Population dynamics of a coral reef ascidian in a deteriorating environment

In the coral reefs of Eilat, a newly described colonial ascidian, Botryllus eilatensis, has been observed overgrowing dead coral skeletons and rapidly colonizing artificial substrates. To study overgrowth dynamics and the relationship between coral morphology, size, and orientation and ascidian overgrowth we monitored (monthly) the interaction of 50 to 70 tagged coral colonies with B. eilatensis. The rate of B. eilatensis overgrowth on corals was influenced by the coral's morphology and orientation combined with a seasonal appearance of the ascidian. During spring, the average ascidian cover was 27 ± 4.6 % (mean ± SE), decreasing rapidly during the summer months when sexual reproduction took place. This seasonality is linked to the vertical mixing phenomenon that occurs in Eilat every winter. Moreover, fragments of B. eilatensis translocated to a nutrient-rich site increased 3-fold in size and exhibited higher survivorship rates in comparison to the fragments at the original site. Increased anthropogenic activity, particularly the eutrophication occurring along the Israeli coasts, creates favorable conditions for filter-feeding organisms such as ascidians, providing them with an advantage in competition for space over reef-building corals and other organisms.