Development of reproductive structures in the introduced green alga, Codium fragile ssp. tomentosoides, in the northern Adriatic Sea

Biological invasions are among the most important drivers of global change. The introduced green alga, Codium fragile ssp. tomentosoides, has become an important component of low-shore assemblages on hard coastal defence structures along the north-west coast of the Adriatic Sea. We investigated the temporal dynamics, the spatial scales of variation and the effects of wave exposure on the abundance of reproductive structures of C. fragile. Furthermore, we assessed whether the reproductive output of C. fragile was correlated with the density of thalli. On the landward side of breakwaters, gametangia were found on thalli of C. fragile from June to September in both 2003 and 2004. Although there was some variation among breakwaters, the abundance of gametangia of C. fragile was greater on the landward than on the seaward side of breakwaters. Scales of variation in the number of gametangia and density of thalli of C. fragile were similar, with most of the variability occurring at a scale of tens of cm. Little variation emerged at larger (tens or hundreds of m) or smaller scales (within thalli, cm). The results of this study indicate that the provision of sheltered rocky habitats has been crucial for the establishment of C. fragile in the northern Adriatic Sea, enabling the alga to grow and reproduce. Processes operating at small spatial scales are responsible for patterns of variation in the number of gametangia and density of thalli of C. fragile. The tendency for a positive correlation between density of thalli and reproductive output in C. fragile in August suggests that greater densities of plants could ameliorate harsh environmental conditions during daytime low tides, fostering reproduction