranges for egg development and larval settlement of C. robusta were estimated to be 16−20°C and 32−34 psu, respectively. This study suggests that the development of C. robusta was affected by both salinity and temperature and that these factors may have a significant effect on the occurrence of C. robusta in Korea.