Didemnid ascidians: Rapid colonizers of artificial reefs in Eilat (Red Sea)

Ascidians frequently occur on man-made surfaces submerged in marine waters and constitute a major technical and economic problem worldwide. In Eilat (Red Sea), artificial reefs constructed for attractive diving are frequently fouled by ascidians. In this study we examined aspects of temporal and spatial fouling of two common ascidians, Didemnum granulatum and Didemnum sp., on two modular experimental artificial reefs. Habitat partitioning, growth rate and survivorship of the two species were monitored along these artificial reefs for 1 yr. D. granulatum was the first ascidian to appear, and the most prominent species on both artificial reefs. Its initial recruitment was on the lower sides of the deepest horizontal plates of both artificial reefs, and it gradually recruited towards the lower sides of the shallower horizontal plates. Didemnum sp. colonies recruited at different depths along the artificial reef, while following a similar pattern of gradual recruitment towards the shallower plates. Both species successfully foul on the artificial reefs prior to the appearance of long lived organisms such as corals. Transplantation experiments further confirmed the depth and position required by D. granulatum along the artificial reefs. We suggest that the initial recruitment of D. granulatum on the lower sides of the deepest horizontal plates is an outcome of its negatively phototactic and geotropic bottom swimming larvae, which are able to delay settlement in order to locate a suitable settlement site.