Ecological and life history characteristics of Botryllus schlosseri (Tunicata) populations inhabiting undersurface shallow-water stones

Populations of Botryllus schlosseri (PALLAS 1772), a cosmopolitan colonial ascidian, were examined on the undersurface of stones for the first time in three adjoining localities along the Israeli Mediterranean coast. A 2 yr study of 1589 stones which were inhabited by 1345 colonies, revealed that colony coverage was <1.5% of the total available substrate area; values were highest in spring, lowest in winter. The 2 yr differed significantly in the number of colonies.m^-2, and these numbers were correlated with mean seawater temperatures. Most of the stones contained 1-5 Botryllus colonies year-round; no significant correlation was found between number and stone size. The brown morph was dominant (80%) at all three localities. The distribution of other colour morphs differed between localities. Reproductive colonies were either hermaphroditic or contained male gonads only. Peak reproduction was in the spring, but did not correlate with seawater temperatures. While colony size ranged between 1 and 1155 zooids, sexually mature colonies consisted of 171-273 zooids on the average, compared with 37-90 zooids for sterile colonies. The three populations differed significantly in several ecological and life history characteristics. This further confirmed past studies indicating that Botryllus populations are characteristically divided into local subpopulations exhibiting microgeographic differences in life history patterns. The results are compared with the accumulated data on other world-wide Botryllus populations residing in other habitats.