Combined effects of temperature and salinity on colonies of Botryllus schlosseri and Botrylloides leachi (Ascidiacea) from the Venetian Lagoon.

Combined effects of temperature and salinity on colonies of Botryllus schlosseri and Botrylloides leachi from the Venetian Lagoon have been studied in two series of experimental samples, consisting respectively of 'young' colonies (from oozoid to 5th blastogenetic generation) and 'adult' colonies (from 5th to 9th blastogenetic generation), at several combinations of temperature (from 3° to 28 °C) and salinity (from 16 ‰ to 44 ‰ S). In both series percentage of survival, index of colonial growth, index of quality of life and mean length of zooids at the end of treatment have been calculated and processed in order to obtain response surfaces. B. leachi is less eurytherm and less euryhaline than B. schlosseri; however, the former exhibits a higher regulative capacity. Colonial growth does not differ noticeably between young and adult colonies in B. leachi; in B. schlosseri growth is much higher in young colonies than in adult ones. Generally, adult colomes of B. schlosseri exhibit a greater range of tolerance to experimental conditions than young ones; the opposite is true for B. leachi.