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ABSTRACT
Populations of botryllid ascidians (Botrylloides sp.) were investigated during October 1988-October 1990 along the subtidal of Akko (Acre) Bay, Israel. The studied area (600 m2) was visited every 1.5–2 months, and 50 stones per visit were randomly sampled. A total of 2,067 colonies were collected, of which 2.0% were found in a state of zooid regression. All colonies were found on the lower surface of stones. They were recorded year-round but showed marked seasonal fluctuations, from as many as 410 colonies per visit in the summer months to 13 colonies per visit in the winter. Botrylloides coverage area varied from 4.4% (summer) to 0.9% (winter) of total stone undersurface area. During the autumn and winter months, 49.2–63.2% of the stones were free of Botrylloides colonies, compared to 4.5% of the summer collections. Only 2–2.7% of the stones collected in winter were populated by 6 or more colonies, compared to 50.5% of the summer stones. Reproductive activity was correlated with monthly mean water temperatures and colony size. During January-April, only a few colonies were fertile, while in August more than half were sexually mature. Examinations of all studied colonies revealed the existence of three distinctive morphs (subpopulations) which differ in their colors, system organization, zooid orientation, relative abundance, and reproductive seasons. The possible existence of only one Botrylloides species (probably B. leachi) in this area or of three different species is discussed. The taxonomic difficulties in distinguishing between closely related species of botryllid ascidians prevent final conclusions.