Effect of temperature, salinity and delayed attachment on development of the solitary ascidian Styela plicata (Lesueur)

Abstract

The solitary ascidian Styela plicata (Lesueur) is a common member of epibenthic marine communities in Hong Kong, where seawater experiences extensive seasonal changes in temperature (18–30 °C) and salinity (22–34‰). In this investigation, the relative sensitivity of different developmental stages (i.e., duration of embryonic development, larval metamorphosis and post-larval growth) to various temperature (18, 22, 26 and 30 °C) and salinity (22‰, 26‰, 30‰ and 34‰) combinations is reported. Fertilized eggs did not develop at lower salinities (22‰ and 26‰). At higher salinities (30‰ and 34‰), the duration of embryonic development increased with decreasing temperature (18 °C: 11.5±0.3 h; 30 °C: 8.5±0.3 h). More than 50% of larvae spontaneously attached and metamorphosed at all the levels of temperature and salinity tested. At higher temperatures (22, 26 and 30 °C) and salinities (30‰ and 34‰), functional siphon developed in about 72 h after hatching, whereas at low temperature (18 °C), siphon developed only in <30% of individuals in about 90 h. However, none of the metamorphosed larvae developed subsequently at low salinity (22‰). When forced to swim (or delayed attachment), larvae lost about 0.27 mJ after 48 h (about 22% of the stored energy). Such a drop in energy reserves, however, was not strong enough to cause a significant impact on post-larval growth. This study suggests that temperature and salinity reductions due to seasonal monsoon may have significant effect on the embryo and post-larval growth of S. plicata in Hong Kong.