Solitary ascidians, especially Styela plicata (Lesueur...

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 jC) and salinity (22–34x). 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 jC) and salinity (22x, 26x, 30xand 34x) combinations is reported. Fertilized eggs did not develop at lower salinities (22xand 26x). At higher salinities (30xand 34x), the duration of embryonic development increased with decreasing temperature (18 jC: 11.5F 0.3 h; 30 jC: 8.5F 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 jC) and salinities (30xand 34x), functional siphon developed in about 72 h after hatching, whereas at low temperature (18 jC), siphon developed only in < 30 % of individuals in about 90 h. However, none of the metamorphosed larvae developed subsequently at low salinity (22x). 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 enoug