Abstract
Two populations of Pyura tessellata, living on vertical subtidal rock walls at 20 m depth in the central and inner part of the fjord were studied by aid of stereo-photographic recordings through a 10-year period, supplemented by extra sampling over a further 2 years. The density of both populations showed no fluctations throughout the 10-year period of study but with a declining trend. The generation times, however, were longer than the period of study. Settling and mortality were sparse and without any apparent correlation to known environmental variables. Growth was slow and maximal size was not attained within the 10 years surveyed. Longevity is probably related to the structure of the cuticle and positive correlation was found between the maximal longevity of five ascidian species and the cuticle-body ratio. Reproduction took place in August–September. The number of ova found in gonads in August was low and the total number of spawned eggs in an average-sized animal was 6 600–16 400. Reproduction coincided with the highest water temperature at 20 m depth. P. tessellata is an example of high persistence stability, generated by long generation times and the ability to withstand abiotic and biotic perturbations.
