Ascidiacea

The colonial ascidians Diplosoma macdonaldi Herdman and Botryllus schlosseri (Pallas) were prominent on both long-and short-term blocks. On the long-term blocks, D. macdonaldi reached maximum development covering large areas of both horizontal and vertical blocks in the winter months of June and July. Some colonies were 9·5 cm. by 12·0 cm. across at this time. A second heavy set was shown by the short-term blocks to occur in the summer and early autumn. B. schlosseri was present to a greater or lesser degree during the whole of the experiment except for the first month. Its peak for surface coverage was early spring, when it replaced D. macdonaldi. Colonies up to 7·0 cm. by 5·5 cm. were present at this time. B. schlosseri did not develop to an identifiable size on the short-term blocks. Three species of solitary ascidians—namely, Corella eumyota Traustedt, Cnemido-carpa nisiotis (Sluiter), and Asterocarpa cerea (Sluiter) appeared during the experiment. On the short-term blocks, C. eumyota was present during October and December-January, and Cn. nisiotis in December-January and March. A. cerea did not appear on the short-term blocks, but was present in some numbers in late spring and early summer, when it replaced C. eumyota as the simple ascidian element on the long-term blocks. By April, 1950, the largest specimen was 6·5 cm. in length. C. eumyota was common on the long-term blocks during winter, and a few small animals were present in October, December, and January, reflecting the sets shown for the species on the short-term blocks. Cn. nisiotis was at its peak for size and numbers in May, and was present also in June, July, and October, but in much smaller numbers. The evidence indicates that the ascidians, as also the page 13 polyzoa, spawn over many months, with a heavy set in spring and summer, and that the species with the longest life span within the group takes approximately four months to attain maturity and maximum growth.