Life histories and senescence of Botryllus schlosseri (Chordata, Ascidiacea) in Monterey Bay.

The colonial ascidian Botryllus schlosseri is a model organism for research on invertebrate histocompatibility, development, and evolutionary biology. Nonetheless, the basic life history of Pacific Ocean populations of the species remains unknown. We determined field rates of growth, reproduction, and senescence in four cohorts of B. schlosseri colonies in Monterey Bay, California. Colonies grew exponentially as juveniles and reached sizes of up to 1400 zooids within 69 days. After a juvenile phase lasting at least 49 days, the colonies began to reproduce sexually. Each zooid produced up to 10 clutches, each with a maximum of 5 eggs, resulting in very high fecundity of up to 8000 eggs per colony. Following a short period (maximum 70 days) of continuous sexual reproduction, colonies abruptly senesced and died while still bearing a full clutch of eggs. Senescence progressed through four distinct stages over 1-2 weeks, and inevitably led to the simultaneous death of all zooids in the colony. Although senescence was the main cause of mortality, some colonies died as a result of predation or undermined causes. Certain life history traits varied significantly between cohorts that settled at different times of year. For example, lifespans in the field varied from about 3 months for spring to 8 months for fall-born colonies, but the lifetime fecundity of colonies did not vary between cohorts. The morphologies and life histories of colonies monitored in the field and reported here differed from those of colonies cultured previously in the laboratory.