Reproductive patterns and population dynamics of Ascidia mentula O.F. Müller on the Swedish west coast

Abstract

A population of Ascidia mentula O.F. Müller on the Swedish west coast has been studied over an 8-yr period by aid of non-destructive (photographic) recordings of a vertical rock face at a depth of 25 m. Supplemental sampling by diving at a depth of 25 m was carried out over a 1-yr period. A demographical analysis revealed elevated mortality during the late summer and during periods of especially low temperature. The mortality was found to be relatively independent of age and was also rather uniform between different years and generations. The mortality is discussed in relation to reproduction and ambient temperature. Larval settling took place all the year round with maximum activity in August and September, but was subject to high interannual variability. It was concluded that variation in recruitment was the most important factor regulating populations of A. mentula. The growth was measured from settling to death and showed great variability between single individuals and between different generations. Negative growth was recorded especially when the ambient temperature reached a high level. Histological examinations showed mature gonads over the full year of study, but with a quantitative difference. Throughout the year, mature oocytes and sperms were present in the gonoducts in 50 to 80% of the collected animals and in close to 100% during the peak of activity.